Atomic Structure | ICSE Class 8 Chemistry Notes

This note covers atoms and subatomic particles, the development of atomic models, chemical symbols, atomic number and mass number, electron arrangements, valency, isotopes, average atomic mass, and isobars.

What is an atom, and how did ideas about it develop?

Atoms are the tiny building blocks of matter, the material making up living beings and non-living things. They are too small to see with the naked eye. Studying atomic structure means examining the smaller particles inside atoms and how those particles are arranged.

How did the idea of indivisible particles begin?

More than 2,000 years ago, Acharya Kanada suggested that repeatedly dividing matter would eventually produce particles that could not be divided further. He called them parmanus. His ideas are recorded in the Sanskrit text Vaisesika Sutras.

In this account, parmanus combine into dyads, groups of two, and triads, groups of three. However, the description does not specify the proportions in which they combine to produce different substances.

The Greek thinkers Leucippus and Democritus proposed indivisible particles called atomos, meaning indivisible. These early ideas were imagined explanations rather than conclusions based on experiments.

What changed with experimental evidence?

In 1808, John Dalton proposed an atomic theory based on scientific experiments. He described matter as being made of indivisible atoms. Later discoveries showed that atoms contain smaller components, so the idea that they could not be divided needed revision.

An atomic model is a representation used to explain the arrangement and behaviour of an atom's parts. Models changed as new experiments revealed evidence that earlier explanations could not account for.

Which particles make up an atom?

The three subatomic particles, meaning particles that form parts of an atom, are electrons, protons and neutrons. An electron carries negative electrical charge. A proton carries positive charge. A neutron has no electrical charge.

The nucleus is the extremely small, dense central region containing protons and neutrons. Electrons occupy the space around it. Most of an atom's mass is concentrated in its nucleus.

How do their charges and masses compare?

Relative charge compares charges using the size of a proton's charge as one unit. The symbols e⁻, p⁺ and n⁰ stand for an electron, proton and neutron respectively; their raised signs indicate negative, positive and zero charge.

Electrically neutral means having no overall charge. Protons and electrons have equal but opposite charges, so equal numbers balance. A helium atom contains two protons and two electrons; a sodium atom contains eleven of each.

Number of electrons = number of protons for a neutral atom. Neutrality does not mean that the atom contains no charged particles. It means that its total positive and negative charges balance.

Why was the neutron important?

Helium has two protons, yet its atomic mass, meaning the mass of an atom, is about four times that of hydrogen. Chadwick's discovery in 1932 explained the additional mass: neutrons contribute mass without adding electrical charge. He received the Nobel Prize in Physics in 1935.

Lighter atoms often have equal numbers of protons and neutrons, as in carbon with six of each and oxygen with eight of each. This is not a universal equality: the iron example has 26 protons and 30 neutrons.

How did Thomson discover electrons and model the atom?

In 1897, J. J. Thomson studied electric current, the flow of electric charge, passing through gases at very low pressure. His apparatus was a glass tube with two electrodes, the conductors through which the electrical supply was connected, and a high voltage. Voltage describes the difference in electrical energy per unit charge between the electrodes.

The negative electrode was the cathode; the positive electrode was the anode. Rays travelled from the cathode towards the anode. These cathode rays were streams of negatively charged particles later called electrons.

Their nature did not depend on the cathode material or the gas in the tube. This showed that electrons were components of atoms of every element. An element has atoms with the same number of protons, which distinguishes it from other elements.

What was the plum pudding model?

Thomson proposed a sphere of positive charge with electrons distributed through it. The positive and negative charges balanced, accounting for the atom's overall neutrality. His model was called the plum pudding model.

A watermelon provides a comparison: the red pulp represents the positive material, while the seeds represent the electrons spread through it. This is an analogy for arrangement, not a claim that an atom has coloured pulp or seeds.

A coloured circular representation of the positive sphere contains small dots. The labels identify an electron and the positive sphere. The electrons are distributed within the sphere.

Thomson received the 1906 Nobel Prize in Physics for studies of electrical conductivity in gases. His model explained charge balance, but later scattering experiments showed that the atom's positive charge was arranged differently.

What did the gold foil experiment reveal?

In 1911, Geiger and Marsden, working under Ernest Rutherford, directed a narrow beam of alpha particles at extremely thin gold foil. An alpha particle, written α-particle using the Greek letter alpha, is positively charged and contains two protons and two neutrons.

