If the number of electrons is equal to the number of protons then the atom is uncharged and is electrically neutral.
However, atoms can gain or lose electrons: increasing or decreasing the negative charge.
Ionisation is the addition or removal of an electron to create an ion.
Losing an electron creates a positive ion.
Gaining an electron creates a negative ion.
An atom's charge can only change through gaining or losing electrons. Protons cannot be added or removed because they are bound up with neutrons in the nucleus.
Electrons can be lost because an ionising radiation (something that you will study later in this chapter) comes close to or collides with the atom and forces the electron away from the atom.
If the atom is hit by an ionising radiation, it may lose an electron.
This turns the atom into a positively charged ion - it has more protons than electrons.