Electrovalent and covalent bonding | ICSE Class 9 Chemistry Notes

This note covers valence electrons, duplets and octets, ion formation, electrovalent bonding, covalent bonding, orbit and electron-dot structures, single, double and triple bonds, and radical formation when a covalent bond breaks.

Why do atoms form chemical bonds?

A chemical bond is the attractive force holding atoms or ions together in a chemical substance. An atom is a particle of an element. Its nucleus, or central region, contains positively charged protons; negatively charged electrons occupy shells around it.

An electron shell is an energy level occupied by electrons. In simple orbit diagrams, shells are drawn as circles. The letters K, L, M and N name the first, second, third and fourth shells respectively, starting nearest the nucleus.

What makes the outer shell important?

The electronic configuration gives the distribution of electrons among shells. For example, sodium has the configuration 2,8,1: two electrons in the first shell, eight in the second and one in the third. Commas separate successive shells.

Valence electrons are electrons in the outermost occupied shell, called the valence shell. They take part in chemical combination. Inner-shell electrons are generally not involved. Thus, the outer-shell count is the starting point for explaining the bonding examples below.

Noble gases are elements with completely filled valence shells and little chemical activity. Helium has two electrons in its first shell. Neon has the configuration 2,8, while argon has 2,8,8. Reactivity is explained through the tendency to attain a completely filled valence shell.

Definition: A duplet is a completed first shell containing two electrons. An octet is an arrangement of eight electrons in the outermost shell.

The octet rule explains combination through transfer or sharing of valence electrons to attain eight outer-shell electrons. Hydrogen achieves a duplet when it shares electrons. The octet rule is useful but not universal; it must not be treated as a rule without exceptions.

Valency means combining capacity. In the simple examples here, count how many electrons an atom loses, gains or shares to obtain the completed outer-shell arrangement. Distinguish this combining capacity from the total number of electrons in the atom.

How do electrons form positive and negative ions?

An ion is an atom or group of atoms carrying a net electric charge. A neutral atom has equal numbers of protons and electrons. Losing or gaining electrons changes that balance without changing the number of protons in its nucleus.

Definition: A cation is a positively charged ion formed by electron loss; an anion is a negatively charged ion formed by electron gain.

The notation e⁻ represents one electron. A superscript positive or negative sign shows an ion's charge. A numeral before the sign gives the number of unit charges; when that numeral is one, it is normally omitted.

For example, Na is the symbol for sodium and Na⁺ means a sodium ion with one positive charge. Cl is chlorine; Cl⁻ is a chloride ion with one negative charge. An arrow, →, means “forms” in the electron-change statements below.

Atomic number means the number of protons in an atom's nucleus. For a neutral atom, it also gives the electron count.

Which starting configurations are needed?

Sodium initially has eleven electrons. Losing one leaves ten electrons but eleven protons, producing the positive charge on Na⁺.

Chlorine initially has seventeen electrons and seventeen protons. Gaining one electron leaves eighteen electrons but seventeen protons, producing Cl⁻. The negative sign records an extra electron, not an extra proton. Electron gain makes the charge more negative.

Electrovalence is expressed by the number of unit charges on an ion. Magnesium loses two electrons and forms Mg²⁺; calcium forms Ca²⁺ in the same way. Oxygen gains two electrons and forms the oxide ion, O²⁻.

Note: A positive ion has lost electrons. A negative ion has gained electrons. Do not reverse these changes merely because the charge sign looks like an instruction to add or subtract.

How does electrovalent bonding produce sodium chloride?

An electrovalent bond, also called an ionic bond, is the electrostatic attraction between oppositely charged ions. Electrostatic attraction means attraction arising from electric charges. Electron transfer creates the ions; their attraction holds the ionic substance together.

What happens during electron transfer?