It is the nucleus of a helium atom. Certain radioactive elements emit particles and energy; this emission is called radioactivity. The experiment used the paths of these particles to investigate the arrangement of charge inside atoms.

How did the observations differ from the prediction?

If positive charge were spread throughout each atom as Thomson proposed, the particles were expected to pass through or be deflected only slightly. Scattering means a change from the original straight path.

A beam approaches thin gold foil from the left. The foil is represented by circles with central dots. Most arrows continue straight, while some paths bend and a path turns back.

These observations led to the idea of the atomic nucleus. Thomson's model could not explain the observed pattern, especially the large deflections of some particles.

Note: Keep the observations distinct: most passed through, some were deflected sharply, and a few bounced back. Saying that most bounced back reverses the evidence for an atom being mostly empty space.

The experiment established a small central concentration of charge and mass. It did not itself establish the existence of neutrons, which were discovered later.

How did Rutherford's and Bohr's models differ?

In Rutherford's model, electrons revolved around a small, positively charged nucleus, somewhat like planets around the Sun. The nucleus contained most of the mass, while most of the atom was empty space.

The atom's diameter was estimated as about 10⁻¹⁰ m, and the nucleus's diameter as about 10⁻¹⁵ m. Here m means metre; 10⁻¹⁰ means one divided by ten raised to the tenth power. The nucleus was about 10⁵, or one lakh, times smaller.

Why did Rutherford's explanation need improvement?

Motion in a circle continuously changes direction and therefore involves acceleration, a change in velocity. A charged electron moving this way was expected to lose energy, spiral inward and fall into the nucleus. Rutherford's model could not explain why atoms remain stable.

In 1913, Niels Bohr proposed allowed circular paths called shells, or energy levels. Each shell has a definite electron energy. These paths are also called stationary states or orbits in his model.

1. Shells are named K, L, M and N, proceeding outwards from the nucleus.
2. The shell number is written n: K has n = 1, L has n = 2, M has n = 3 and N has n = 4.
3. An electron in an allowed shell does not lose energy while revolving in that shell.
4. An electron changes shells by absorbing or releasing an amount of energy equal to the difference between their energies.

The K-shell is nearest the nucleus and has the least energy. Shell energy increases with distance from the nucleus. Bohr explained stability by proposing that electron energy remains constant in an allowed stationary state.

A central dot labelled nucleus is surrounded by four concentric circles. From inside out, the circles are labelled K-shell (n = 1), L-shell (n = 2), M-shell (n = 3) and N-shell (n = 4).

Bohr received the Nobel Prize in 1922. His model explained many observations but also had limitations. The modern description uses electron clouds, regions where electrons are most likely to be found, rather than well-defined circular paths.

Worked example: how many atoms span a sheet's thickness?

Worked example 1. A sheet is 0.1 mm thick, where mm means millimetre. Its thickness is 10⁻⁴ m, and an atom's diameter is about 10⁻¹⁰ m. Estimate how many atoms would be stacked across the thickness.

Answer: Number of atoms ≈ thickness ÷ atomic diameter = 10⁻⁴ m ÷ 10⁻¹⁰ m = 10⁶. Here ≈ means approximately equal to, and ÷ means divided by. About one million atoms would span the thickness.

How do chemical symbols and atomic numbers identify elements?

A chemical symbol is an internationally recognised abbreviation for an element. Dalton introduced pictorial symbols in 1803. In 1813, Berzelius suggested deriving symbols from Latin names, helping establish the use of letters.

The International Union of Pure and Applied Chemistry, abbreviated IUPAC, approves element names and symbols. Many symbols use one or two letters. The first letter is capitalised; a second letter, when present, is lowercase.

How should symbols be written?

Hydrogen is H, aluminium is Al, and cobalt is Co. Write Al, not AL, and Co, not CO. Some symbols use a later letter from the element's name, such as Cl for chlorine and Zn for zinc.

What is atomic number?

Definition: The atomic number, represented by Z, is the number of protons in an atom's nucleus. It identifies the element.

Z = number of protons. Since a neutral atom has equal numbers of protons and electrons, its atomic number also gives its electron count. Hydrogen has one proton, so its atomic number is 1; helium has two, so its atomic number is 2.

Use the proton count to identify the element. The electron count equals it under the stated condition of a neutral atom. Neutrons do not determine the atomic number.

Do not confuse the symbol K for potassium with the name K-shell. One represents an element; the other names the innermost electron shell. The surrounding words show which meaning is intended.

How do mass number and atomic number give particle counts?

