Chapter 4: Chemical Bonding

Atoms can bond when joining lowers their total energy. Lower energy usually means greater stability.

Valence electrons usually take part in bonding.

Atoms may lose, gain or share electrons. In a metal, some outer electrons move through the whole material. These are called delocalized electrons.

Many atoms reach an outer electron arrangement like a noble gas. The octet rule helps explain this, but it does not work for every substance.

A coordinate covalent bond is a covalent bond formed in a particular way: one atom supplies both electrons of the shared pair.

Electrons in the outermost shell containing electrons are valence electrons. They largely determine an atom's chemical properties and bonding capacity.

Helium has a full first shell with 2 electrons: a duplet. Neon and argon have 8 outer electrons: an octet. Noble gases usually react very little.

Hydrogen can share one electron with another atom to gain access to two electrons in its first shell.

Many main-group atoms lose, gain or share electrons to reach eight electrons in their outermost shell.

Sodium commonly loses one electron: Na → Na⁺ + e⁻.

Chlorine commonly gains one electron: Cl + e⁻ → Cl⁻.

The rule has exceptions. H needs only 2 electrons. Boron in BF₃ has fewer than 8 around it. Phosphorus in PCl₅ has more than 8 around it.

Use the octet rule to help explain many formulas. It is a guide, not a rule for every bond.

Na has arrangement 2,8,1. It loses its outer electron and becomes Na⁺, with arrangement 2,8. Cl has arrangement 2,8,7. It gains that electron and becomes Cl⁻, with arrangement 2,8,8.

Na → Na⁺ + e⁻ | Cl + e⁻ → Cl⁻

Na⁺ and Cl⁻ have opposite charges. They attract and form a crystal containing many ions.

Cation: an ion with positive charge, usually formed by electron loss (Na⁺, Mg²⁺).

Anion: an ion with negative charge, usually formed by electron gain (Cl⁻, O²⁻).

Electron transfer makes the ions in this example. The attraction between opposite ions is the ionic bond. Solid NaCl contains a large pattern of ions, not separate NaCl molecules.

Mg (2,8,2) loses 2 electrons to form Mg²⁺ (2,8). O (2,6) gains 2 electrons to form O²⁻ (2,8). One +2 charge balances one −2 charge. The formula is MgO.

Mg → Mg²⁺ + 2e⁻ | O + 2e⁻ → O²⁻

Choose enough positive and negative ions to make total charge zero.

Mg²⁺ + O²⁻ gives MgO.

Mg²⁺ + 2Cl⁻ gives MgCl₂.

2Al³⁺ + 3O²⁻ gives Al₂O₃.

Mg²⁺ and O²⁻ attract each other strongly. A lot of energy is needed to separate them. MgO therefore melts at a much higher temperature than NaCl. Ion size and the crystal arrangement also affect the attraction.

Each H atom supplies 1 electron. The atoms share the pair. Each H now has access to 2 electrons: H—H.

Each Cl starts with 7 outer electrons. The atoms share 1 pair, giving each access to 8 electrons. Each Cl also has 3 lone pairs.

One shared pair makes a single bond, as in H₂, Cl₂, HCl and each C—H bond of methane.

A double bond contains 2 shared pairs. O₂ is written O=O. Each O also has 2 lone pairs. Ethene, H₂C=CH₂, has a double bond between its carbon atoms.

A triple bond contains 3 shared pairs. N₂ is written N≡N. Each N also has 1 lone pair. Ethyne is H—C≡C—H.

For comparable bonds between the same kinds of atoms, a triple bond is usually shorter and stronger than a double bond. A double bond is usually shorter and stronger than a single bond.

Each bond line stands for 1 shared pair of electrons. It does not stand for an atom.

The N≡N bond is very strong. This helps explain why nitrogen reacts slowly under ordinary conditions.

Two electrons shared between bonded atoms. A single covalent bond contains one bond pair.

Two valence electrons located mainly on one atom that do not form a particular bond.

Lone pairs affect molecular shape. An atom can also donate a lone pair to form a coordinate covalent bond.

Electronegativity is the ability of an atom in a bond to attract the shared electrons towards itself.

Two atoms share electrons equally or nearly equally. H—H, O=O, Cl—Cl and N≡N are examples.

Atoms share electrons unequally. The more electronegative atom develops partial negative charge (δ−), and the other develops partial positive charge (δ+).

In HCl, Cl pulls the shared pair closer. H becomes slightly positive, δ+. Cl becomes slightly negative, δ−.

A larger difference in electronegativity usually gives a more polar bond, with more ionic character.

Some books use a difference of 1.7 as a rough guide. It is not an exact boundary for every bond.

δ+ and δ− mean partial charges. The + and − signs on ions show their net charges.

