Counting Protons, Neutrons, and Electrons in Ions

An ion is a charged substance

• A neutral substance has equal numbers of protons and electrons.

– Na has 11 protons and 11 electrons

– Cl has 17 protons and 17 electrons

• An ion either has more or fewer electrons than protons

– Na+ has 11 protons and 10 electrons

– Cl- has 17 protons and 18 electrons

Ions and Atoms have very different properties.

Ions are formed by gaining or losing electrons.

• Sodium loses an electron to become the sodium ion:

• Chlorine gains an electron to become the chloride ion:

Na → Na+ + e-

p+ 11 11

e- 11 10 1

Cl + e- → Cl-

p+ 17 17

e- 17 1 18

Misconception Note!

• Atoms only gain and lose electrons

– If an atom gains an electron, it gets a negative charge, because it now has more negatives than positives.

– If an atom loses an electron, it gets a positive charge, because it now has more positives than negatives.

• Atoms do not gain and lose protons. The number of protons stays the same for an atom.

Some examples – determine the answer then turn the page to check.

• How many protons and electrons are in the following?

Substance Number of p+ Number of e-

Mg 12

Mg2+

S 16

S2-

Fe 26

Fe3+

N 7

N3-

Some answers

• How many protons and electrons are in the following?

Substance Number of p+ Number of e-

Mg 12 12

Mg2+ 12 10

S 16 16

S2- 16 18

Fe 26 26

Fe3+ 26 23

N 7 7

N3- 7 10

Try the exercise.