Chapter 2: Atoms, elements and compounds — Part 2

01 Isotopes

Isotopes are atoms of the same element with the same number of protons but different numbers of neutrons. They have the same atomic number but different mass numbers.

Isotopes are written using nuclide notation, e.g. ¹²₆C and ¹⁴₆C (both carbon, Z = 6).

Chemical properties

Isotopes of the same element have identical chemical properties because chemical behaviour depends on the number of electrons (and hence protons), not neutrons.

Relative atomic mass (A_r)

The weighted average mass of the isotopes of an element, taking into account their relative abundances. It is calculated using:

A_r = (mass of isotope 1 × % abundance) + (mass of isotope 2 × % abundance) + … (then divide by 100 if abundances are percentages).

02 Ions and ionic bonds

Ion formation

An ion forms when an atom gains or loses electrons, giving it a net positive or negative charge. Only electrons are transferred — the nucleus (protons) is unchanged.

Cations

Positively charged ions formed when metals lose electrons (e.g. Na ? Na⁺, Mg ? Mg²⁺).

Anions

Negatively charged ions formed when non-metals gain electrons (e.g. Cl ? Cl^-, O ? O^2-).

Ionic bond

A strong electrostatic attraction between oppositely charged ions in a chemical compound. It forms when a metal transfers electrons to a non-metal.

Lattice structure

Ionic compounds exist as a giant three-dimensional lattice of alternating positive and negative ions held together by strong ionic bonds in all directions.

03 Dot-and-cross diagrams

Dot-and-cross diagrams show how electrons are transferred during ionic bond formation. Dots and crosses represent electrons from different atoms.

In magnesium chloride (MgCl₂), each Mg atom (2, 8, 2) loses two electrons to form Mg²⁺ (2, 8). Each Cl atom (2, 8, 7) gains one electron to form Cl^- (2, 8, 8). Two Cl^- ions are needed for each Mg²⁺ ion.

04 Properties of ionic compounds

High melting and boiling points

Large amounts of energy are needed to overcome the strong electrostatic forces between oppositely charged ions throughout the lattice.

Electrical conductivity

Ionic compounds conduct electricity when molten or dissolved in water because the ions are free to move and carry charge. They do not conduct when solid because the ions are fixed in position in the lattice.

Explanation

Conductivity requires mobile charged particles. In the solid state, ions are locked in the lattice; when melted or in solution, they become mobile.