Atoms can lose or gain electrons due to collisions or other interactions. When they do, they form charged particles called ions:
A helium atom has two electrons in an energy level outside the nucleus. The atom is neutral as it has two positive protons and two negative electrons.
A helium atom that has lost or gained an electron is an ion.
If the ion has two positive protons but one negative electron, it is a positive ion.
If the ion has two positive protons but three negative electrons, it is a negative ion.
Even though Rutherford had proven the existence of the nucleus, scientists were unsure how electrons fitted into this new model.
In 1913, Niels Bohr revised Rutherford’s model by suggesting that the electrons orbited the nucleus in different energy levels or at specific distances from the nucleus.
By doing this, he was able to explain that since particular chemicals burn with certain-coloured flames; the pattern of energy released by electrons in the chemical reaction must be the same for every single atom of that element.
Therefore, electrons cannot be arranged at random, but they must have fixed levels of energy within each type of atom.
Bohr’s ‘solar system’ model of the atom is the way that most people think about atoms today.
When atoms absorb energy, perhaps by absorbing electromagnetic radiation, the electrons at a particular level are pushed up to higher levels (at bigger distances from the nucleus) - they become ‘excited’. In time, they jump back down to a lower level releasing light of definite frequencies.