Isotopes (Edexcel A Level Chemistry): Revision Note

Atomic Structure Calculations

• An atom is neutral and has no overall charge

• Ions on the other hand are formed when atoms either gain or lose electrons, causing them to become charged

• The number of subatomic particles in atoms and ions can be determined given their atomic (proton) number, mass (nucleon) number and charge

Properties

• Isotopes have similar chemical properties but different physical properties

Chemical properties

• Isotopes of the same element display the same chemical characteristics

• This is because they have the same number of electrons in their outer shells

• Electrons take part in chemical reactions and therefore determine the chemistry of an atom

Physical properties

• The only difference between isotopes is the number of neutrons

• Since these are neutral subatomic particles, they only add mass to the atom

• As a result of this, isotopes have different physical properties such as small differences in their mass, density, melting point and boiling point 

  • For example samples of uranium hexafluoride gas, UF₆, can undergo uranium enrichment by using a gas centrifuge cylinder

  • This is because nuclear reactors require higher amounts of the lighter U²³⁵ isotope 

  • UF₆ gas is placed in a gas centrifuge cylinder and rotated at a high speed

  • This rotation creates a strong centrifugal force so that the heavier gas molecules (UF₆ containing the heavier U²³⁸ isotope) move towards the outside of the cylinder

  • The lighter gas molecules (containing the U²³⁵ isotope) collect closer to the centre

Protons

• The atomic number of an atom and ion determines which element it is

• Therefore, all atoms and ions of the same element have the same number of protons (atomic number) in the nucleus

  • E.g. lithium has an atomic number of 3 (three protons) whereas beryllium has atomic number of 4 (4 protons)

• The number of protons equals the atomic (proton) number

• The number of protons of an unknown element can be calculated by using its mass number and number of neutrons:

Mass number = number of protons + number of neutrons

Number of protons = mass number - number of neutrons

Electrons

• An atom is neutral and therefore has the same number of protons and electrons

• Ions have a different number of electrons to their atomic number depending on their charge

  • A positively charged ion has lost electrons and therefore has fewer electrons than protons

  • A negatively charged ion has gained electrons and therefore has more electrons than protons

Neutrons

• The mass and atomic numbers can be used to find the number of neutrons in ions and atoms:

Number of neutrons = mass number (A) - number of protons (Z)

Defining & Calculating Isotopes

• The symbol for an isotope is the chemical symbol (or word) followed by a dash and then the mass number

  • E.g. carbon-12 and carbon-14 are isotopes of carbon containing 6 and 8 neutrons respectively

• Isotopes are atoms of the same element that contain the same number of protons and electrons but a different number of neutrons

  • For example, three isotopes of hydrogen are protium, deuterium and tritium

The atomic structure and symbols of the three isotopes of hydrogen