chemistry - topic 1: particles

Describe the forces between the three states of matter.

• Solid: Strong forces of attraction between particles, which hold them in fixed positions. Particles vibrate around their positions.

• Liquid: Weaker forces of attraction than solids, allowing particles to slide past one another, resulting in a definite volume but no fixed shape.

• Gas: No forces of attraction, allowing particles to move freely and occupy the entire volume of their container, resulting in neither a definite shape nor volume. Gases either expand when heated, or their pressure increases.

What was John Dalton’s theory at the start of the 19th Century?

Atoms were ‘solid spheres’ and different spheres made up the different elements. Atoms cannot be subdivided, created, or destroyed.

What was Thomson’s model called, and what did he conclude?

Plum Pudding Model: An atom contains smaller, negatively charged particles — electrons — distributed like a charged ‘soup’

What infamous experiment did Ernest Rutherford conduct and what did it conclude?

The Gold Foil experiment: they were expecting the particles to pass through, although even though some went straight through, some were deflected at large angles. This concludes that there must be a positively charged nucleus at the centre, surrounded by a ‘cloud’ of electrons. He also concluded that most of the atom is empty space.

What did Neils Bohr propose?

Neils Bohr proposed that electrons orbit the nucleus in shells and exist in fixed orbits with fixed energy.

What are two reasons scientists publish their theories?

To share findings and allow peer review.

What does the atomic number and mass number tell us about an atom?

The atomic number shows us the number of protons, and the mass number shows us the total number of protons and neutrons.

What is an ion?

An ion is formed when atoms (or a group of atoms) lose or gain electrons.

What is an isotope?

Different forms of the same element, which have the same number of protons but a different number of neutrons.

What is a chemical reaction and give an example?

The breaking of bonds between atoms. These reactions are hard to reverse. For example, iron reacting with sulfur to create iron sulfate — Fe(s) + S(s) → FeS(s).

What is a physical reaction and give an example?

When a substance changes from one state of matter to another — e.g. by melting, bioling, condensing, or freezing.