Skip to content

Electrons

Radiation and the atom · level 4, lesson 3 of 6

A question to start

A balloon rubbed on your hair picks up a charge. Where does the charge come from?

The idea

Electrons are the light, negatively charged particles around the nucleus. J. J. Thomson found them in 1897. An electron is about 1,836 times lighter than a proton, but its charge is just as big, the other way round.

  • A whole atom has as many electrons as protons, so the charges cancel and the atom is neutral.
  • If an atom loses an electron, it is left positive. If it gains one, it becomes negative. A charged atom is an ion.
  • In a simple picture, electrons sit in shells around the nucleus. The first holds 2, the next 8, and the third holds 8 for the first twenty elements. (Really, electrons are spread out in clouds, not tiny dots on rings.)

Rubbing a balloon moves electrons from your hair to the balloon: that is static electricity. Knocking electrons out of atoms is also what makes some radiation harmful, as the next level shows.

nucleus: 8 protonsinner shell: 2 electronsouter shell: 6 electrons

Worked examples

1. Counting electrons. Sodium has 11 protons, so a neutral sodium atom has 11 electrons: 2, then 8, then 1.

2. An ion. Sodium easily loses that one outer electron. Then it has 11 protons and 10 electrons: a charge of +1.

3. The mass. An electron is \(9.11 \times 10^{-31}\) kg; a proton \(1.67 \times 10^{-27}\) kg. \(1.67 \times 10^{-27} \div 9.11 \times 10^{-31} \approx 1{,}836\).

Try it

  1. What charge does an electron have?
  2. Why is a whole atom neutral?
  3. Oxygen has 8 electrons. How many are in each shell?
  4. A chlorine atom (17 protons) gains an electron. What is its charge now?
Answers
  1. Negative.
  2. It has as many electrons as protons, and their charges cancel.
  3. 2 in the first shell, 6 in the second.
  4. 18 electrons and 17 protons: a charge of −1.

Common mistake

"An ion has a different number of protons." An ion has gained or lost electrons. Its protons, and so its element, stay the same.

Where it leads