A cation is an ion with a positive charge, meaning it has more protons than electrons. This idea is important because ions are the building blocks of many salts, minerals, and biological processes. The mnemonic "a cation is paws-itive" helps connect the word cation with a positive charge using the image of a cat raising its paw toward a plus sign.
Atoms become cations when they lose one or more electrons, since electrons carry negative charge. Metals commonly form cations, such as Na+, Mg2+, and Al3+. In ionic formulas, the cation is usually written first, so in NaCl the Na+ is the cation and the Cl- is the anion.
Recognizing the cation helps you name ionic compounds and predict how charges balance.
Understanding Chemistry: Cation carries a positive charge
Electron loss is not random. The outer electrons of an atom are held less tightly than inner electrons because they are farther from the nucleus and are partly shielded by other electrons. For many metals, removing one, two, or three outer electrons leads to a more stable arrangement of the remaining electrons.
This is why sodium usually forms an ion with one positive charge, magnesium usually forms one with two positive charges, and aluminium usually forms one with three positive charges. Removing further electrons needs much more energy because the next electron comes from a fuller inner shell.
The charge on an ion tells you how many electrons were removed, not how many protons it has in total. A magnesium atom has twelve protons. A magnesium ion with two positive charges still has twelve protons, but it has only ten electrons.
The identity of an element depends on its proton number, so magnesium remains magnesium after ion formation. Students often mix up atomic number, mass number, and ion charge. Keep these ideas separate.
Atomic number counts protons. Mass number counts protons plus neutrons. Charge compares protons with electrons.
Cations attract negative ions through electric forces. In a solid ionic substance, huge numbers of positive and negative ions arrange themselves in a repeating pattern called a crystal lattice. The attraction in this lattice is strong, which helps explain why many salts are hard solids with high melting points.
When an ionic solid melts or dissolves in water, its ions can move. Moving charged particles carry electric current.
This is why molten salts and salt solutions can conduct electricity, while the same salt as a dry solid usually cannot. The ions are present in the solid, but they are locked in place.
Charge balancing becomes more important with ions that have different charge sizes. One calcium ion has two positive charges, so it needs two chloride ions, each with one negative charge, to make a neutral compound. A compound containing aluminium ions and oxide ions needs two aluminium ions for every three oxide ions.
The smallest whole number ratio is used in a formula. Some metals, especially transition metals, can form more than one cation. Iron can form ions with two positive charges or three positive charges.
Names use Roman numerals to show which charge is present, such as iron two ion or iron three ion. This detail prevents mistakes when writing formulas or predicting reactions.
Key Facts
- A cation is a positively charged ion.
- Cation charge = number of protons - number of electrons.
- Atoms form cations by losing electrons.
- Metals often form cations, such as Na+, Ca2+, and Fe3+.
- In ionic formulas, the cation is usually written first, such as Na+ in NaCl.
- Ionic compounds are electrically neutral overall, so total positive charge + total negative charge = 0.
Vocabulary
- Cation
- A cation is an ion with a positive charge because it has lost one or more electrons.
- Anion
- An anion is an ion with a negative charge because it has gained one or more electrons.
- Ion
- An ion is an atom or group of atoms that has an electric charge because it has gained or lost electrons.
- Electron
- An electron is a negatively charged particle found outside the nucleus of an atom.
- Ionic compound
- An ionic compound is a substance made from positive and negative ions held together by electrostatic attraction.
Common Mistakes to Avoid
- Confusing cation with anion: a cation is positive, while an anion is negative. Use the memory aid "a cation is paws-itive" to keep them separate.
- Thinking a cation gains electrons: gaining electrons makes an ion more negative. A cation forms when an atom loses electrons.
- Writing the anion first in an ionic formula: ionic formulas usually list the cation first. In NaCl, Na+ comes before Cl- because sodium is the cation.
- Ignoring charge balance in ionic compounds: the total charge of a stable ionic compound must be zero. For example, Mg2+ needs two Cl- ions to form MgCl2.
Practice Questions
- 1 A sodium atom has 11 protons and loses 1 electron, leaving it with 10 electrons. What is its ion charge, and is it a cation or an anion?
- 2 A magnesium ion has 12 protons and 10 electrons. Calculate its charge and write its ion symbol.
- 3 In the formula CaCl2, identify the cation and explain how the formula shows that the total charge is neutral.