1. Sodium begins with the electronic configuration 2,8,1 and one valence electron.
2. It loses that electron and forms Na⁺, whose configuration is 2,8. The formerly inner second shell becomes its outermost occupied shell.
3. Chlorine begins with 2,8,7 and gains the electron lost by sodium, forming Cl⁻ with configuration 2,8,8.
4. The oppositely charged sodium and chloride ions attract one another, forming sodium chloride.

The electron-change statements are Na → Na⁺ + e⁻ and Cl + e⁻ → Cl⁻. A plus sign between separate particles means “and” or “combines with”; it is distinct from a superscript plus sign denoting ionic charge.

The chemical formula NaCl represents sodium chloride. A formula uses element symbols to show composition. Here, one sodium ion balances one chloride ion because their charges have equal size and opposite signs. The compound as a whole is electrically neutral.

An arrow shows sodium's outer electron moving towards chlorine. The products are marked Na⁺ and a bracketed Cl⁻ with eight outer electrons. Electron dots and crosses distinguish their starting origins.

See Fig. 3.5 in your NCERT textbook

For a full orbit structure, draw sodium with three occupied shells and chlorine with three. After transfer, show two occupied shells for Na⁺ and three for Cl⁻. Label both charges and show the completed outer shells.

Sodium chloride does not exist as separate molecules. It consists of aggregates of oppositely charged ions. Therefore, its formula expresses the simplest ion ratio; a drawing of one sodium ion beside one chloride ion represents the transfer and ratio, not an isolated molecule.

How do magnesium chloride and calcium oxide form?

The same electron-transfer method explains compounds in which two electrons move. The crucial checks are the electrons each atom loses or gains, the resulting charges, and the ratio needed to make the compound electrically neutral.

Why does magnesium need two chlorine atoms?

Magnesium chloride has the formula MgCl₂. The small lowered numeral ₂ is a subscript: here it indicates two chloride ions for each magnesium ion. It does not mean that one chloride ion has a charge of two.

1. Magnesium starts with 2,8,2 and loses its two valence electrons.
2. The resulting Mg²⁺ ion has configuration 2,8 and two positive unit charges.
3. Each chlorine atom starts with 2,8,7 and accepts one electron. Two chlorine atoms therefore accept magnesium's two electrons.
4. Each chloride ion has configuration 2,8,8. One Mg²⁺ balances two Cl⁻ ions, giving MgCl₂.

The electron changes are Mg → Mg²⁺ + 2e⁻ and 2Cl + 2e⁻ → 2Cl⁻. A numeral written before a symbol counts particles: 2Cl here means two chlorine atoms in the electron-transfer account.

Two arrows lead from magnesium's two outer electrons towards two chlorine atoms. The product symbols show Mg²⁺ and chloride with a complete outer octet; a subscript two records the two chloride ions.

See Fig. 3.6 in your NCERT textbook

Why is calcium oxide a one-to-one combination?

Calcium oxide, CaO, contains calcium and oxide ions. Calcium begins with 2,8,8,2. Losing two electrons produces Ca²⁺ with 2,8,8. Oxygen begins with 2,6. Gaining those two electrons produces O²⁻ with 2,8.

The electron changes are Ca → Ca²⁺ + 2e⁻ and O + 2e⁻ → O²⁻. One calcium ion's two positive charges balance one oxide ion's two negative charges. The simplest ratio is one calcium ion to one oxide ion, giving CaO.

Draw calcium's four occupied shells and oxygen's two. Transfer both outer calcium electrons to oxygen. Redraw Ca²⁺ as 2,8,8 and O²⁻ as 2,8, placing the charges clearly beside the ions.

Magnesium chloride and calcium oxide both involve the loss of two electrons by a metal atom. Their formulae differ because chlorine accepts one electron per atom, whereas oxygen accepts two. Matching the electron gain and loss explains the different ratios.

How does covalent bonding differ from electrovalent bonding?

Definition: A covalent bond is formed by the sharing of an electron pair between two atoms. The shared electrons count towards the outer-shell arrangement of both bonded atoms.

