Charged atoms: ions

The idea

An atom that loses electrons has more protons than electrons and becomes a positively charged cation; an atom that gains electrons becomes a negatively charged anion; cations and anions are together called ions.

Losing an electron

A sodium atom has 11 protons and 11 electrons. It loses one electron.

What do you think happens to the charge of the sodium atom?

• It becomes positive
• It becomes negative
• It stays neutral

Count protons and electrons

Choose a state and compare the bars.

Ions

A sodium cation has 11 protons and 10 electrons, so it has more positive than negative charge. A chloride anion has 17 protons and 18 electrons, so it has more negative than positive charge.

An atom that loses electrons has more protons than electrons and becomes a positively charged cation. An atom that gains electrons becomes a negatively charged anion. Cations and anions are together called ions.

Notes

An atom that loses electrons becomes a positive cation; one that gains electrons becomes a negative anion. Both are ions.

Check yourself

A sodium cation (Na⁺) has 11 protons. How many electrons does it have?

Answer: 10

The atom had 11 electrons and lost one, so the cation has 10.

What does an atom become when it gains electrons?

What are cations and anions together called?

A chloride ion (Cl⁻) has 17 protons. How many electrons does it have?

Answer: 18

The chlorine atom had 17 electrons and gained one, so the anion has 18.

• A positively charged cation. A cation forms when an atom loses electrons.
• A neutral molecule. Gaining electrons gives the atom a negative charge, so it is an ion.
• A negatively charged anion — correct. Yes!
• Ions — correct. Yes!
• Molecules. A molecule is an electrically neutral entity of more than one atom. Cations and anions are charged.
• Isotopes. Cations and anions together are called ions.