Mass Number & Isotopes (AQA A Level Chemistry)

• The mass of an atom is concentrated in the nucleus, because the nucleus contains the heaviest subatomic particles (the neutrons and protons)
  • The mass of the electron is negligible

• The nucleus is also positively charged due to the protons
• Electrons orbit the nucleus of the atom, contributing very little to its overall mass, but creating a ‘cloud’ of negative charge
• The electrostatic attraction between the positive nucleus and negatively charged electrons orbiting around it is what holds an atom together

The mass of the atom is concentrated in the positively charged nucleus which is attracted to the negatively charged electrons orbiting around it

• 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

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)

What are isotopes?

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

Isotopes of the same element have different numbers of neutrons

• Isotopes have the same 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 and density

• Isotopes are different atoms of the same element that contain the same number of protons and electrons but a different number of neutrons.
  • These are atoms of the same elements but with different mass numbers

• Because of this, the mass of an element is given as relative atomic mass (A_r) by using the average mass of the isotopes
• The relative atomic mass of an element can be calculated by using the relative abundance values
  • The relative abundance of an isotope is either given or can be read off the mass spectrum

A_r =