Ions are formed when atoms gain or lose electrons, giving them a positive or negative charge. Sodium loses an electron to become Na+, giving it 11 protons and 10 electrons. Chlorine gains an electron to become Cl-, giving it 17 protons and 18 electrons. Atoms only gain or lose electrons to become ions, keeping the number of protons the same. The document provides examples of ions formed from magnesium, sulfur, iron and nitrogen atoms and lists their number of protons and electrons.

1. Counting Protons, Neutrons, and Electrons in Ions

2. 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

3. Ions and Atoms have very different properties.

4. Ions are formed by gaining or losing electrons.
• Sodium loses an electron to become the sodium ion:
Na → Na+ + e-p+
11 11
e- 11 10 1
• Chlorine gains an electron to become the chloride
ion:
Cl + e- → Cl-p+
17 17
e- 17 1 18

5. 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.

6. 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-

7. 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

8. Try the exercise.