Atomic Structure (Edexcel IGCSE Physics)

• 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 over 10,000 times smaller than the whole atom, but it contains almost all of the mass of the atom

Atomic structure of lithium

Diagram showing the structure of a Lithium atom. If drawn to scale then the electrons would be around 100 metres away from the nucleus!

Particles in 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 properties of each of the particles are shown in the table below:

Table of particle properties

Particle	Location	Relative charge	Relative mass
proton	in the nucleus	+1	1
neutron	in the nucleus	0	1
electron	orbiting the nucleus	−1	1/2000 (negligible)

 

Charge in the atom

• Although atoms contain particles of different charge, the total charge within an atom is zero
  • This is because the number of electrons is equal to the number of protons
• The following table sets out the calculation of the total charge in the lithium atom in the diagram above:

Calculating total charge table

Particle	Relative charge	Number of particles in lithium atom	number × relative charge	Total charge
proton	+1	3	+3	(+3) + 0 + (−3) = 0
neutron	0	4	0
electron	−1	3	−3

 

• If an atom loses electrons, then it is said to be ionised
• Symbols are used to describe particular nuclear by their element symbol, atomic number and mass number
  • This notation is called nuclear notation

Carbon 12 in nuclear notation

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 in order to have no overall charge

Mass number

• The total number of particles in the nucleus of an atom is called its mass number (it can also be called the nucleon 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, then 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

• Here are three examples:

Examples of nuclear notation for atoms of Hydrogen, Sodium and Uranium

• 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 number of each type of atom in the examples above is shown in this table:

Number of protons, neutrons & electrons table

Atom	Number of protons (atomic number)	Number of neutrons (mass number − atomic number)	Number of electrons (same as atomic number)
hydrogen	1	1	1
sodium	11	12	11
uranium	92	143	92