Atoms and isotopes

All substances contain atoms. There is a nucleus at the centre of each atom. Atomic nuclei consist of two types of subatomic particles:

This means that all atomic nuclei are positively charged.

Atomic nuclei are surrounded by negatively charged electrons. Electrons are important for understanding chemical reactions.

Elements

There are over 100 chemical elements. Each element has a unique number of protons in the nuclei of its atoms. This means that the nucleus of each element has a characteristic positive charge. For example:

  • hydrogen nuclei have one proton, so each carry a +1 charge
  • lithium nuclei have three protons, so each carry a +3 charge

The atomic number of an atom is the number of protons in its nucleus. For example:

  • the atomic number of hydrogen is 1 because its nuclei each contain one proton
  • the atomic number of lithium is 3 because its nuclei each contain three protons

Isotopes

Atoms of the same element can have different numbers of neutrons in the nuclei of their atoms. This means that atoms of the same element can differ in the mass of their nucleus.

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Isotopes of an element have atoms with the same number of protons but different numbers of neutrons.

The mass number of an atom is the total number of protons and neutrons in its nucleus. Isotopes of an element will have the same atomic number but a different mass number.

The mass number of an atom is never smaller than its atomic number. It can be the same as the atomic number but is usually greater.

There is a conventional way to represent the nuclei of different isotopes. The chemical symbol of the element is given with:

  • mass number at the top left
  • atomic number at the bottom left

For example: _{17}^{35}Cl is the symbol for a chlorine-35 nucleus.

Calculating protons and neutrons

The number of neutrons in a nucleus can be calculated:

  • neutrons = (mass number) - (atomic number)

The number of neutrons in a chlorine-35 nucleus is (35 - 17) = 18.

Example

Lithium isotopes

Question

Lithium atoms exist naturally as two different isotopes, _{3}^{6}Li and _{3}^{7}Li. Complete the table to show the numbers of subatomic particles in each isotope and the nuclear charges.

IsotopeName of isotopeNumber of protonsNumber of neutronsNuclear charge
IsotopeName of isotopeNumber of protonsNumber of neutronsNuclear charge
_{3}^{6}LiLithium-63(6 - 3) = 3+3
_{3}^{7}LiLithium-73(7 - 3) = 4+3
Question

Certain uranium isotopes are useful as a nuclear fuel. The atomic number of uranium is 92. Calculate the numbers of neutrons in the isotopes uranium-235 and uranium-238.

Uranium-235: (235 - 92) = 143 neutrons

Uranium-238: (238 - 92) = 146 neutrons