Atoms, elements and bonding
The structure of atoms, isotopes and electron shells, and how ionic, covalent and metallic bonding explain the properties of substances.
📌 Key points
- An atom has a nucleus of protons (relative charge +1, relative mass 1) and neutrons (charge 0, mass 1), surrounded by electrons (charge −1, mass 11840) in shells.
- Proton number (atomic number) = number of protons; mass number (nucleon number) = protons + neutrons. In a neutral atom, electrons = protons.
- Isotopes are atoms of the same element with the same number of protons but different numbers of neutrons. They have the same chemical properties because they have the same number of outer-shell electrons.
- Electrons fill shells 2, 8, 8. The number of outer electrons gives the group (I to VII) and the number of occupied shells gives the period.
- Ionic bonding is the strong electrostatic attraction between oppositely charged ions, formed when a metal transfers electrons to a non-metal.
- Ionic compounds form giant lattices, have high melting and boiling points, and conduct electricity when molten or in aqueous solution (ions free to move) but not as solids.
- A covalent bond is a pair of electrons shared between two atoms. Simple molecular substances have low melting and boiling points (weak intermolecular forces) and do not conduct electricity.
- Diamond: each carbon bonded to four others, very hard, used in cutting tools, does not conduct. Graphite: each carbon bonded to three others in layers with delocalised electrons, conducts, soft and slippery (electrodes, lubricant).
- Metallic bonding is the electrostatic attraction between positive metal ions and a sea of delocalised electrons. It makes metals good conductors and malleable.
- Relative atomic mass (Ar) is the average mass of the isotopes of an element compared with 112 of the mass of a carbon-12 atom.
📖 Key terms
- Isotopes
- Atoms of the same element with the same proton number but different numbers of neutrons.
- Ion
- An atom or group of atoms with an electrical charge, formed by losing or gaining electrons.
- Ionic bond
- Strong electrostatic attraction between oppositely charged ions.
- Covalent bond
- A pair of electrons shared between two atoms.
- Metallic bond
- Electrostatic attraction between positive metal ions and a sea of delocalised electrons.
- Compound
- A substance made of two or more elements chemically combined.
- Proton number
- The number of protons in the nucleus of an atom.
✏️ Worked example
Chlorine is 75% chlorine-35 and 25% chlorine-37. Calculate the relative atomic mass of chlorine.
- Multiply each mass number by its percentage: 35 × 75 = 2625 and 37 × 25 = 925.
- Add them: 2625 + 925 = 3550.
- Divide by 100: 3550 ÷ 100 = 35.5.
- Ar of chlorine = 35.5 (closer to 35 because chlorine-35 is more common).
When explaining melting points, name what is broken: strong ionic bonds in a giant lattice, but only weak intermolecular forces (not covalent bonds) in simple molecules.
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