Isotopes and isobars

Isotopes are atoms of the same element (same number of protons) with different numbers of neutrons, so different mass numbers. Think of them as twins of different weights.

Hydrogen has three isotopes, each with 1 proton and 1 electron:

• protium ¹₁H: no neutron; about 99.98% of hydrogen;
• deuterium ²₁H: 1 neutron; about 0.015%;
• tritium ³₁H: 2 neutrons; only traces.

Carbon has ¹²C (the most common), ¹³C and ¹⁴C, each with 6 protons and 6 electrons.

Isotopes have the same electrons, so they react in the same way: the same chemical properties. But their masses differ, so some physical properties differ, such as melting and boiling points.

Isotopes at work:

• Uranium-235: fuel in nuclear power reactors.
• Cobalt-60: radiation treatment for cancer.
• Iodine-131: treating goitre and thyroid cancer.
• Carbon-14: finding the age of fossils and old objects.

Average atomic mass. Chlorine is 75% chlorine-35 and 25% chlorine-37. The average is weighted by how common each one is: 35 × 0.75 + 37 × 0.25 = 35.5 u. It is not 36 u (the simple average), because there is more of the lighter one. (u is the unified atomic mass unit.) Note: 35.5 is an average atomic mass; mass numbers are always whole numbers, like 35 or 37.

Isobars are the opposite: atoms of different elements with the same mass number. Calcium (Z = 20), potassium (Z = 19) and argon (Z = 18) can all have A = 40: they weigh the same but are different elements.