A molecule is a group of chemically bonded atoms forming a particle of a substance. The simple covalent molecules considered here contain shared pairs. They are represented differently from the separate charged ions used to explain electrovalent compounds.

How are shared electrons counted?

A bond pair is a shared electron pair joining two atoms. A lone pair is a pair of valence electrons not shared in a bond. In an electron-dot structure, dots represent valence electrons; crosses may distinguish electrons originally contributed by another atom.

Dots and crosses do not represent different kinds of electron. A shared pair may contain one dot and one cross. Count that pair towards the completed outer shell of each bonded atom, but count it only once when checking the molecule's total electrons.

The symbol H means hydrogen. In H₂, the subscript two counts hydrogen atoms within one molecule. A bond can also be represented by a line: H-H represents one shared pair between two hydrogen atoms.

Use single bond for one shared pair, double bond for two shared pairs and triple bond for three shared pairs between the same two atoms. These correspond to two, four and six shared electrons respectively.

How do hydrogen, chlorine and hydrogen chloride form single bonds?

How does hydrogen complete its first shell?

A hydrogen atom has one electron in its first shell and needs one more for a duplet. Two hydrogen atoms contribute one electron each to a shared pair. Each atom then counts two electrons in its first shell, giving the molecule H₂.

Hydrogen is diatomic, meaning that each molecule contains two atoms. Its structure H-H contains one single covalent bond. Neither hydrogen atom needs eight electrons: the completed first shell contains two, like the first shell of helium.

Two hydrogen circles overlap, with the two electrons drawn in the shared region. The second figure also shows H-H as a line representation of the single bond.

See Figs. 4.1 and 4.2 in your NCERT textbook

How does chlorine obtain an octet?

Each chlorine atom has configuration 2,8,7. Two chlorine atoms share one electron each, forming Cl₂, a chlorine molecule. Each atom counts the shared pair plus six unshared electrons, making eight electrons around that chlorine.

The structure Cl-Cl therefore contains one bond pair and three lone pairs on each chlorine atom. Sharing completes both octets without forming a sodium-like cation and chloride anion. The molecular formula counts two chlorine atoms joined by a single bond.

What changes when the two atoms are different?

Hydrogen chloride, HCl, contains one hydrogen atom bonded to one chlorine atom. Hydrogen contributes one electron and chlorine contributes one to their shared pair. Hydrogen obtains a duplet, while chlorine obtains an octet and retains three lone pairs.

In a full orbit drawing, show hydrogen's first shell overlapping chlorine's outermost, third shell. Place one shared pair in the overlap. Chlorine's inner-shell configuration remains 2,8, and six of its outer electrons remain outside the shared region.

All three molecules have one single bond, but their completed shells differ. H₂ gives two duplets; Cl₂ gives two octets; HCl gives one duplet and one octet. The number of shared pairs is one in each case.

Why do oxygen and nitrogen form multiple bonds?

Some atoms require more than one additional electron to complete an octet. When the same two atoms share two or three pairs, they form a multiple bond. The number of shared pairs must match the electron requirements of both atoms.

How does the oxygen double bond form?

Oxygen has electronic configuration 2,6. Each oxygen atom needs two more electrons to complete its outer shell. Each contributes two electrons to sharing with another oxygen atom, producing two shared pairs in the oxygen molecule, O₂.

The notation O=O represents a double covalent bond; the equals-shaped bond sign stands for two shared pairs here. Each oxygen also has two lone pairs. Counting four shared electrons and four unshared electrons gives an octet around each oxygen.

Two oxygen circles overlap with four electrons in the shared region. The remaining electrons lie outside the overlap. An electron-dot structure below shows two shared pairs between the oxygen symbols.

See Fig. 4.3 in your NCERT textbook

How does the nitrogen triple bond form?

N is the symbol for nitrogen. Its electronic configuration is 2,5, so each atom needs three more electrons for an octet. Each nitrogen contributes three electrons to sharing with another nitrogen, producing the molecule N₂ with three shared pairs.

