3.1.2 Atomic Structure 2

• Ernest Rutherford discovered the nucleus by firing a beam of alpha particles at thin metal foils (only a few atoms wide).
• Most alpha particles passed straight through the foil, suggesting that most of the atom is made up of empty space.
• Some particles bounced back towards the source, indicating that there is a positively charged mass in the atom repelling the particles.
• Rutherford's conclusions led to the model of the atom with negatively charged electrons orbiting a positively charged nucleus.
• Our understanding of atoms has improved over time.
• In 1897, an English physicist called J.J. Thomson discovered electrons.
• Thomson modelled the atom as a 'plum pudding' - a ball of positive charge (dough), with negatively charged electrons (currants) mixed in with the 'dough'.
• In 1909, Ernest Rutherford discovered that alpha particles could bounce back off atoms, indicating that an atom's mass is concentrated in the atom's centre.
• Rutherford's discovery led to the concept of the nucleus, which contains positively charged particles called protons.
• Neils Bohr discovered that electrons orbit (fly around) the nucleus at fixed distances.
• In 1932, James Chadwick discovered that some particles in the nucleus have no charge at all, which he called neutrons.
• Electrons are arranged in shells around a nucleus, each shell having a different energy level.
• When atoms absorb or emit electromagnetic radiation, their 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 to the nucleus.