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Atomic Structure Notes

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Unit 1; 1 Atomic Structure

ATOMIC STRUCTURE Fundamental Particles; Protons, Neutrons and Electrons

The actual values of the masses and charges of the subatomic particles are so small that it is more meaningful to consider their relatives masses; Proton (p)

Neutron (n)

Electron (e)

Relative mass




Relative charge




The atoms of different elements contain different sub-atomic particles.

Mass number and Isotopes

Atomic number (Z) - number of protons (also known as proton number) In an element, the number of protons = the number of electrons and it is the electronic structure of the atom that determine its chemical properties. It is the number of outer electrons that control the chemical properties. Mass Number (A) - sum of protons and neutrons in the nucleus of an atom It is the mass of an atom that determines its physical properties. Isotopes are atoms of the same element with the SAME NUMBER OF PROTONS BUT DIFFERENT NUMBER OF NEUTRONS or atoms of the same element with the same atomic number but different mass number.

Uses of isotopes

• Most isotopes are stable however some are unstable; their nuclei bread apart, emitting alpha particles, beta particles or gamma rays which can be detected. the property of radioactive decay in isotopes makes them useful in traces (usually in medicine to obtain information without surgery - useful isotopes are Technetium-99, Iodine-131, Iron-59 and Sodium-24)

• For dating (carbon dating)

The isotopes of an elements have different physical properties because they have different number of neutrons and therefore DIFFERENT MASSES. Isotopes with few neutrons will have;

1. lower masses

2. lower densities because the atoms are the same size so the mass per unit volume of the isotope is less

3. lower melting and boiling points because of the weaker forces of attraction between particles as the mass decreases

4. faster rate of diffusion; the rate of diffusion of a gas depends upon temperature and mass, so if two gaseous isotopes of an element are at the same temperature, then the lighter isotope will diffuse faster



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