The Particle Model

P1-1-1 Structure of an atom

In 1897 J.J. Thompson discovered the electron is much lighter than the atom, and hence realised the atom is not the smallest particle. So he proposed the Plum Pudding model:

Figure 1 Plum Pudding model, source.

The pudding is positive, and the plums in it are negative! 

JJ Thompson did not think of neutrons! 

Nuclear Model (Rutherford Model): 

In 1909 Geiger and Marsden fired some alpha particles towards a thin sheet of gold (alpha particle is made of two protons and two neutrons, so it has a positive charge!).

Because they thought material is made of atoms that are like a plum pudding and hence it is soft like a pudding, they expected all of the alpha particles to pass through the metal sheet. But they saw some alpha particles bounce back! So their supervisor, Ernest Rutherford, said there is a positive nucleus at the centre of the atom which repels the positive alpha particles. He said electrons randomly orbit around the nucleus. 

Figure 2 alpha scattering experiment (Rutherford), source: Link

Figure 3 Rutherford Model - Image source: Link.

Bohr model:

It was shown if the negative electrons orbit randomly around the nucleus (as Rutherford suggested), they would be attracted and absorbed by the positive nucleus. So in 1913 Bohr said electrons only orbit in specific circles called energy levels! 

Figure 4: Bohr model, source

P1.1b

Later Rutherford discovered proton in 1920, and neutron by James Chadwick 1932.

The Bohr’s model is the accepted theory for the model of an atom today: proton and neutron at the centre forming the nucleus, with electrons orbiting the nucleus. Almost all of the mass of an atom is in its nucleus. 

In general a theory becomes accepted when it can explain results of different experiments. 

P1.1c

Some numbers to remember:

Radius of an atom: 10^-10 m.

Radius of nucleus: of radius of atom

Radius of smallest molecule is almost the same as an atom, e.g. radius of water molecule: 2.8 x 10^-10

P1-1-2 Density

Density: is the mass of unit volume of a substance. unit for density: kg/m³ (or g/cm³)

If material A is denser than B, it means molecules of A are packed closer together than B.

Solids usually are denser than liquids, and liquids are usually denser than gases. Because molecules in a gas are more spread out compared to liquid, and same goes for liquid compared to solid (usually! One exception is water: water is denser than ice!)

Description of three states of matter:

Solid: has a fixed shape and volume. Particles cannot move from a point to another, they just vibrate in their position. The bond between particles (atoms or molecules) is strong.

Liquid: has fixed volume, but not a fixed shape, it takes shape of the container. Intermolecular bonds is less strong than in a solid.

Gas: no fixed shape or volume! They have no spine! Particles move randomly and fill up the container. There is almost no intermolecular bond. Particles collide with each other and with walls of the container. 

Figure 5: Solid, liquid, gas; source: wikimedia

Example 1: (P1.1f):

Q1. Density of air is 1.3 kg/m³. Calculate the mass of air in a room with dimensions of 2 m x 3 m x 5 m.

         Answer: Volume of the air in the room =

Q2. Discuss how density of the air in tyres of a lorry changes when the vehicle is fully loaded. 

         Answer: Density increases, because mass remains the same, but volume decreases. 

Source of the icon image: Source.