The notation N≡N represents this triple covalent bond. The sign ≡ stands for three shared pairs. Each nitrogen retains one lone pair, so six shared electrons and two unshared electrons complete its outer octet.

Two nitrogen circles overlap, enclosing six shared electrons. One unshared pair remains beside each nitrogen. The lower representation shows three pairs between the nitrogen symbols.

See Fig. 4.4 in your NCERT textbook

Both O₂ and N₂ are diatomic, although they have different numbers of bonds between their atoms. A subscript two counts atoms, not shared pairs. Read the bond sign or electron-dot structure to distinguish a double bond from a triple bond.

How are the bonds in water and ammonia formed?

Water and ammonia contain hydrogen bonded to a central atom. A central atom is the atom to which the surrounding atoms are attached in the structure. Each hydrogen forms a single bond and completes a duplet through sharing.

Why does water contain two hydrogen atoms?

Water has the formula H₂O: two hydrogen atoms and one oxygen atom per molecule. Oxygen starts with six valence electrons. It shares one electron with each of two hydrogen atoms, and each hydrogen contributes one electron to its own shared pair.

1. Place oxygen between the two hydrogen symbols in the bonding diagram.
2. Show one shared pair between oxygen and the first hydrogen, then another shared pair between oxygen and the second hydrogen.
3. Place oxygen's four remaining valence electrons as two lone pairs.
4. Check that oxygen counts eight outer electrons and that each hydrogen counts two.

The bonding arrangement H-O-H shows two single covalent bonds. It does not show a double bond to one hydrogen. Oxygen uses two shared pairs with two different neighbouring atoms, whereas O₂ uses two shared pairs between the same two atoms.

Draw oxygen's two shells and the first shell of each hydrogen. Overlap each hydrogen shell with oxygen's outer shell. Show one pair in each overlap and two lone pairs remaining on oxygen.

Why does ammonia contain three hydrogen atoms?

Ammonia has the formula NH₃: one nitrogen atom and three hydrogen atoms per molecule. Nitrogen starts with five valence electrons and shares one with each hydrogen. The three hydrogen atoms each contribute one electron, producing three single bonds.

Nitrogen has three bond pairs and one lone pair. The six shared electrons and the two electrons of its lone pair complete its octet. Each hydrogen counts only the pair in its own bond to nitrogen and therefore obtains a duplet.

Draw nitrogen with two shells and three surrounding hydrogen atoms with one shell each. Show three separate shared pairs in the overlaps with nitrogen's outer shell. Leave one lone pair on nitrogen and label all atoms.

How does carbon bond in methane and carbon tetrachloride?

C is the symbol for carbon. A carbon atom has atomic number six and electronic configuration 2,4. Its four valence electrons can form four shared pairs. Carbon is tetravalent, meaning that it has a combining capacity of four.

Gaining four electrons would make it difficult for carbon's nucleus to hold the extra electrons. Losing four would require a large amount of energy. Carbon overcomes this problem by sharing its valence electrons with other atoms.

How are methane's four bonds formed?

Methane has the formula CH₄. Carbon shares one electron with each of four hydrogen atoms. Each hydrogen contributes one electron to its shared pair. This produces four single carbon-hydrogen bonds, completing carbon's octet and each hydrogen's duplet.

Carbon is drawn at the centre with four hydrogen circles around it. Each hydrogen overlaps the carbon circle, and one shared pair is marked in each of the four overlapping regions.

See Fig. 4.5 in your NCERT textbook

The four carbon electrons and four hydrogen electrons supply eight valence electrons altogether. They form four bond pairs. Carbon has no lone pair in this structure. When checking the total, do not count each shared pair separately for both atoms.

How does carbon tetrachloride complete five octets?

