Size & Mass of Atoms (AQA GCSE Chemistry: Combined Science)

How big is an atom?

• 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

What is the atomic number of an atom?

• 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

What is the mass number of an atom?

• 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

What is an isotope?

• 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

Table to show the structure of isotopes of hydrogen

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 

The symbol key is represented like this 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!