Number of Subatomic Particles
Specification Point 1.10:
Calculate the numbers of protons, neutrons and electrons in atoms given the atomic number and mass number.
- The atomic number is equal to the number of protons in an atom.
- Since atoms are neutral, then it is also the same as the number of electrons.
- The mass number is the number of protons plus neutrons.
- The number of neutrons can thus be calculated by subtracting the atomic number from the mass number.
- Beryllium for example has an atomic mass 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 symbol key for beryllium as represented on the Periodic Table
Isotopes & Relative Atomic Mass
Specification Point 1.11:
Explain how the existence of isotopes results in relative atomic masses of some elements not being whole numbers.
Relative Atomic mass
- The symbol for the relative atomic mass is Ar and it is sometimes written as R.A.M.
- It is calculated from the mass number and relative abundances of all the isotopes of a particular element.
- Since isotopes have the same atomic number but a different mass number, they are all atoms of the same element but have different atomic masses.
- The relative atomic mass is an average of these masses, hence it is not always a whole number.
Symbol, mass number and atomic number of chlorine
Edexcel GCSE Chemistry Notes
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Morgan’s passion for the Periodic Table begun on his 10th birthday when he received his first Chemistry set. After studying the subject at university he went on to become a fully fledged Chemistry teacher, and now works in an international school in Madrid! In his spare time he helps create our fantastic resources to help you ace your exams.
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