What Are Ions? How Atoms Become Charged

An ordinary atom has equal numbers of positively charged protons and negatively charged electrons, so its total charge is zero. The number of protons identifies the element and normally stays fixed during chemical reactions. Electrons are the particles that are more easily transferred. If an atom loses one or more electrons, it is left with more protons than electrons and becomes positively charged.

A positive ion is called a cation. A negative ion is called an anion. An atom becomes an anion when it gains electrons, giving it more electrons than protons. For example, a sodium atom can lose one electron to form a sodium ion with a charge of plus one, while a chlorine atom can gain one electron to form a chloride ion with a charge of minus one.

This charge difference is the basic answer to what ions are. The nucleus has not suddenly become positive or negative. The imbalance comes from the number of electrons compared with the number of protons.

For many main-group elements, ion formation is connected to valence electrons, the electrons in the outer part of the atom. Metals on the left side of the periodic table often lose a small number of outer electrons relatively easily. Group 1 metals commonly form plus one ions, while Group 2 metals commonly form plus two ions.

Nonmetals near the right side of the periodic table often gain electrons in simple ionic compounds. Halogens commonly form minus one ions, while oxygen-group elements often form minus two ions. These patterns are useful shortcuts, but they are not universal rules for every element. Transition metals, for example, can often form ions with more than one possible charge.

When studying ions, do not confuse charge with the number of protons. A magnesium ion with a plus two charge is still magnesium because it still has 12 protons. It simply has two fewer electrons than a neutral magnesium atom.

Oppositely charged ions attract each other. In an ionic compound such as sodium chloride, positive sodium ions and negative chloride ions are held together by strong electrostatic attraction in a repeating lattice. The overall compound is electrically neutral because the total positive and negative charges balance.

Charge balance helps you write ionic formulas. A magnesium ion has a plus two charge and a chloride ion has a minus one charge, so two chloride ions are needed for every magnesium ion. The formula is therefore MgCl2. The subscripts show the simplest whole-number ratio that makes the total charge zero.

Ions can also move freely when many ionic substances are melted or dissolved in water. That movement allows electric current to pass through the liquid and is important in electrolysis. In biological systems, ions such as sodium, potassium, calcium, and chloride also help with nerve signals, muscle function, and fluid balance. To remember what ions are, connect the definition to a simple check: compare protons and electrons. More protons means a positive ion, more electrons means a negative ion, and equal numbers mean a neutral particle.

Some ions are not single atoms but small groups of atoms held together by covalent bonds, carrying one overall charge. Common examples include hydroxide, OH with a charge of minus one, nitrate, NO3 with minus one, carbonate, CO3 with minus two, sulfate, SO4 with minus two, and ammonium, NH4 with plus one. In reactions these groups usually stay together as a unit, so it helps to learn them as fixed packages with their charges rather than working them out from their atoms each time.

Writing formulas uses the same charge-balancing rule as for single ions, but brackets now matter. Calcium has a plus two charge and nitrate a minus one charge, so two nitrate groups are needed, written Ca(NO3)2. Ammonium sulfate needs two ammonium ions to balance one sulfate, written (NH4)2SO4. The brackets show that the subscript applies to the whole group rather than only the last atom. Recognising these units also makes compounds easier to name, because the group names appear directly, as in sodium hydroxide or copper sulfate.

Ions in a sample can be identified with simple laboratory tests. Many metal ions give a characteristic colour when a little of the compound is held in a hot flame on a clean wire loop: lithium gives crimson, sodium a strong yellow-orange, potassium lilac, calcium orange-red and copper blue-green. The heat lifts electrons into higher energy levels, and as they fall back they release light of particular energies, which you see as particular colours. Adding sodium hydroxide solution identifies other metal ions by the precipitate formed: copper(II) ions give a blue solid, iron(II) green and iron(III) orange-brown.

Negative ions have their own tests. Add dilute nitric acid and then silver nitrate solution, and halide ions form precipitates: white for chloride, cream for bromide and yellow for iodide. For sulfate, add dilute hydrochloric acid and then barium chloride solution, and a white precipitate of barium sulfate appears. Carbonate ions fizz when acid is added, releasing carbon dioxide that turns limewater milky. The acid in the first two tests has a purpose. It removes any carbonate ions, which would otherwise react with the silver or barium ions as well and give a misleading result.

Gaining or losing electrons changes an atom's size as well as its charge. A sodium atom has a radius of roughly 186 picometres, but a sodium ion is only about 102. Losing the single outer electron removes the whole outer shell, and the remaining electrons are pulled in more tightly, because the nucleus still has 11 protons holding only 10 electrons. Chlorine goes the other way. A chlorine atom is about 99 picometres in radius, but a chloride ion swells to about 181, because the extra electron increases the repulsion between electrons while the nuclear charge stays the same. Positive ions shrink, and negative ions grow.

A neat comparison uses ions that all have the same number of electrons. The oxide, fluoride, sodium, magnesium and aluminium ions each have 10 electrons, arranged like those of neon. Yet their sizes fall steadily, from about 140 picometres for oxide to about 54 for aluminium, because the number of protons rises from 8 to 13 and pulls the same electrons closer. Size matters in practice. Small, highly charged ions such as aluminium attract water molecules and other ions very strongly, which affects how compounds dissolve and melt, while the relative sizes of positive and negative ions help decide which crystal structures they can form.

Ions are charged atoms or groups of atoms formed when electrons are gained or lost. Losing electrons creates positive cations, while gaining electrons creates negative anions. Their charges explain ionic bonding, formula ratios, electrical conduction in solutions, and many important chemical and biological processes.