The mass number, represented by A, counts the protons and neutrons in one atom's nucleus. These particles are collectively called nucleons. Electrons have almost negligible mass compared with them and are not included in this count.

A = number of protons + number of neutrons. Because Z already counts the protons, Number of neutrons = A − Z. The sign − means subtraction. Both atomic number and mass number are particle counts, not measurements in grams.

How is an atom written in standard notation?

Place the mass number at the upper left of the chemical symbol and the atomic number at the lower left. For example, ¹²₆C represents carbon with mass number 12 and atomic number 6. It therefore has six protons and six neutrons.

What sequence should a particle calculation follow?

1. Read the atomic number and use it as the proton count.
2. If the atom is neutral, use the same count for electrons.
3. Subtract the atomic number from the mass number to find neutrons.
4. Check that adding protons and neutrons reproduces the given mass number.

Worked example 2. A neutral atom has atomic number 26 and 56 nucleons. Find its protons, electrons and neutrons.

Answer: Its 56 nucleons give A = 56, while Z = 26. It has 26 protons and 26 electrons. Neutrons = 56 − 26 = 30. Check: 26 + 30 = 56 nucleons.

Worked example 3. A nucleus contains 20 protons and has mass number 41. Calculate its neutron count.

Answer: Atomic number = 20. Neutrons = 41 − 20 = 21. Adding 20 protons and 21 neutrons gives the stated mass number of 41.

These examples show why the two numbers must be kept separate. Atomic number tells you which element is present; mass number tells you how many nucleons that particular atom contains.

How are electrons arranged in shells?

The electronic configuration of an atom describes the distribution of its electrons among shells. In the shell model, electrons fill levels beginning nearest the nucleus. The arrangements below concern the first eighteen elements.

What do the Bohr-Bury rules tell us?

Maximum electrons in a shell = 2n², where n is the shell number and n² means n multiplied by itself. The multiplication sign × shows a product. Thus the shell number determines its maximum capacity.

Worked example 4. Calculate the maximum capacities of the K, L and M shells, using n = 1, 2 and 3 respectively.

Answer: K-shell: $2 \times 1^{2} = 2$ electrons. L-shell: $2 \times 2^{2} = 8$ electrons. M-shell: $2 \times 3^{2} = 18$ electrons.

The outermost shell can accommodate at most eight electrons in these rules, except the first shell, whose maximum is two. Fill K before L, and proceed outwards step by step. Distinguish a shell's capacity from the electrons actually present.

What are the configurations of the first eighteen elements?

In a configuration such as 2, 8, 1, successive numbers give the electron counts in K, L and M. Only occupied shells are listed. For each neutral atom, adding these numbers gives its atomic number.

The drawings show central nuclei, circular shells and electron dots for the first eighteen elements. Hydrogen and helium have one occupied shell; sodium through argon have three occupied shells.

For magnesium, twelve electrons are distributed as two in K, eight in L and two in M. Their sum is twelve. Argon has eight electrons in M, although the formula gives that shell a maximum capacity of eighteen.

Note: Do not write 2, 10 for magnesium. The L-shell can hold only eight electrons, so the two electrons remaining after K and L are filled occupy M.

How do valence electrons determine valency?

The valence shell is the outermost occupied shell. Its electrons are called valence electrons. An octet is a set of eight electrons in that shell. Helium has a complete first shell with two electrons.

Elements with a complete octet, or two outer electrons in helium, are largely unreactive and more stable. Atoms with incomplete valence shells are usually more reactive. They can lose, gain or share electrons to complete their outer arrangement.

What does combining capacity mean?

Valency means combining capacity. It can be expressed as the number of electrons lost, gained or shared to obtain a stable configuration. It can also be found from the number of hydrogen or chlorine atoms combining with one atom of the element.

Hydrogen and chlorine each have a combining capacity of one. A chemical formula uses element symbols and numbers to show the atoms combined in a substance. A lowered number after a symbol counts those atoms; an omitted number means one.

In water, H₂O, one oxygen atom combines with two hydrogen atoms, so oxygen's combining capacity is two. In ammonia, NH₃, nitrogen combines with three hydrogen atoms. In magnesium chloride, MgCl₂, magnesium combines with two chlorine atoms.

How does electron arrangement explain the examples?

Generally, an element with fewer than four valence electrons tends to lose electrons. With more than four, it tends to gain electrons. Carbon cannot easily gain or lose its four valence electrons and can share them instead.