A polar bond has a slightly positive end and a slightly negative end. This is a bond dipole. Think of each bond dipole as a small arrow. If the arrows cancel, the molecule is nonpolar. If they do not cancel, the molecule is polar.

A bond can be polar even when the whole molecule is nonpolar. Check both the bonds and the shape.

NH₃ has a lone pair on nitrogen. Nitrogen donates it to H⁺:

NH₃ + H⁺ → NH₄⁺.

After formation, all 4 N—H bonds in NH₄⁺ are equivalent.

Water has lone pairs on oxygen. Oxygen can donate one to H⁺:

H₂O + H⁺ → H₃O⁺.

NH₃ + HCl → NH₄Cl.

The product contains NH₄⁺ and Cl⁻ ions. Nitrogen’s lone pair forms a new N—H bond. Chlorine is not directly bonded to nitrogen.

After it forms, a coordinate bond behaves like an ordinary covalent bond. Supplying both electrons does not, by itself, make the bond polar.

Moving electrons carry charge, so metals conduct electricity.

Electrons and vibrations of the metal cores carry heat through the metal.

Many metals can be beaten into sheets. Their layers can move while the bonding remains.

Many metals can be drawn into wires.

Many metals have high melting points and densities. There are exceptions: mercury is liquid at room temperature.

Metallic bonding acts throughout a metal. It is not just a covalent bond between a pair of atoms.

Intramolecular means within a molecule: these bonds join its atoms. Intermolecular means between molecules: these forces attract separate molecules.

In a polar molecule, one end is slightly positive and the other is slightly negative. The positive end attracts a negative end on a nearby molecule.

Hδ+—Clδ− ··· Hδ+—Clδ−

The dotted line shows attraction between molecules. It is not another covalent bond.

Electrons keep moving. At one moment, more electrons may be on one side of a particle. That side becomes slightly negative. This can change the electron distribution in a nearby particle and cause attraction.

These are London dispersion forces. They occur in all atoms and molecules.

For molecular substances, attractions between molecules help determine boiling point and physical state.

Hydrogen bonding is an attraction involving hydrogen already bonded to N, O or F. This hydrogen is attracted to an electron-rich region, usually a lone pair, on another atom. The attraction may act between molecules or within a molecule.

Hydrogen is bonded directly to N, O or F.

Another N, O or F atom has a suitable lone pair to attract the δ+ hydrogen.

Water, ammonia and HF can form hydrogen bonds. Their hydrogen atoms are bonded to O, N or F. A polar H—Cl bond does not give HCl the same strong type of hydrogen bonding.

Water needs a lot of energy to separate its molecules. Its boiling point is unusually high for such a small molecule.

Ice has a more open arrangement of molecules than liquid water. It is less dense, so it floats.

Hydrogen bonds help hold DNA base pairs together. They also help proteins keep their shapes.

They affect how thick some liquids are, how substances dissolve and how some polymers stick.

Hydrogen bonds are stronger than many ordinary dipole attractions. They are usually weaker than covalent bonds.

An ionic crystal has a repeating pattern of positive and negative ions. NaCl tells us their ratio: 1 Na⁺ for each Cl⁻. It does not mean that the crystal contains separate NaCl molecules.

Usually form crystalline solids at room temperature.

Ionic solids are often hard but brittle. If layers shift, ions with the same charge may come close together. They repel, and the crystal can break.

The ions attract strongly. Separating them takes a lot of energy, so melting and boiling points are often high.

In a solid, the ions cannot move through the crystal. Solid ionic compounds therefore usually do not conduct.

When melted, the ions can move and carry current. Dissolved ions can also carry current in water.

Many ionic substances dissolve in polar solvents such as water. Some, such as AgCl, dissolve only slightly.

For example, NaCl melts at about 801 °C, while MgO has a much higher melting point.

Simple molecular substances contain separate molecules. Attractions between these molecules hold them together.

Often have low melting/boiling points compared with many ionic solids.

Most do not conduct in pure form. They have no charged particles that can move freely through them.

Some are soluble in water, depending on polarity and interactions with water.

Diamond and SiO₂ have large networks of strong covalent bonds. They have very high melting points. Graphite has bonded layers and mobile electrons. It conducts along its layers.

“Like dissolves like” is a useful guide. Polar substances often dissolve in polar solvents. Nonpolar substances often dissolve in nonpolar solvents. There are exceptions.

Dissolving does not always make a solution conduct. HCl forms ions in water, so its solution conducts. Sugar dissolves without forming many ions, so its solution conducts poorly.

Boiling usually pulls molecules apart. It does not break the covalent bonds inside them. This is why strong bonds can exist in a substance with a low boiling point.

