Atomic Structure and Elements

Atoms are incredibly small and dense particles with a relative mass of just 1/2000th compared to everyday objects. Each atom contains a nucleus (made of protons and neutrons) surrounded by electrons in shells.

An element is simply a substance made entirely of one type of atom - think of pure gold or pure oxygen. Isotopes are atoms of the same element that have different numbers of neutrons, but they behave identically in chemical reactions because electrons control reactivity.

When atoms gain or lose electrons, they become ions - charged particles. Cations are positively charged (lost electrons) whilst anions are negatively charged (gained electrons).

Key Point: Isotopes don't affect chemical behaviour because electrons, not neutrons, determine how atoms react.

Relative Atomic Mass

Scientists measure atomic mass using relative isotopic mass - comparing each isotope to 1/12th of a carbon-12 atom (called 1u, roughly equal to one proton or neutron). Relative atomic mass is the weighted average of all isotopes of an element.

Mass spectrometers help us find percentage abundance of isotopes. The sample gets vaporised and ionised (made positive), then ions are accelerated and separated by mass - heavier ions move slower through the drift region.

You can calculate relative atomic mass using: relative atomic mass = Σ(percentage abundance × relative isotopic mass) ÷ Σ percentage abundance. Notice there are no units and it's rarely a whole number.

Key Point: Mass spectrometers separate ions by mass because heavier particles travel slower when accelerated to the same speed.

Simple and Polyatomic Ions

Simple ions form when single atoms lose or gain electrons. Groups 1, 2, and 3 typically lose electrons $1+, 2+, 3+ charges$, whilst groups 5, 6, and 7 gain electrons $3-, 2-, 1- charges$. Negative ions end in -ide.

Key exceptions to remember: hydrogen can form hydride ion (H⁻), zinc forms Zn²⁺, silver forms Ag⁺, and scandium forms Sc³⁺.

Polyatomic ions are groups of different atoms that gain or lose electrons together. Essential ones include ammonium (NH₄⁺), nitrate (NO₃⁻), carbonate (CO₃²⁻), sulfate (SO₄²⁻), and phosphate (PO₄³⁻).

Key Point: Group numbers often predict ion charges - Group 1 forms 1+ ions, Group 2 forms 2+ ions, and so on.