Valency is therefore not automatically equal to the number of valence electrons. Oxygen has six valence electrons but a valency of two. Some compounds appear to violate the usual valency rule, so this introductory pattern should not be treated as exceptionless.

What are isotopes, and how can their properties differ?

Definition: Isotopes are atoms of the same element with the same atomic number but different mass numbers. Their proton counts match, while their neutron counts differ.

Dalton proposed that atoms of an element have the same mass. Isotopes showed that this part of his theory needed revision. Changing the neutron count changes the mass number without changing the element's identity.

What are the isotopes of hydrogen?

Hydrogen has three isotopes: protium, deuterium and tritium. Each has one proton. Each neutral atom also has one electron. Their neutron counts differ, as the table shows.

Naturally occurring hydrogen contains about 99.98% protium, about 0.015% deuterium, and tritium in traces. The sign % means per hundred. These proportions describe abundance, meaning how common each isotope is.

Three labelled drawings show protium, deuterium and tritium, each with one electron dot on a circular shell. Their central nuclei contain one, two and three coloured particles respectively.

Carbon also has three isotopes: ¹²₆C, ¹³₆C and ¹⁴₆C. Each has six protons and, when neutral, six electrons. Carbon-12 is the most abundant. The neutron counts are six, seven and eight respectively.

Isotopes have similar chemical properties because they have the same electronic configuration. Chemical properties depend mainly on valence electrons. Physical properties, such as melting and boiling points, differ. Do not describe all their properties as identical.

Why is chlorine's average atomic mass 35.5?

Atomic masses are expressed using the unified atomic mass unit, written u, a unit suited to tiny particles. Average atomic mass describes the average for an element's naturally occurring isotopes, taking account of how common each isotope is.

Chlorine occurs in two isotopic forms with masses of 35 u and 37 u, in the ratio 3:1. The lighter form makes up about 75%, while the heavier form makes up about 25%. Equal weighting would misrepresent this mixture.

How is a weighted average calculated?

A weighted average gives each value a contribution proportional to its abundance. Multiply each isotope's mass by its fraction of the sample and add the contributions. A percentage is converted to a fraction by dividing it by 100.

Worked example 5. Calculate chlorine's average atomic mass using isotopic masses 35 u and 37 u and abundances 75% and 25% respectively.

Answer: Average atomic mass = (35 × 75/100) + (37 × 25/100) = (105/4) + (37/4) = 142/4 = 35.5 u. Here / means division. The more abundant 35 u isotope contributes more to the average.

The simple mean, (35 + 37)/2 = 36, ignores the unequal abundances and gives 36 u. It is unsuitable for the stated natural mixture. The weighted result is closer to the mass of the more common isotope.

A value of 35.5 u does not mean that each chlorine atom has that mass. For one million atoms in this example, 7.5 lakh are chlorine-35 and 2.5 lakh are chlorine-37. Their combined average is 35.5 u.

Note: A mass number counts nucleons in a particular atom. Average atomic mass describes an isotopic mixture and can have a fractional value. Do not use chlorine's average mass as its neutron-plus-proton count.

How do isobars differ from isotopes, and why are isotopes useful?

Isobars are atoms of different elements with equal mass numbers but different atomic numbers. Their total numbers of nucleons match, but their proton counts do not. Equal mass numbers therefore do not establish that two atoms belong to the same element.

Calcium, potassium and argon provide an example. Their atomic numbers are 20, 19 and 18 respectively, yet each has an atom with mass number 40. Subtracting the atomic number from 40 gives 20, 21 and 22 neutrons respectively.

Which number should be compared first?

First compare proton counts to establish whether the element is the same. Then compare mass numbers. Isotopes differ through their neutron counts; isobars retain the same combined count of protons and neutrons despite different proton counts.

What applications do particular isotopes have?

• Uranium-235, written ²³⁵₉₂U, is used as fuel in a nuclear reactor, where nuclear energy is used to generate electricity.
• Cobalt-60, written ⁶⁰₂₇Co, is a radioactive isotope used in radiation treatment for cancer.
• Iodine-131, written ¹³¹₅₃I, is used to treat goitre, an enlargement of the thyroid gland in the neck, and thyroid cancer, cancer affecting that gland.
• Carbon-14, written ¹⁴₆C, helps determine the ages of ancient fossils and artefacts.

These applications concern particular isotopes with useful properties. They do not mean that every isotope has the same use. The element name and mass number together identify which isotope is being discussed.

