Atom vs Ion

Quick Answer

An atom is electrically neutral. An ion is an atom that has gained or lost electrons and carries a charge.

What Each One Really Means

Let me put it plainly. An atom is neutral. The number of protons in the nucleus equals the number of electrons around it. That balance is what keeps the atom neutral. But atoms are not fixed. They can lose or gain electrons. When they do, the balance breaks and the particle becomes charged. That is what we call an ion. Lose an electron, the positive charges now outnumber the negative ones, so the ion is positive. We call that a cation. Gain an electron, the negative charges outnumber the positive ones, so the ion is negative. We call that an anion. The mnemonic 'cat has paws' helps you remember that cations are positive. It sounds childish, but I have seen final-year students still use it. Here is a subtle point. Atoms and ions do not only differ in charge. They also differ in size. A cation is always smaller than its parent atom, because losing electrons reduces electron repulsion and also means you may have lost an entire shell. An anion is always larger, because adding electrons increases repulsion and spreads the electron cloud out. That is why sodium ions are smaller than sodium atoms, and chloride ions are larger than chlorine atoms. This size change matters in crystal structures and in periodic trends, so do not ignore it. On exams, always check whether the question is asking about an atom or an ion. The numbers change and the size changes.

Side-by-Side Comparison

Memory Device

Cat has paws. Cation is positive. Also, an ion has an 'i' for 'ion is charged'.

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