AQA A Level Chemistry

Revision Notes

1.1.2 Mass Number & Isotopes

Subatomic Structure of Atoms & Ions

  • 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

Atomic Structure Mass and Charge Distribution, downloadable AS & A Level Chemistry revision notes

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

Worked Example

Determine the number of protons of the following ions and atoms:

  1.  Mg2+ ion
  2.  Carbon atom
  3.  An unknown atom of element X with mass number 63 and 34 neutrons

Answers

Answer 1: The atomic number of a magnesium atom is 12 indicating that the number of protons in the magnesium element is 12

    • Therefore the number of protons in a Mg2+ ion is also 12

 

Answer 2: The atomic number of a carbon atom is 6 indicating that a carbon atom has 6 protons in its nucleus

 

Answer 3: Use the formula to calculate the number of protons

Number of protons = mass number – number of neutrons

Number of protons = 63 – 34

Number of protons = 29

    • Element X is therefore copper

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

Worked Example

Determine the number of electrons of the following ions and atoms:

  1. Mg2+ ion
  2. Carbon atom
  3. An unknown atom of element X with mass number 63 and 34 neutrons

Answers

Answer 1: The atomic number of a magnesium atom is 12 suggesting that the number of protons in the neutral magnesium atom is 12

    • However, the 2+ charge in Mg2+ ion suggests it has lost two electrons
    • It only has 10 electrons left now

Answer 2: The atomic number of a carbon atom is 6 suggesting that the neutral carbon atom has 6 electrons orbiting around the nucleus

Answer 3: The number of protons of element X can be calculated by:

Number of protons = mass number – number of neutrons

Number of protons = 63 – 34

Number of protons = 29

    • The neutral atom of element X  therefore also has 29 electrons

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)

Worked Example

Determine the number of neutrons of the following ions and atoms:

  1. Mg2+ ion
  2. Carbon atom
  3. An unknown atom of element X with mass number 63 and 29 protons

Answers

Answer 1: The atomic number of a magnesium atom is 12 and its mass number is 24

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

Number of neutrons = 24 – 12

Number of neutrons = 12

    • The Mg2+ ion has 12 neutrons in its nucleus

Answer 2: The atomic number of a carbon atom is 6 and its mass number is 12

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

Number of neutrons = 12 – 6

Number of neutrons = 6

    • The carbon atom has 6 neutrons in its nucleus

Answer 3: The atomic number of an element X atom is 29 and its mass number is 63

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

Number of neutrons = 63 – 29

Number of neutrons = 34

    • The neutral atom of element X has 34 neutrons in its nucleus

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

Atomic Structure Hydrogen Isotopes, downloadable AS & A Level Chemistry revision notes

The atomic structure and symbols of the three isotopes of hydrogen

RAM from Mass Spectra

  • 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 (Ar) 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

Calculating Relative Atomic Mass equation 1

Worked Example

Calculating relative atomic mass of oxygen

A sample of oxygen contains the following isotopes:

What is the relative atomic mass of oxygen in this sample, to 2dp?

  1. 16.00
  2. 17.18
  3. 16.09
  4. 17.00

 

Answer 

The correct answer is option 1

Calculating Relative Atomic Mass worked example equation 1

= 16.0044

= 16.00

Worked Example

Calculating relative atomic mass of boron

Calculate the relative atomic mass of boron using its mass spectrum, to 2dp:

Analytical Techniques Mass Spectrum Boron, downloadable AS & A Level Chemistry revision notes

Answer

 Calculating Relative Atomic Mass worked example equation 2

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