Chemical Bonding lessons

Three ways to a full shell

The noble gases, such as neon and argon, hardly react at all. Their outer electron shells are full.

Other atoms bond so that each ends up with a full outer shell, like a noble gas. There are three ways to do it.

Transfer, share and pool: three ways atoms reach full outer shells.

Ionic bonds form between a metal and a non-metal. Covalent bonds form between non-metals, and metallic bonds hold metal atoms together.

Practice: Name the bond type

Pick ionic, covalent or metallic for each substance.

Ionic bonding

In ionic bonding, a metal atom gives its outer electrons to a non-metal atom. Both are left with full outer shells.

An atom that loses electrons becomes a positive ion. An atom that gains electrons becomes a negative ion.

Electrons move from metal to non-metal for NaCl, MgO and MgCl2, leaving charged ions.

Opposite charges attract strongly. That attraction between the ions is the ionic bond.

Ion charges and formulae

An ion's charge comes from its group in the periodic table. Metals in groups 1, 2 and 3 lose 1, 2 or 3 electrons, forming 1+, 2+ and 3+ ions.

Non-metals in groups 5, 6 and 7 gain 3, 2 or 1 electrons, forming 3-, 2- and 1- ions. In a compound, the charges must add up to zero.

1. \text{Mg}^{2+}\text{ and }\text{Cl}^{-} (magnesium is in group 2, chlorine in group 7)
2. (+2)+2\times(-1)=0 (two chloride ions balance one magnesium ion)
3. \text{MgCl}_{\textcolor{#8b5cf6}{2}} (the small 2 counts the chloride ions)

So magnesium chloride is \text{MgCl}_2. Aluminium oxide needs two \text{Al}^{3+} and three \text{O}^{2-}, so it is \text{Al}_2\text{O}_3.

Practice: Balance the charges

Set how many of each ion it takes for the charges to cancel, then read off the formula.

Giant ionic lattices

Ions don't pair off one by one. Huge numbers of them pack into a giant ionic lattice, a regular pattern repeated in every direction.

A 3D tour of a sodium chloride lattice: the block, one sodium ion and its six neighbours, and the repeating pattern.

Each ion is pulled by all its neighbours, so separating them takes a lot of energy. Salt melts at 801 °C, and magnesium oxide, with double charges, at 2852 °C.

Solid ionic compounds don't conduct electricity, because their ions are locked in place. Melted or dissolved in water, the ions can move and carry charge.

Covalent bonding

Two non-metal atoms both need extra electrons, so neither gives any away. Instead they share pairs of electrons in a covalent bond.

Each shared pair is one bond, held by both nuclei at once. A dot-and-cross diagram shows one atom's electrons as dots and the other's as crosses.

Dot-and-cross diagrams for H2, HCl, H2O, NH3 and CH4, with the bonding and lone pairs counted.

Outer electrons that are paired up but not shared are called lone pairs. Only the outer shells are drawn, because inner electrons take no part.

Practice: Count the pairs

Count the bonding pairs and lone pairs in a dot-and-cross diagram.

Double and triple bonds

Atoms can share more than one pair. Two shared pairs make a double bond, and three make a triple bond.

Oxygen, \text{O}=\text{O}, has a double bond. Nitrogen, \text{N}\equiv\text{N}, has a triple bond.

Single, double and triple carbon to carbon bonds, drawn to scale with their bond energies.

More shared pairs pull the atoms closer and hold them tighter. The bond energy is the energy needed to break one mole of that bond.

Practice: Energy to break the bonds

Count the bonds in a displayed formula and add up their bond energies.

Molecules and giant structures

Most covalent substances are simple molecules, like water and methane. The bonds inside each molecule are strong, but the forces between molecules are weak.

Boiling only pulls whole molecules apart, so it takes little energy. Water boils at 100 °C and methane at −161 °C.

In a giant covalent structure, every atom is bonded into one huge network. Diamond and graphite are both pure carbon, and neither melts until well above 3500 °C.

Simple molecules, diamond and graphite side by side, and what heating each one has to break.

Practice: Sort the structures

Sort substances into giant ionic, simple molecular and giant covalent.

Metallic bonding

In a metal, each atom releases its outer electrons into the whole structure. These delocalised electrons move freely between a lattice of positive ions.

The metallic bond is the attraction between the positive ions and this sea of electrons. It is strong, so most metals melt at high temperatures.

Positive metal ions in a sea of moving electrons: drifting, carrying a current, and layers sliding.

The moving electrons carry charge and heat, so metals conduct well. Layers of ions can slide over each other, so metals bend instead of shattering.

Practice: Explain the property

Match each property to the bonding that explains it.

Electronegativity

Electronegativity is how strongly an atom pulls on a shared pair of electrons. Fluorine pulls hardest, at 3.98 on the Pauling scale.

When two atoms differ, the shared pair sits closer to the stronger puller. That atom gets a small negative charge, \delta-, and the other a small positive charge, \delta+.

A tug of war over the shared pair for six bonds, from H-H to Na-Cl, with the difference measured.

A bond with uneven sharing is a polar bond. A difference of about 1.8 or more pulls the electrons right across, and the bond is ionic.

Practice: Order the bonds

Order bonds from least polar to most polar using electronegativity values.

Shapes from electron pairs

Electron pairs around a central atom repel each other. They spread out as far apart as possible, and that sets the molecule's shape.

Count the bonding pairs and lone pairs on the central atom. A double bond counts as one group, because its two pairs point the same way.

Electron pairs spread out round a central atom: linear, trigonal planar, tetrahedral, pyramidal and bent.

Lone pairs repel a little more than bonding pairs. They squeeze the bonds closer, from 109.5° in methane to 107° in ammonia and 104.5° in water.

Practice: Predict the shape

Pick the shape of a molecule from its bonding and lone pairs.

Polar and non-polar molecules

A molecule with polar bonds can still be non-polar overall. It depends on whether the pulls of its bonds cancel out.

Carbon dioxide is linear, so its two bonds pull in opposite directions and cancel. Water is bent, so its two bond pulls add up, and the molecule is polar.

Water, carbon dioxide and methane in 3D: their shapes, bond angles and polarity.

Methane's four bonds point to the corners of a tetrahedron. The shape is symmetrical, so methane is non-polar.

Practice: Polar or non-polar?

Sort molecules into polar and non-polar from their bonds and shapes.

Forces between molecules

Intermolecular forces are weak attractions between separate molecules. The stronger they are, the higher the boiling point.

London forces act between all molecules and grow as molecules get bigger. Polar molecules also attract each other through their \delta+ and \delta- ends.

The strongest are hydrogen bonds, which form when hydrogen is bonded to nitrogen, oxygen or fluorine. They are why water boils at 100 °C.

Boiling points of the hydrides of groups 14 to 17, with water, hydrogen fluoride and ammonia far above the trend.

Practice: Read the boiling points

Tap the hydride on the boiling point chart that answers each question.