Glossary

• Atom — A tiny building block of matter containing smaller particles arranged within and around a central nucleus.
• Electron — A negatively charged subatomic particle found around the nucleus, much lighter than a proton.
• Proton — A positively charged particle in the nucleus whose charge balances that of one electron.
• Neutron — An uncharged particle in the nucleus with a mass nearly equal to a proton's mass.
• Nucleus — The extremely small, dense central region containing an atom's positive charge and most of its mass.
• Atomic number — The number of protons in an atom's nucleus, represented by Z and identifying its element.
• Mass number — The combined count of protons and neutrons in one nucleus, represented by the letter A.
• Nucleons — The protons and neutrons present in a nucleus and counted together by its mass number.
• Electronic configuration — The distribution of an atom's electrons among its occupied shells or energy levels.
• Valence electrons — Electrons occupying the outermost occupied shell of an atom and mainly determining its chemical properties.
• Octet — A group of eight electrons occupying the outermost shell of an atom.
• Valency — Combining capacity, expressed through the electrons lost, gained or shared to achieve a stable configuration.
• Isotopes — Atoms of one element with equal atomic numbers but different neutron counts and mass numbers.
• Isobars — Atoms of different elements having equal mass numbers but different numbers of protons.
• Weighted average atomic mass — The average mass calculated by weighting each naturally occurring isotope's mass according to its abundance.

Common errors and misconceptions

• Misconception: A neutral atom contains no charged particles. Correct: Its protons and electrons are charged, but equal numbers give balanced positive and negative charges.
• Misconception: All atoms have equal proton and neutron counts. Correct: Lighter atoms often do, but the iron example contains 26 protons and 30 neutrons.
• Misconception: Most alpha particles bounced back from gold foil. Correct: Most passed through undeflected; a few bounced back.
• Misconception: Mass number includes electrons. Correct: It counts only protons and neutrons; electrons are not nucleons.
• Misconception: The M-shell's capacity of eighteen means argon has eighteen electrons in M. Correct: Argon's arrangement is 2, 8, 8, with eighteen electrons in total.
• Misconception: Valency equals the valence-electron count in every case. Correct: Oxygen has six valence electrons but valency two because it can gain two electrons.
• Misconception: Isotopes are different elements. Correct: Their proton counts and atomic numbers match; their neutron counts and mass numbers differ.
• Misconception: Every chlorine atom has mass 35.5 u. Correct: This is the weighted average for its naturally occurring isotopic mixture.

Exam-style questions with model answers

Q1. A neutral helium atom contains two protons and two electrons. Use relative charges +1 per proton and −1 per electron to explain why it has no overall charge. [2 marks]

1. Each proton has relative charge +1, so two protons provide a total positive charge of +2.
2. Each electron has relative charge −1, so two electrons provide −2. The equal and opposite charges balance.

Q2. A neutral atom has atomic number 26 and mass number 56. Calculate its numbers of protons, electrons and neutrons, explaining each result. [3 marks]

1. It has 26 protons because atomic number is defined as the number of protons in the nucleus.
2. It has 26 electrons because a neutral atom contains equal numbers of negatively charged electrons and positively charged protons.
3. It has 56 − 26 = 30 neutrons because mass number counts protons and neutrons together, so subtracting protons leaves neutrons.

Q3. In the gold foil experiment, most positively charged alpha particles passed straight through, some were sharply deflected, and a few bounced back. State three conclusions about atomic structure and explain why Thomson's model was inadequate. [4 marks]

1. Most of the atom is empty space, as shown by most alpha particles passing through without any deflection.
2. Positive charge is concentrated in a small region, since some positively charged particles experienced sharp changes in direction.
3. The central region is dense and contains most of the mass, accounting for the few particles that bounced back.
4. Thomson spread positive charge throughout the atom. This model could not explain the observed scattering, particularly the large deflections.

Q4. A neutral magnesium atom has atomic number 12 and mass number 24. Using a maximum of two electrons in K and eight in L, find its protons, neutrons, electrons, electronic configuration and number of valence electrons. Explain each answer. [5 marks]

1. The atom contains 12 protons. Atomic number gives the proton count directly, so the stated atomic number of 12 determines this result.
2. Its neutron count is 24 − 12 = 12. Mass number counts protons and neutrons together; subtracting the proton count leaves neutrons.
3. The atom has 12 electrons. Since it is neutral, its number of negative electrons must equal its number of positive protons.
4. Its electronic configuration is 2, 8, 2. Two electrons occupy K, eight occupy L, and the remaining two occupy M.
5. It has two valence electrons. M is its outermost occupied shell, and the configuration shows that this shell contains two electrons.

