Structure of the Atom — The Three Particles, the 2n² Rule and Isotopes

Three Particles Decide Every Atom Question

Open any general science paper and you will find one or two atom questions. They are not chemistry so much as bookkeeping — count the particles correctly and the mark is yours.

An atom has exactly three particles worth knowing:

Particle · Charge · Mass (u) · Found by

Electron · –1 · 1/1840 · J. J. Thomson (1897)

Proton · +1 · 1 · E. Goldstein / Rutherford

Neutron · 0 · 1 · James Chadwick (1932)

Protons and neutrons sit packed in the nucleus; electrons circle it in shells. Because the nucleus holds both heavy particles, it carries almost the entire mass of the atom while occupying almost none of its volume.

A neutral atom always has electrons = protons. The moment those two numbers stop matching, you are no longer looking at an atom — you are looking at an ion.

1. Who discovered the neutron?

2. How many neutrons are present in an atom of sodium, ²³₁₁Na?

3. In Rutherford's gold foil experiment, most alpha particles passed straight through the foil without deflection. What does this prove?

4. What is the electronic configuration of magnesium (atomic number 12)?

5. According to the 2n² rule, what is the maximum number of electrons the M shell can hold?

6. Isotopes of an element have the same number of which particle, and a different number of which?

7. Which of the following pairs are isobars?

8. Which isotope is used in the treatment of cancer?