Atomic Structure and Chemical Bonding
This domain tests the particles inside an atom, how electrons are arranged in shells, and how that arrangement explains valency and the formation of ionic and covalent bonds.
Core rules
- An atom has protons and neutrons in the nucleus and electrons in shells around it. Atomic number (Z) equals the number of protons; mass number (A) equals protons plus neutrons.
- Isotopes have the same Z but different A because they have different numbers of neutrons. They show the same chemical properties because chemical behaviour depends on electrons, not neutrons.
- Electrons fill shells in the order 2, 8, 8 for the first three shells in simple treatments. Atoms tend to reach a stable outer shell, often eight electrons, by losing, gaining, or sharing electrons.
- An ionic bond forms by electron transfer and electrostatic attraction between oppositely charged ions. A covalent bond forms by sharing electron pairs.
Worked example
Explain the bonding in sodium chloride. Sodium (Z = 11) has the configuration 2, 8, 1. It loses one electron to form Na+ with 2, 8. Chlorine (Z = 17) has 2, 8, 7 and gains one electron to form Cl− with 2, 8, 8. The oppositely charged ions attract strongly and form an ionic lattice. The formula is NaCl because one Na+ balances one Cl−.
Traps that cost marks
Do not confuse mass number with relative atomic mass; mass number is a count of nucleons. Do not say isotopes have different chemical properties. Do not write that covalent compounds contain no charges at all; many covalent bonds are polar. When drawing electronic configurations, keep the shell counts correct.
Key points
- Z = protons; A = protons + neutrons.
- Isotopes differ in neutrons only.
- Ionic bonding involves transfer; covalent bonding involves sharing.
- Valency follows from the electrons needed to complete the outer shell.