An adhesive joins two surfaces. First, it spreads over them. It may stick by attractions between particles, by filling tiny gaps, or by forming chemical bonds. The surfaces being joined are called substrates.

A two-part epoxy contains resin and hardener. They react and make a network of polymer chains. A polymer has many repeating units. Links between chains are called crosslinks. Hardening by chemical change is called curing. Clean, well-prepared surfaces help make a strong join.

Epoxy can bond parts in cars, aircraft, boats and electronic equipment.

Use the mixing ratio and curing time stated for that product. Different epoxies need different times.

Hydrogen bonding is not the only reason glue adheres.

Choose the glue to suit the materials and the job. Strength, temperature and contact with water all matter.

A coordinate covalent bond is not necessarily polar. Donor origin and bond polarity are different ideas.

Covalent bonds are not weak simply because some molecular substances melt easily: intermolecular forces are overcome.

Not all covalent compounds are insoluble in water. Not all dissolved polar compounds form conducting ions.

NaCl is an extended ionic lattice; one NaCl formula is a formula unit.

Classical strong hydrogen bonding is associated with H attached to N, O or F, rather than ordinary HCl molecules.

A neutral sodium atom has 11 electrons, arranged 2,8,1. Always check that the electron numbers add to the atomic number.

Hydrogen and fluorine share one pair to form polar covalent HF. Potassium gives one electron to fluorine to form K⁺ and F⁻, giving ionic KF.

Octet: eight valence electrons, common among stable main-group species. Duplet: two electrons fill the first shell, as in helium and bonded hydrogen.

Al → Al³⁺ + 3e⁻, changing electron arrangement from 2,8,3 to 2,8.

O (2,6) gains two electrons: O + 2e⁻ → O²⁻ (2,8).

A lone pair stays on one atom and does not form the bond; a bond pair is shared between the two bonded atoms.

Strong electrostatic attraction acts throughout the ionic crystal lattice.

Increasing electronegativity difference generally increases bond polarity and ionic character; there is no perfectly sharp boundary.

Magnesium has two valence electrons and can form Mg²⁺ with stable 2,8 configuration.

Many metals tend towards delocalized metallic bonding or ion formation with nonmetals. Some metal compounds also contain covalent bonds.

A neutral atom has equal proton and electron counts. An ion has net charge due to losing or gaining electrons.

Attraction between δ+ and δ− regions of neighbouring polar molecules.

PVA for paper or wood, epoxy for strong joins and coatings, polyurethane for footwear and wood.

They attract separate molecules, whereas covalent bonds within molecules share electrons strongly between their nuclei.

Metal cores occupy a lattice and attract delocalized electrons; metals can often conduct, bend and stretch.

Melting separates molecules by overcoming intermolecular attractions, rather than breaking their strong internal covalent bonds. Giant covalent solids are exceptions.

Usually crystalline, hard, brittle and high-melting; conduct when molten or when mobile ions are present in solution.

Ions are strongly attracted in a stable repeating lattice at ordinary temperatures.

Hydrogen bonds can raise boiling points and influence density, solubility and biological molecule structures.

Na: 2,8,1; Mg: 2,8,2; Al: 2,8,3; Si: 2,8,4; P: 2,8,5; S: 2,8,6. Each neutral atom has protons = electrons = atomic number.

Sodium loses one electron: Na → Na⁺ + e⁻. Chlorine gains it: Cl + e⁻ → Cl⁻. Opposite charges attract throughout an extended lattice; NaCl is the simplest whole-number ion ratio.

A chemical bond may lower total energy. Many common main-group arrangements resemble filled outer shells, although the octet rule has important exceptions.

Nonmetal atoms share one or more pairs of valence electrons. Example: each H adds one electron to the shared H—H pair.

Single: one electron pair, H—H. Double: two pairs, O=O. Triple: three pairs, N≡N. Ethene contains C=C; ethyne contains C≡C.

H₂O: H—O—H (two lone pairs on O); N₂: :N≡N: (one lone pair on each N); CH₄: four C—H bonds; C₂H₂: H—C≡C—H; Cl₂: Cl—Cl (three lone pairs on each Cl); H₂: H—H.

Positive metal cores arranged in a lattice are held together by attraction to delocalized valence electrons.

Unequal sharing of bonded electrons leads to partial charges. A greater electronegativity difference generally increases ionic character.

A polar molecule has a net dipole (H₂O, HCl). A nonpolar molecule has no net dipole (H₂, CH₄, CO₂). Overall geometry affects dipole cancellation.

PVA bonds paper and wood; epoxy resins cure into durable crosslinked materials used in vehicles, boats, repairs, flooring and coatings.

Count valence electrons; share pairs between bonded atoms; assign remaining pairs; complete suitable duplets and octets; use multiple bonds if necessary; check total electrons and charges.