Isotopes (Edexcel GCSE Chemistry: Combined Science)

• 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

Deducing protons, neutrons & electrons

Finding the 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

Finding the electrons

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

Finding the neutrons

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

Number of neutrons = mass number – number of protons

Are mass number and relative atomic mass the same thing?

• On a GCSE periodic table you will see that lithium has a relative atomic mass of 7
• Although it seems that this is the same as the mass number, they are not the same thing because the relative atomic mass is a rounded number
• Relative atomic mass takes into account the existence of isotopes when calculating the mass
• Relative atomic mass is an average mass of all the isotopes of that element
• For simplicity relative atomic masses are often shown to the nearest whole number

The relative atomic mass of lithium to two decimal places is 6.94 when rounded to the nearest whole number, the RAM is 7, which is the same as the mass number shown on this isotope of lithium