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Protons and neutrons

Radiation and the atom · level 4, lesson 2 of 6

A question to start

A carbon atom and an oxygen atom are both made of the same three kinds of particle. So what makes carbon carbon?

The idea

The nucleus is made of two kinds of particle:

  • Protons, with a positive charge.
  • Neutrons, with no charge. They were found by James Chadwick in 1932.

They have almost the same mass: a neutron is only about 0.1% heavier than a proton.

The number of protons is the atomic number. It decides which element the atom is. Every atom with 6 protons is carbon; every atom with 8 is oxygen. Change the number of protons, and you have a different element.

Element Symbol Protons (atomic number)
Hydrogen H 1
Helium He 2
Carbon C 6
Oxygen O 8
Silicon Si 14
Iron Fe 26
Uranium U 92
++++++++ helium: 2 + 2carbon: 6 + 6

The particles marked + are protons; the grey ones are neutrons.

Worked examples

1. Which element? An atom has 26 protons. From the table, it is iron.

2. Changing element. If a nucleus of carbon (6 protons) gained one proton, it would have 7: it would become nitrogen.

3. The masses. A proton is \(1.6726 \times 10^{-27}\) kg and a neutron \(1.6749 \times 10^{-27}\) kg. The neutron is heavier by \(0.0023 \div 1.6726 \approx 0.14\%\).

Try it

  1. What two particles are in a nucleus?
  2. What does the atomic number count?
  3. An atom has 8 protons. What element is it?
  4. Can two atoms of the same element have different numbers of protons?
Answers
  1. Protons and neutrons.
  2. The number of protons.
  3. Oxygen.
  4. No. The number of protons decides the element.

Common mistake

"Neutrons are negative." Neutrons have no charge at all. The negative particles are electrons, outside the nucleus.

Where it leads