P1 - History of the Atom & Atomic Structure

  • 1897 JJ Thompson figured out that atoms weren't solid spheres. Measurements of charge and mass showed that an atom must contain smaller, negatively charged particles - electrons. From this, he created a model of the atom known as the 'Plum Pudding Model', where negative electrons were spread through the positive model that made up most of the atom.
  • In 1909 E Rutherford along with H Geiger & E Marsden, conducted the 'Gold Foil Experiment'. They fired positively charged alpha particles at a thin sheet of gold. They expected the particles to pass straight through the sheet or for it to be slightly deflected. Most particles went straight through the sheet, some were deflected more than they had expected, a few had deflected back the way they had come - something the pudding model couldn't explain.
  • Rutherford came up with a new theory of the nuclear atom to explain the new evidence. In his model, most of the mass of an atom concentrated in a small, positively charged nucleus at the centre surrounded by a 'cloud' of negative electrons - most of the atom is empty space.
  • Scientists realised that the electrons in a 'cloud' around the nucleus of an atom like this would be attracted to the nucleus, causing the atom to collapse.
  • N Bohr proposed a new model where the electrons are in shells, suggesting that electrons can only exist in these shells or fixed orbits and not anywhere else. Each shell has fixed energy and his theory was the closest.
1 of 8

P1 - History of the Atom & Atomic Structure

  • The nucleus contains protons, which are positively charged and neutrons, which are neutral, which gives the atom an overall positive charge. The nucleus is very small - the nucleus radius is about 1 x 10 -15m. Almost the whole mass of the atom is about 1 x 10 -23g and is concentrated in the nucleus.
  • The rest of the atom is mainly empty space. The negative electrons move around the outside of the nucleus really quickly, in electron shells. They give the atom its overall size - the diameter of an atom is around 1 x 10 -10m - so the nuclear radius is around 10000 times smaller than the atomic radius.
  • The particle Proton has a relative mass of 1 and a relative charge of +1.
  • The particle Neutron has a relative mass of 1 and a relative charge of 0.
  • The particle Electron has a relative mass of 0.0005 and a relative charge of -1.
2 of 8

P1 - Density

  • Density is a measure of mass in a given volume of a substance. It relates the mass of a substance to how much space it takes up.
  • The equation for density is: DENSITY = MASS/VOLUME
  • The unit for mass is kg and volume is m3.
  • The units of density are kg/m3 or g/cm3.
  • The density of an object depends on what it is made of and density doesn't vary with size or shape.
  • The average density of an object determines whether it floats or sinks - a solid object will float on a fluid if it has a lower average density than the fluid.


3 of 8

P1 - Particle Theory & States of Matter

Three states of matter are solid (ice), liquid (water) and gas (water vapour). The particles of a substance in each state are the same - only the arrangement and energy of the particles are different. If you reverse a change of state, the particles go back to how they were before. So physical changes (melting or boiling) are from different from chemical reactions

  • Solid - Strong forces of attraction hold the particles close together in a fixed, regular position. The particles don't have much energy in their kinetic energy stores so they can only vibrate about their fixed position.
  • Liquid - Forces of attraction between the particles are weaker and they move closer together but can move past each other and form irregular arrangements. They have more energy in their kinetic energy stores than the particles in a solid - they move in random directions at low speeds.
  • Gas - There are almost no forces of attraction between the particles. Particles have more energy in their kinetic energy stores than those in liquids and are free to move - travel in random directions at high speeds.
4 of 8

P1 - Particle Theory & States of Matter

  • The process of a solid turning into a liquid is melting, but the process of a liquid turning into a solid is freezing.
  • The process of a liquid turning into a gas is evaporating, but the process of a gas turning into a liquid is condensing.
  • The process of a solid turning into a gas is sublimating and a gas turning into a solid is still sublimating.
  • The energy in a substances thermal energy is held by particles in their kinetic energy stores.
  • When a liquid is heated, the extra energy passes into the particles' kinetic energy stores, making them move faster. Eventually, when enough of the particles have enough energy to overcome their attraction to each other the liquid starts to boil.
  • This process is similar when heating a solid. The extra energy makes the particles vibrate faster until eventually the forces between them are partly overcome and the particles start to move around - melting.
5 of 8

P1 - Specific Heat Capapcity

Specific heat capacity relates temperature and energy.

  • Heating a substance increases the energy in its thermal energy store, or the kinetic stores of the particles. This is sometimes referred to as the internal energy of a substance.
  • In kinetic theory, the temperature is a way of measuring the average internal energy of a substance.
  • However, it takes more energy to increase the temperature of some materials than others. Materials which need to gain lots of energy to warm up also release loads of energy when they cool down again. They store a lot of energy for a given change in temperature.
  • The change in the energy stored in a substance when you heat it is related to the change in its temperature by its specific heat capacity. The specific heat capacity of a substance is the change in energy in the substance's thermal store needed to raise the temperature of 1kg of that substance by 1°c.
  • The equation for specific heat capacity that relates temperature, energy, mass and specific heat capacity is: CHANGE IN THERMAL ENERGY = MASS X SPECIFIC HEAT CAPACITY X CHANGE IN TEMPERATURE
  • The unit for ΔTE is J, mass is kg, specific heat capacity is J/kg°c and ΔT is °c.
6 of 8

P1 - Intermolecular Bonds

  • When you heat a solid or liquid, you're transferring energy to the kinetic energy stores of the particles in the substance, making the particles vibrate or move faster.
  • When a substance is melting or boiling, you're still putting in energy, but the energy's used for breaking intermolecular bonds rather than raising the temperature.
  • When a substance is condensing or freezing, bonds are forming between particles, which releases energy. This means the temperature doesn't go down until all the substance has turned into a liquid (condensing) or a solid (freezing).


7 of 8

P1 - Specific Latent Heat

  • The specific latent heat of a change of state for a substance is the change of energy in its thermal energy store when 1kg of the substance changes state without changing its temperature.
  • Specific latent heat is different for different materials and for different changes of state,
  • The specific latent heat for changing between a solid and a liquid (melting or freezing) is called the specific latent heat of fusion. The specific latent for changing between a liquid and a gas (boiling or condensing) is called the specific latent heat of vaporisation. The equation for this is:


  • The unit for thermal energy for a Δs is J, mass is kg and specific latent heat is J/kg.
8 of 8


No comments have yet been made

Similar Physics resources:

See all Physics resources »See all Particle Model & Changes of State (Matter) resources »