Carbon tetrachloride has the formula CCl₄. Carbon shares one electron with each of four chlorine atoms. Each chlorine contributes one electron to its bond with carbon. Four single carbon-chlorine bonds give carbon an outer octet.

Each chlorine counts its shared pair and three lone pairs, also completing an octet. Carbon has four bond pairs and no lone pair. The chlorine atoms retain their inner-shell electrons, which are omitted when drawing only the valence-electron structure.

Put carbon at the centre with four chlorine atoms around it. Draw two shells for carbon and three for each chlorine. Show one shared pair in each outer-shell overlap and three lone pairs on each chlorine.

CH₄ and CCl₄ both have four single bonds around carbon. Their outer atoms differ: hydrogen reaches a duplet, while chlorine reaches an octet. Neither structure requires carbon to transfer all four of its valence electrons to another atom.

How can breaking a covalent bond form radicals?

Bond formation involves a shared electron pair; understanding bond breaking requires following those same electrons. In homolytic cleavage, a covalent bond breaks so that each bonded atom takes one electron from the shared pair. “Cleavage” means breaking a bond.

Definition: A free radical is an atom or group of atoms containing an unpaired electron. Homolytic cleavage produces free radicals; the neutral radicals considered here do not carry an ionic charge.

An unpaired electron is an electron not paired with another electron in the structure. A single dot beside a chemical symbol denotes it. Thus, Cl• represents a chlorine radical. The dot is neither a negative charge nor a multiplication sign.

What happens to a chlorine molecule?

A chlorine molecule contains a single covalent bond with two shared electrons. In the presence of light or heat, that bond can undergo homolysis, another name for homolytic cleavage. Each chlorine atom takes one electron from the former shared pair.

The change is Cl₂ → 2Cl•, with light or heat supplied. One chlorine molecule forms two chlorine radicals. Each radical has seven outer electrons, including one unpaired electron. The two electrons from the former bond are divided equally between the atoms.

These radicals are very reactive. Do not describe the products as two chloride ions: chloride, Cl⁻, has gained an electron and has eight outer electrons. A chlorine radical is neutral and has an unpaired electron instead.

The electron destination is the essential explanation: transfer produces the ions discussed earlier, sharing produces covalent bonds, and equal division of a broken shared pair produces radicals. A radical symbol and an ion symbol therefore convey different information.

Glossary

• Chemical bond — An attractive force holding atoms or ions together within a chemical substance.
• Electronic configuration — The distribution of an atom's or ion's electrons among its occupied shells.
• Valence electrons — Electrons in the outermost occupied shell that take part in chemical combination.
• Duplet — A completed first electron shell containing a total of two electrons.
• Octet — An arrangement containing eight electrons in an atom's outermost shell.
• Ion — An atom or group of atoms carrying a net positive or negative electric charge.
• Cation — A positively charged ion formed when an atom loses one or more electrons.
• Anion — A negatively charged ion formed when an atom gains one or more electrons.
• Electrovalent bond — The electrostatic attraction between oppositely charged ions formed through electron transfer.
• Covalent bond — A chemical bond formed when two atoms share an electron pair.
• Bond pair — A pair of electrons shared between two atoms in a covalent bond.
• Lone pair — A pair of valence electrons not shared with another atom in bonding.
• Valency — The combining capacity of an element, related to the electrons involved in bonding.
• Free radical — An atom or group of atoms containing an unpaired electron in its structure.
• Homolytic cleavage — Breaking a covalent bond so each bonded atom takes one electron from the shared pair.

