Particles and Atomic Structure: notes

States of Matter

• Solid: particles are close together in fixed positions and vibrate. A block of ice keeps its shape.
• Liquid: particles are close but can slide past each other. Water takes the shape of its container.
• Gas: particles are far apart and move quickly in all directions. The air in a tyre is a gas.
• Changes of state: melting (solid to liquid), freezing (liquid to solid), boiling and evaporation (liquid to gas), condensation (gas to liquid). Sublimation turns a solid straight into a gas, as dry ice does.
• Diffusion: particles spread from high concentration to low concentration. Perfume smell spreading across a room.

Atoms

• Protons: positive charge, relative mass 1, found in the nucleus.
• Neutrons: no charge, relative mass 1, found in the nucleus.
• Electrons: negative charge, almost no mass, move around the nucleus in shells.
• Atomic number: the number of protons in an atom. Sodium has atomic number 11.
• Mass number: protons plus neutrons. Sodium 23 has 12 neutrons.
• Isotopes: atoms of the same element with the same protons but different neutrons. Carbon 12 and carbon 14 are isotopes.
• Electron arrangement: the first shell holds 2 electrons, the next two hold 8 each. Sodium is 2, 8, 1.

Elements, Compounds and Mixtures

• Element: one type of atom only. Oxygen and iron are elements.
• Compound: two or more elements chemically joined in a fixed ratio. Water is H2O.
• Mixture: substances mixed but not chemically joined. Sand and salt can be separated by adding water and filtering.