P1.1 Particle Model of Matter

An atom has a small, positively-charged nucleus surrounded by orbiting negatively-charged electrons

The Nucleus contains protons and neutrons in the centre. Electrons orbit outside of the nucleus

Protons = positively charged/ +1

Neutrons = neutral/ 0

Electrons = negatively charged/ -1

Atoms have a radius of 1 × 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 a radius of an atom

Most of the mass of an atom is concentrated in the nucleus

Protons: charge of +1, found in the nucleus, an elements atomic number is the number of protons it possesses, all atoms of the same element have the same number of protons.

Electrons: charge of -1, found in fixed orbits around the nucleus, in any atom the total negative electrons equals the number of positive protons meaning atoms have no overall charge.

Neutrons: charge of 0, found in the nucleus

Rutherford’s experiment: 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, this suggested that most of the atom is made up of empty space. However some particles bounced back towards the source. The large deflections suggested that some positively charged mass in the atom was repelling the particles. This led to the model of the atom with negatively charged electrons orbiting a positively charged nucleus.

Thomson’s plum pudding model: in 1897, Thomson discovered electrons. he modelled the atom as a ‘plum pudding’ - a ball of positive charge (dough), with negatively charged electrons (currants) mixed in with the ‘dough’

Rutherford’s nuclear model: in 1909, Rutherford discovered that alpha particles could bounce back off atoms. He concluded that an atom’s mass is concentrated in the atom’s centre. This was called the ‘nucleus’ and it contained positively charged particles called protons.

The modern model: Bohr discovered that electrons orbit the nucleus at fixed distances. In 1932, Chadwick discovered that some particles in the nucleus have no charge at all. he called them neutrons.

Electrons are arranged in shells around a nucleus. Each shell has a different energy level. When an atom absorbs or emits electromagnetic radiation, its electron arrangements can change.

Absorb radiation: when atoms absorb electromagnetic radiation electrons move to a higher energy level further away from the nucleus

Emit radiation: when atoms emit electromagnetic radiation, electrons can drop to a lower energy level, closer to the nucleus.