Size & Mass of Atoms (AQA GCSE Chemistry)

• Atoms are extremely small with a radius of about 1 x 10^-10 metres or 0.1 nanometres
• The central nucleus contains protons and neutrons only which are packed close together in a small region of space
• The radius of the nucleus is about 10 000 times smaller than that of the atom, around 1 x 10^-14 m, so it is an extremely small region of space compared to the overall size of the atom
• This means that rather than being evenly spread out throughout the atom, virtually all of the atom's mass is concentrated inside the nucleus
• Electrons have a much smaller mass than protons and neutrons (1 proton has the same mass of around 1840 electrons) and move in the space outside the nucleus in orbits

Sub-Atomic Particle Mass Comparison Table

• The atomic number (or proton number) is the number of protons in the nucleus of an atom
  • The symbol for this number is Z

• It is also the number of electrons present in an atom and determines the position of the element on the periodic table
• The proton number is unique to each element, so no two elements have the same number of protons
• Electrons can be lost, gained, or shared during chemical processes but the proton number of an atom does not change in a chemical reaction

• The mass number (or nucleon number) is the total number of protons and neutrons in the nucleus of an atom
  • The symbol for this number is A

• The nucleon number minus the proton number gives you the number of neutrons of an atom
• Note that protons and neutrons can collectively be called nucleons
• The atomic number and mass number for every element is on the periodic table

Diagram showing the notation used on the periodic table

• 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
• So, C-14 is the isotope of carbon which contains 6 protons and 6 electrons, but the 14 signifies that it has 8 neutrons (14 - 6 = 8)
  • It can also be written as ¹⁴C

• Isotopes display the same chemical characteristics
• This is because they have the same number of electrons in their outer shells, and this is what determines their chemistry
• The difference between isotopes is the neutrons which are neutral particles within the nucleus and add mass only

The atomic structure and symbols of the three isotopes of hydrogen

• The atomic number is equal to the number of protons (p) in an atom
• Since atoms are neutral, then it is also the same as the number of electrons (e)
• The mass number is the number of protons plus neutrons
• The number of neutrons (n) can thus be calculated by subtracting the atomic number from the mass number
• For example, beryllium has an atomic number of 4, therefore it has 4 protons and 4 electrons.
• The mass number of beryllium is 9, so it has 9 - 4 = 5 neutrons
• The PEN numbers for beryllium are thus:
  • p = 4
  • e = 4
  • n = (9 - 4 =) 5

The symbol key for beryllium as represented on the periodic table

• The same process can be applied to ions but the charge of the ion has to be considered
  • For example, the fluoride ion can be represented as 
  • The fluoride ion has an atomic number of 9, therefore it has 9 protons and 9 electrons
  • But, it has a 1- charge which means that it has gained one more electron
  • The mass number of the fluoride ion is 19, so it has 19 - 9 = 10 neutrons
  • The PEN numbers for the fluoride ion are thus:
    • p = 9
    • e = (9 + 1 =) 10
    • n = (19 - 9 =) 10
  • For example, the magnesium ion can be represented as  
  • The magnesium ion has an atomic number of 12, therefore it has 12 protons and 12 electrons
  • But, it has a 2+ charge which means that it has lost two electrons
  • The mass number of the magnesium ion is 24, so it has 24 - 12 = 12 neutrons
  • The PEN numbers for the magnesium ion are thus:
    • p = 12
    • e = (12 - 2 =) 10
    • n = (24 - 12 =) 12

• To put the size of an atom into perspective, if an atom were magnified to the size of a football stadium, then:
  • The nucleus would be the size of a small pea at the centre of the pitch where kick-off takes place
  • The electrons would be orbiting the pea at the outermost seats of the stadium
  • In between the nucleus and the electrons is nothing but empty space

• Another way to think about the size of an atom is to consider that the size of a full stop on a piece of paper is usually 0.003 m = 3 x 10^-3 The width of a hydrogen atom is about 1.06 x 10^-10
• This means that you could fit almost 28,301,887 hydrogen atoms across the diameter of a full stop!