Atomic structureIB MYP Sciences: Revision notes

Atoms are made of three types of subatomic particle.

• Proton: relative charge +1, relative mass 1, found in the nucleus.
• Neutron: relative charge 0, relative mass 1, found in the nucleus.
• Electron: relative charge -1, relative mass negligible (very small), found in shells around the nucleus.

The nucleus is tiny and in the centre, but contains almost all of the atom's mass.

• The atomic number is the number of protons in an atom. It identifies the element.
• The mass number is the total number of protons + neutrons in the nucleus.

So: number of neutrons = mass number - atomic number.

In a neutral atom, the number of electrons equals the number of protons.

Worked example: an atom of sodium has atomic number 11 and mass number 23.

• protons = atomic number = 11
• electrons = protons in a neutral atom = 11
• neutrons = 23 - 11 = 12

The nuclide notation writes the mass number above and the atomic number below the symbol, for example ²³₁₁Na.

An atom has equal numbers of protons (+1 each) and electrons (-1 each), so the charges cancel and it is neutral.

An ion is an atom (or group of atoms) that has gained or lost electrons, so it is charged.

• Losing electrons gives a positive ion: Na (11 electrons) becomes Na⁺ (10 electrons).
• Gaining electrons gives a negative ion: Cl (17 electrons) becomes Cl⁻ (18 electrons).

The number of protons never changes when an ion forms.

Isotopes are atoms of the same element (same number of protons) with different numbers of neutrons, so they have different mass numbers. For example, chlorine-35 has 18 neutrons and chlorine-37 has 20 neutrons. They behave the same way chemically because they have the same number of electrons.

The relative atomic mass (Ar) of an element is the average mass of its atoms, taking into account the amount of each isotope.

Worked example: chlorine is 75% Cl-35 and 25% Cl-37. Ar = (75 × 35 + 25 × 37) ÷ 100 = 35.5.

This is why relative atomic masses are often not whole numbers.