Losing an electron leaves an atom with more protons than electrons, giving it an overall positive charge and turning it into a cation. Gaining an electron gives an atom more electrons than protons, producing a negative charge and forming an anion. Several things can trigger ionization, including heat, light in a process called photoionization, or a collision with another fast-moving particle.
Ionization can also happen when a molecular, or covalent, compound reacts with a polar solvent like water, creating ions that did not previously exist in the pure substance. Hydrogen chloride gas ionizing in water to form H+ and Cl- ions is a clear example of this.
This process differs from dissociation, where an already-ionic solid such as table salt simply separates into ions it already contained once it dissolves. The energy needed to remove the most loosely held electron from a gaseous atom is called its first ionization energy. This value generally rises across a period and falls down a group, following changes in nuclear charge and atomic size.
Instruments called mass spectrometers deliberately ionize sample molecules so their resulting ions can be sorted and identified by mass, making ionization a practical tool as well as a theoretical concept.