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Isotopes

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

A question to start

All carbon atoms have 6 protons. Do they all have the same number of neutrons too?

The idea

No. Atoms of the same element can have different numbers of neutrons. These different kinds are called isotopes of the element.

Isotopes are named by their total number of protons and neutrons:

Isotope Protons Neutrons Share of the carbon on Earth
Carbon-12 6 6 98.93%
Carbon-13 6 7 1.07%
Carbon-14 6 8 a tiny trace

Hydrogen has three: ordinary hydrogen (no neutron), deuterium (one neutron) and tritium (two neutrons).

Isotopes of an element have the same electrons, so they behave the same way in chemistry. But the extra neutrons make them heavier, and some isotopes, like carbon-14 and tritium, are unstable: their nuclei break down over time. They are radioactive.

+++ hydrogendeuteriumtritium

Worked examples

1. Counting neutrons. Carbon-14 has 14 particles in its nucleus. 6 are protons, so \(14 - 6 = 8\) are neutrons.

2. Heavy water. Water made with deuterium instead of ordinary hydrogen is called heavy water. A litre of it is about 10% heavier than a litre of ordinary water.

3. How rare. Only about 1 hydrogen atom in 8,700 on Earth is deuterium (NIST gives 0.0115%): \(100 \div 0.0115 \approx 8{,}700\).

Try it

  1. What is an isotope?
  2. How many neutrons does carbon-13 have?
  3. Why do isotopes of an element behave the same way in chemistry?
  4. What does it mean that carbon-14 is radioactive?
Answers
  1. An atom of an element with a particular number of neutrons.
  2. \(13 - 6 = 7\).
  3. They have the same number of electrons, which decide how atoms join.
  4. Its nucleus is unstable and breaks down over time.

Common mistake

"Isotopes are different elements." Isotopes of carbon are all carbon, because they all have 6 protons. Only the number of neutrons differs.

Where it leads