Isotopes and ions

Isotopes

Isotopes of an element have the same number of protons, but different numbers of neutrons in their nuclei. Isotopes of an element have:

  • the same atomic number
  • different mass numbers

Three isotopes of hydrogen

All hydrogen atoms contain one proton (and one electron), but they can contain different numbers of neutrons. Hydrogen-1 is the most abundant isotope of hydrogen.

Three isotopes of hydrogen, and their symbols, and numbers of protons, neutrons and electronsThree isotopes of hydrogen, and their symbols, and numbers of protons, neutrons and electrons

All three isotopes of hydrogen have identical chemical properties. This is because the number of electrons determines chemical properties, and the atoms of all three isotopes have one electron.

Ions

Ions are charged particles formed when atoms, or groups of atoms, lose or gain electrons:

  • positively charged ions form when electrons are lost
  • negatively charged ions form when electrons are gained

The charge on an ion is shown as a superscript. For example, an aluminium ion Al3+ carries 3 positive charges. It forms when an aluminium atom loses 3 electrons.

When you calculate the number of subatomic particles in an ion, you need to take into account the number of electrons lost or gained.

Question

The atomic number of a chloride ion, Cl-, is 17 and its mass number is 35. Calculate the number of protons, neutrons and electrons it contains.

  • number of protons = 17
  • number of electrons = 17 + 1 = 18
  • number of neutrons = 35 - 17 = 18
Move on to Test
next