Atomic Structure (Oxford AQA IGCSE Physics)

Atomic Structure

• Atoms are the building blocks of all matter

• They are incredibly small, with a radius of only 1 × 10^-10 m

  • This means that about one hundred million atoms could fit side by side across your thumbnail

• Atoms have a tiny, dense nucleus at their centre, with electrons orbiting around the nucleus

• The radius of the nucleus is much smaller (over 10,000 times smaller) than the whole atom, but it contains almost all of the mass of the atom

Parts of the atom

• The nucleus contains:

  • Protons - positively charged particles with a relative atomic mass of one unit

  • Neutrons – no charge, and also with a relative atomic mass of one unit

• Almost all of the atom is empty space, but moving around the nucleus there are:

  • Electrons – negative charge with almost no mass (1/2000 the mass of a proton or neutron)

The structure of an atom

• The properties of each of the particles are shown in the table below:

Table of the Relative Masses and Charges

Atomic & Mass Number

Atomic number

• The number of protons in an atom is called its atomic number (it can also be called the proton number)

  • Elements in the periodic table are ordered by their atomic number

  • Therefore, the number of protons determines which element an atom is

• The atomic number of a particular element is always the same

• For example:

  • Hydrogen has an atomic number of 1. It always has just one proton

  • Sodium has an atomic number of 11. It has 11 protons

  • Uranium has an atomic number of 92. It has 92 protons

• The atomic number is also equal to the number of electrons in an atom

  • This is because atoms have the same number of electrons and protons to have no overall charge

  • For more information, see the revision note on Atoms & Ions

Mass number

• The total number of particles in the nucleus of an atom is called its mass number

• The mass number is the number of protons and neutrons in the atom

• The number of neutrons can be found by subtracting the atomic number from the mass number

Number of Neutrons = Mass Number - Atomic Number

• For example, if a sodium atom has a mass number of 23 and an atomic number of 11:

  • It has 23 particles in its nucleus

  • 11 of them are protons

  • So the number of neutrons would be 23 – 11 = 12

Nuclear notation

• The mass number and atomic number of an atom are shown by writing them with the atomic symbol

  • This is called nuclear notation

Examples of nuclear notation

• The top number is the mass number

  • This is equal to the total number of particles (protons and neutrons) in the nucleus

• The lower number is the atomic number

  • This is equal to the total number of protons in the nucleus

• The atomic and mass numbers of each type of atom in the examples above are shown in this table:

Calculating the Number of Protons, Neutrons and Electrons