Q5. Chlorine has isotopic masses 35 u and 37 u with abundances 75% and 25% respectively. Give each isotope's weighted contribution, calculate the average, explain why the simple mean is unsuitable, and interpret the result. Here u means unified atomic mass unit. [5 marks]

1. The lighter isotope contributes 35 × 75/100 = 26.25 u to the weighted average. Its 75% abundance gives it the greater weight.
2. The heavier isotope contributes 37 × 25/100 = 9.25 u. Its smaller contribution reflects its lower abundance in the stated mixture.
3. Adding the two contributions gives 26.25 + 9.25 = 35.5 u. This is the weighted average atomic mass for the given abundances.
4. The simple mean gives (35 + 37)/2 = 36 u. It incorrectly weights the two isotopes equally despite their different percentages.
5. The answer describes the mixture's average. It does not mean that every individual chlorine atom has a mass of 35.5 u.

Q6. Hydrogen atoms called protium, deuterium and tritium each contain one proton; their neutron counts are zero, one and two respectively. For their neutral atoms, state the common atomic number, give all three mass numbers, explain their isotope relationship, and compare their electronic configurations. [4 marks]

1. All three have atomic number 1 because each nucleus contains exactly one proton, and protons determine atomic number.
2. Their mass numbers are 1, 2 and 3 respectively, obtained by adding the single proton to each stated neutron count.
3. They are isotopes of hydrogen: the same atomic number identifies one element, while different neutron counts produce different mass numbers.
4. Each neutral atom has one electron in the K-shell. Their electronic configurations match, accounting for their similar chemical properties.

Q7. Sodium, oxygen and carbon have electronic configurations 2, 8, 1; 2, 6; and 2, 4 respectively. Explain how each can complete an octet and state its valency. [3 marks]

1. Sodium can lose its single outermost electron, leaving the stable arrangement 2, 8. Since one electron is lost, its valency is 1.
2. Oxygen can gain two electrons to complete eight in its outer shell. Its valency is 2, although it initially has six valence electrons.
3. Carbon cannot easily gain or lose four electrons. It can share four with other atoms to complete its octet, giving valency 4.

Q8. Calcium and argon atoms each have mass number 40, but their atomic numbers are 20 and 18 respectively. Identify their relationship and calculate their neutron counts. [2 marks]

1. They are isobars because they have the same mass number but different atomic numbers, so they belong to different elements.
2. Calcium has 40 − 20 = 20 neutrons; argon has 40 − 18 = 22 neutrons, using mass number minus atomic number.

Key takeaways

• Atoms contain electrons, protons and neutrons; most of their mass lies in a tiny central nucleus.
• Equal numbers of protons and electrons make a neutral atom because their charges are equal and opposite.
• Gold foil scattering supported a small, dense, positively charged nucleus and showed that most atomic space is empty.
• Bohr proposed fixed energy levels where electrons do not lose energy; later models refined this picture.
• Atomic number counts protons, while mass number counts protons plus neutrons; subtracting the former from the latter gives neutrons.
• Electronic configuration describes shell populations, while valency concerns electrons lost, gained or shared to reach a stable configuration.
• Isotopes share atomic numbers but differ in mass numbers; isobars share mass numbers but have different atomic numbers.
• Average atomic mass depends on isotopic abundances, so chlorine's value of 35.5 u describes an average across its mixture.

Test yourself

What observation showed that most of an atom is empty space?

Most alpha particles passed through the thin gold foil without being deflected.

What does Z represent, and which particle determines it?

Z represents atomic number, determined by the number of protons in the nucleus.

A neutral atom has 17 protons and 18 neutrons. What is its mass number?

Its mass number is 17 + 18 = 35, the combined number of protons and neutrons.

Why can oxygen have six valence electrons but valency two?

Oxygen can gain two electrons to complete its outer octet, so its valency is two.

Which hydrogen isotope has no neutrons?

Protium has one proton and no neutrons; its mass number is one.

Why do neutral isotopes have similar chemical properties?

They have equal electron counts and the same electronic configuration, including the same valence-electron arrangement.

What distinguishes the neutron from the proton in terms of charge?

A neutron has no electrical charge, while a proton has positive electrical charge.

In Bohr's model, what happens to electron energy within an allowed stationary shell?

Its energy remains constant; the electron does not lose energy while moving within that shell.