the particle model

• an atom has a small, positively charged nucleus surrounded by orbiting negatively charged electrons
• atoms are very small and have a radius of about 1 x 10^-10 metres
• the electrons are arranged at different distances from the nucleus - different energy levels
• the radius of a nucleus is less than 1/10,000 of the radius of an atom
• most of the mass of an atom is concentrated in the nucleus
• ernest rutherford discovered the nucleus by firing a beam of alpha particles at thin metal foils, only a few atoms wide
• rutherford found that most alpha particles passed straight through the foil, suggesting that most of the atom is made up of empty space, however some particles bounced back towards the source
• rutherford concluded that the large deflections suggested that some positively charged mass in the atom was repelling the particles, which led the the model of the atom with negatively charged electrons orbiting a positively charged nucleus
• electrons are arranged in shells around the nucleus, each shell having a different energy level
• when an atom absorbs or emits electromagnetic radiation, its electron arrangements can change
• when atoms absorb electromagnetic radiation, electrons move to a higher energy level further away from the nucleus
• when atoms emit electromagnetic radiation, electrons can drop to a lower energy level, closer the the nucleus
• density = mass / volume
• an object floats in a fluid if the density of the object is lower than the density of the fluid - the force that keeps the object afloat is called upthrust
• the law of displacement says that an object completely submerged in a fluid will replace an amount of fluid equal to its own volume
• differences in the density of solids, liquids and gases can be explained using the particle model of matter
• solids are the densest state of matter as the particles are packed tightly together
• liquids are less dense than solids but more denser than gases as the particles can move around each other
• gases are the least dense state of matter as the particles are free to move with tiny forces between particles
• changes of state are physical changes
• when a material changes state, it is able to recover its old properties if the change is revered
• some chemical reactions are also reversible, however some are irreversible, meaning the material wouldn't be able to return to its original properties
• when a substances changes state, its mass if conserved
• the amount of energy in an object's internal store is the sum of the kinetic and chemical stores of the object's particles
• the energy in an object's kinetic store comes from the random motion of its particles
• the temperature of a body is a measure of the energy in the kinetic stores of its particles
• increasing a body's temperature increases the energy in the kinetic stores of the body's particles, which means that the energy in the body's internal store also increases
• the energy in an object's potential store comes from the potential energy stored in molecules
• the specific heat capacity of a substance is the amount of energy needed to increase the temperature of 1kg of that substance by 1°C
• the heater increases the internal energy of the body and we measure this using a joulemeter
• measure the temperature of the body at the start and measure the maximum temperature of the body at the end
• specific heat capacity = change in internal energy / (mass x maximum temperature rise)
• thermal capacity = mass x specific heat capacity
• a melting point is also known as the fusion temperature
• melting point = freezing point
• boiling point = condensing point
• a liquid boils when the particles have enough energy to completely escape the forces between them
• latent heat is the energy that is transferred to a substance without the substance's temperature changing, which happens when a substance is changing state
• when a substance changes state, this energy is absorbed to create or weaken bonds, rather than transfer energy to the kinetic store of a substance's particles
• if we measure energy using the change in temperature, this energy transfer will not be measured as the energy is transferred without the temperature changing