Common errors and misconceptions

• Misconception: A sodium atom gains an electron to become Na⁺. Correct: It loses one electron. The unchanged proton number then exceeds the electron number by one.
• Misconception: The subscript in MgCl₂ means that chlorine has a double negative charge. Correct: It indicates two chloride ions, each carrying one negative charge, for every magnesium ion.
• Misconception: Sodium chloride consists of separate NaCl molecules. Correct: It consists of aggregates of oppositely charged ions; NaCl expresses their simplest ratio.
• Misconception: Hydrogen must complete an octet. Correct: Its completed first shell is a duplet, achieved by sharing one electron pair in the molecules discussed here.
• Misconception: The two bonds in water make one double bond. Correct: Water contains two single bonds to two different hydrogen atoms; a double bond shares two pairs between the same two atoms.
• Misconception: Every pair around oxygen in water is shared. Correct: Oxygen has two shared bond pairs and two unshared lone pairs, together completing its octet.
• Misconception: A chlorine radical is a chloride ion. Correct: Cl• is neutral and has an unpaired electron; Cl⁻ has gained one electron and carries a negative charge.

Exam-style questions with model answers

Q1. Define an electrovalent bond and a covalent bond, distinguishing attraction between ions from electron sharing. [2 marks]

1. An electrovalent bond is the electrostatic attraction between oppositely charged ions formed through electron transfer.
2. A covalent bond is formed when two atoms share an electron pair.

Q2. Sodium has configuration 2,8,1 and chlorine has 2,8,7. Explain the electron changes, the resulting configurations and the simplest ion ratio in sodium chloride. [3 marks]

1. Sodium loses its single outer electron and becomes Na⁺. Its configuration changes from 2,8,1 to 2,8, leaving a completed outer shell.
2. Chlorine gains that electron and becomes Cl⁻. Its configuration changes from 2,8,7 to 2,8,8, completing an octet.
3. The oppositely charged ions attract. Their equal and opposite single charges balance in a one-to-one ratio, so the compound's formula is NaCl.

Q3. Magnesium has configuration 2,8,2; chlorine has 2,8,7. Explain magnesium chloride formation, giving both ionic configurations, the number of chlorine atoms required and the formula. [4 marks]

1. Magnesium loses two valence electrons and forms Mg²⁺ with configuration 2,8. This gives the magnesium ion a completed outer shell.
2. Each chlorine atom gains one electron and forms Cl⁻ with configuration 2,8,8.
3. Two chlorine atoms are needed because magnesium supplies two electrons but each chlorine accepts one.
4. One Mg²⁺ ion balances two Cl⁻ ions. Their electrostatic attraction forms magnesium chloride, with formula MgCl₂.

Q4. Calcium has configuration 2,8,8,2; oxygen has 2,6. Describe electron transfer, state the resulting ions and configurations, and explain why calcium oxide has formula CaO. [4 marks]

1. Calcium loses its two outer electrons and forms Ca²⁺. Its remaining electrons have configuration 2,8,8.
2. Oxygen gains those two electrons and forms O²⁻. Its resulting configuration is 2,8, completing an octet.
3. The electron loss by calcium equals the electron gain by oxygen, so one atom of each supplies the required transfer.
4. One Ca²⁺ balances one O²⁻ because their charges are equal and opposite. The simplest ratio is one to one, giving CaO.

Q5. Oxygen has configuration 2,6 and nitrogen has 2,5. Explain bonding in O₂ and N₂, giving the electrons contributed by each atom, shared pairs, bond types and lone pairs. Explain why both are called diatomic. [5 marks]

1. Each oxygen has six valence electrons and requires two more for an octet. It contributes two electrons to sharing with the other oxygen atom.
2. O₂ therefore has two shared pairs and a double bond, O=O. Each oxygen retains two lone pairs, which complete its octet together with the shared electrons.
3. Each nitrogen has five valence electrons and requires three more for an octet. It contributes three electrons to sharing with the other nitrogen atom.
4. N₂ therefore has three shared pairs and a triple bond, N≡N. Each nitrogen retains one lone pair and counts eight outer electrons.
5. Both molecules are diatomic because each contains two atoms. The number of atoms does not determine whether the bond is double or triple.

