In chemistry, an atom is the smallest unit of matter containing particles. When a group of atoms join together and the number of electrons is equal to the number of protons it is called an ion. Depending on the ratio between electrons and protons, two different types of ions can be formed. They can form cations or anions. A cation is formed when the ratio between electrons and protons has more protons than electrons, therefore making it positively charged. An anion is formed when the ratio between electrons and protons has more electrons than protons, therefore making it negatively charged. Atoms join together in groups because of electronegativity. Elements on the left side of the periodic table use more energy to bond therefore losing more electrons and creating cations. However, elements on the right side are more energy efficient therefore attracting more electrons and creating anions. Electronegativity increases moving right across the periodic table. As you move down a group, electronegativity decreases. This is because the atomic number increases down a group and thus there is an increased distance between the valence electrons and the positively charged nucleus. This form of organization is credited to Dimitri Mendeleev as he was the founder of the periodic table.
When an atom loses electrons that is called oxidation. When the atom is oxidized it is called the reducing agent. When an anion is oxidized it loses electrons and therefore has the potential to become an cation.
Some examples of an anion would be Cl2, Br2, and I2. An example for cations would be (SO4)2-, (CO3)2-, and (C2O4)2-. The similarities between the cations is that all Alkaline Earth Metals are in Group 2A, which forms cations, while all halogens are elements in group 7A form anions. Cations are usually formed between metals while anions are usually formed between nonmetals.
According to the experiment in the Laboratory of Chemistry, if you can classify Alkaline Earth Metals, you can tell if the ion is an cation or an anion. This experiment consists of aqueous nitrate solutions being mixed with an polyatomic anion. Oxidation is important in terms of cations and anions, because if an anion is oxidized it has the potential to be a cation. In the second experiment it was revealed that the halogen with the more positive electrode would be the stronger reducing agent.