Atoms and Spectra

What do we know ?

• We know that the electrons in an atom orbit the nucleus.

• We also know that a gas will emit a line spectrum when an electric current is passed through it.

How did Mr Bohr explain this ?

• Bohr postulated ( suggested ) that the electrons were arranged around the nucleus in energy levels or shells.

• The energy levels were discrete ( not continuous ). There were gaps between them where no electron could exist.

• The electrons could move from one energy level to another. Such movements are known as electron transitions.

The Bohr Atom ( Neon )

N

nucleus – with protons and neutrons

electrons The electrons in the energy level closest to the nucleus are most tightly bound (tied to) the nucleus. When all the electrons are as close to the nucleus as possible, we say the atom is in its ground state.

The Bohr Atom ( Neon )

N

nucleus – with protons and neutrons

electrons If the atom gains energy, one of the electrons can move to a higher energy level. We say that the atom has become ‘excited’. The atom can gain energy in a number of different ways.

The Bohr Atom ( Neon )

N

nucleus – with protons and neutrons

electrons This leaves a gap in the lower energy level. The electron can jump back down again. But it has to get rid of energy and it gives out a photon as it jumps downwards.

The Bohr Atom ( Neon )

N

nucleus – with protons and neutrons

electrons This leaves a gap in the lower energy level. The electron can jump back down again. But it has to get rid of energy and it gives out a photon as it jumps downwards.

Another way of looking at it

• The energy levels are more commonly represented as horizontal lines. The lowest line represents the ground state ( E0 ) while those above represent excited states ( E1, E2 etc ).

E0

E1

E2

E3

E4

Another way of looking at it

• The highest energy level is known as the ionisation level. If an electron reaches that level, it may escape from the atom all together and the atom will become an ion.

E0

E1

E2

E3

E4