Q6. Water is H₂O and ammonia is NH₃. Hydrogen has one valence electron, oxygen six and nitrogen five. Describe the bond pairs, lone pairs and completed outer shells in both molecules. [5 marks]

1. In water, oxygen shares one electron with each of two hydrogen atoms. Each hydrogen contributes one electron, making two shared pairs and two single bonds.
2. Oxygen's remaining four valence electrons form two lone pairs. These and the two bond pairs complete an octet around oxygen.
3. In ammonia, nitrogen shares one electron with each of three hydrogen atoms. Each hydrogen contributes one electron, producing three shared pairs and three single bonds.
4. Nitrogen's remaining two valence electrons form one lone pair. Three bond pairs and this lone pair together complete its octet.
5. Every hydrogen in both molecules counts the two electrons of its own shared pair. Each therefore achieves a duplet, rather than an octet.

Q7. Methane is CH₄ and carbon tetrachloride is CCl₄. Carbon has four valence electrons, hydrogen one and chlorine seven. Compare carbon's bonds, the completed shells of the surrounding atoms and their lone pairs. [3 marks]

1. Carbon forms four single covalent bonds in each molecule. Four shared pairs give carbon an octet, with no lone pair remaining on carbon.
2. In methane, each hydrogen shares one pair with carbon and achieves a duplet. Hydrogen has no lone pair in this structure.
3. In carbon tetrachloride, each chlorine shares one pair with carbon and retains three lone pairs. Each chlorine therefore completes an octet.

Q8. A chlorine molecule, Cl₂, has one shared electron pair. With light or heat supplied, the bond breaks so each atom takes one electron from that pair. Name the process and identify the products and their charge. [2 marks]

1. The process is homolytic cleavage: the two electrons of the shared pair separate equally between the bonded atoms.
2. Two neutral chlorine radicals, 2Cl•, form. Each has an unpaired electron; the products are not negatively charged chloride ions.

Key takeaways

• Valence electrons take part in bonding; electron transfer or sharing can produce completed outer shells in the examples studied.
• Electron loss forms positive cations, while electron gain forms negative anions without changing the number of protons.
• Electrovalent bonding is the attraction between opposite ionic charges; the resulting compound has an electrically neutral overall composition.
• Sodium chloride and calcium oxide have one-to-one ion ratios, while magnesium chloride requires two chloride ions per magnesium ion.
• A single, double or triple covalent bond contains one, two or three shared electron pairs respectively.
• Hydrogen completes a duplet; oxygen, nitrogen, chlorine and carbon complete octets in the covalent examples discussed.
• Water has two single bonds and ammonia three; methane and carbon tetrachloride each have four single bonds around carbon.
• Homolytic cleavage divides a shared pair equally and produces radicals with unpaired electrons, which must be distinguished from ions.

Test yourself

Why does sodium become positively charged after losing an electron?

Its proton number remains unchanged while its electron number decreases. It therefore has one more proton than electron.

Chlorine has configuration 2,8,7. How many electrons does one chlorine atom accept in forming chloride?

It accepts one electron, becoming Cl⁻ with configuration 2,8,8 and a completed outer octet.

Why does MgCl₂ contain two chloride ions for every magnesium ion?

Magnesium forms Mg²⁺, whereas each chloride ion is Cl⁻. Two single negative charges balance the two positive charges.

How many electrons are shared in a double covalent bond?

Four electrons are shared, arranged as two pairs between the same two bonded atoms.

Why does hydrogen in HCl complete a duplet rather than an octet?

Hydrogen's completed first shell contains two electrons. Its shared pair with chlorine supplies that duplet.

How many lone pairs remain on nitrogen in ammonia?

One lone pair remains on nitrogen, alongside the three bond pairs shared with three hydrogen atoms.

How do carbon's bonds in CH₄ and CCl₄ compare?

Carbon forms four single bonds in each molecule, sharing four electron pairs and completing its outer octet.

What distinguishes Cl• from Cl⁻?

Cl• is a neutral chlorine radical with an unpaired electron. Cl⁻ is a negatively charged chloride ion with a completed octet.