1.05 Neutral Atoms and ions

Neutral atoms
In its natural state an atom behaves like a neutral particle. This is because the atom has an equal number of protons and electrons. Therefore the positive charge of the nucleus (due to protons in the nucleus) is balanced by the negative charge of the orbiting electrons.

Electron shells and stability
The orbits of the electrons in an atom are arranged into different shells. Each shell has a fixed number of possible orbits which electrons can occupy. The shell closest to the nucleus has two possible electron orbits, the next shell has eight, the third also has eight. The electrons tend to fill these orbits from the lowest shell upwards. In the outer shell a few or even many of the orbits may be empty. For the outer shell to be stable it must have all of orbits occupied. Atoms that do not have enough electrons to fill all of the orbits in the outer shell are susceptible to either loosing electrons or accepting electrons from other atoms.
Figure 1.4. shows an atom with the lower (K shell) filled and only four electrons in the next shell (L shell). In this atom the outer shell is only half full as it contains eight possible orbits

If an atom either loses or gains electrons it is said to be ionised. The resulting atom is then called an ion. An ion has either positive or negative charge due to the imbalance of protons and electrons within the atom. If the atom looses electrons it will become a positive ion because there will be more positively charged protons in the nucleus than negatively charged electrons orbiting the nucleus. If the atom gains electrons it becomes a negative ion due to the negative charge of the extra electrons. The more electrons the atom either looses or gains the greater will be the amount of positive or negative charge carried by the ion.