Matter: Particles, Atomic Structure and the Periodic Table
Particles and States
Solids, liquids and gases differ in the arrangement and energy of particles. In solids particles vibrate about fixed positions; in liquids they are close but can slide; in gases they are far apart and move randomly. Diffusion is faster at higher temperature and for lighter particles. Changes of state are physical changes: mass is conserved, but arrangement and energy change.
Atomic Structure
Atoms contain protons (positive, in the nucleus), neutrons (neutral, in the nucleus) and electrons (negative, in shells). Atomic number = number of protons; mass number = protons + neutrons. Isotopes have the same protons but different neutrons. Ions form when atoms gain or lose electrons.
The Periodic Table
Elements are ordered by increasing atomic number. Groups are columns with similar properties; periods are rows. Group 1 alkali metals become more reactive down the group; Group 7 halogens become less reactive down the group; Group 0 noble gases are unreactive because their outer shells are full. Metals are on the left, non-metals on the right.
Traps to Watch
- Confusing atomic number with mass number.
- Thinking particles stop moving in a solid.
- Assuming all elements in a group react identically rather than showing a trend.
Worked Example
An atom has 11 protons and 12 neutrons. Atomic number = 11 (so it is sodium); mass number = 11 + 12 = 23. A neutral atom has 11 electrons, arranged 2, 8, 1, which places it in Group 1, Period 3.