By the end of the subtopic, learners should be able to:
  1. Draw the structure of an atom.
  2. Define and use the relative atomic mass.
  3. Draw the electronic configuration of atoms.
  4. Define an isotope and give specific examples.

Structure of matter

  • Matter can be defined as any substance that has mass and occupies space. It can be in form of solid, liquid or gas.
  • Matter is made up of small particles called atoms.


  • Atoms are the smallest units of matter.
  • Atoms are made up of sub-atomic particles which are:
    • protons
    • electrons 
    • neutrons

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Sub-atomic particles

  • Found in shells or orbitals around the nucleus.
  • They are negatively charged (-).
  • They are found in different energy levels.
  • They have negligible mass so they do not contribute to the mass of the atom.
  • Relative mass of an electron is 1/1836 which is very small.
  • For a neutral atom, the number of protons is equal to the number of electrons.
  • For an ion the number of protons differs from the number of electrons.
Protons (Z)
  • Found in the nucleus (at the centre) of the atom.
  • Protons are positively charged (+).
  •  A protons has a relative mass of 1.
  • The number of protons in the atom helps us to identify the atom.
  • The number of protons is called the proton number or atomic number and is used to arrange atoms in the periodic table.
Atomic number (Z)
  • The number of protons in an atom.
  • The atomic number defines an element's chemical properties because the atom number (Z), is also the number of electrons.
  • The atomic number is unique for each element and it determines the bonding characteristic or behavior or reactivity.
  • Every atom with different atomic number acts in a different way.
  • Number of protons (Z) = number of electrons for a neutral atom.
Neutrons (N)
  • Found in the nucleus (at the centre) of the atom.
  • Neutrons have no charge.
  • Have a relative mass of 1.
  • Nucleons are the particles found in the nucleus, that is, the protons and neutrons.
  • The nucleon number, therefore, is the total number of protons and neutrons.
  • It tells us about the relative atomic mass of the atom.
  • Nucleon number is the mass number (A) = relative mass of protons + neutron.
  • To get the number of neutrons in an atom you have to subtract the number of protons from the mass number (N = A - Z).

Relative atomic mass (Ar)

  • The relative atomic mass is the ratio of the average mass per atom of the naturally occurring form of element to one twelfth the mass of an atom of carbon 12.

Ar =mass of atom112of the mass of carbon 12

  • The relative atomic masses of most elements are found on the periodic table of elements.
  • It is equal to the relative mass of proton plus relative mass of neutrons (Ar=P+N).

For example the relative atomic mass of oxygen is 16.

  •  Ar (O) = 16
  •   Ar (H) = 1
  •   Ar (C) = 12

Relative atomic mass is dimensionless.

The term relative atomic mass is exactly equivalent to atomic weight.

Nuclide notation

Atoms of different elements are shown on the periodic table as follows:

 Atomic number (Z)Mass number (A)Symbol of element (X)

For example:M1224g

The element magnesium has an atomic mass of 24 and atomic number of 12.

      • Number of protons = 12
      • Number of electrons = 12
      • Number of neutrons N = A (24) – Z (12) = 12


  • Isotopes are atoms of the same element that have the same proton number but different nucleon numbers.
  • The difference in the nucleon numbers is due to the different number of neutrons.
  • Isotopes of an element have the same chemical properties because they have the same atomic number hence the same number of electrons.
  • Electrons in the outer most shell (valence electrons) determine the chemical properties of an element because the valence electrons are involved in chemical bonding.
  • Isotopes have different physical properties due to different atomic masses.
Examples of isotopes

Carbon has several isotopes, the most common of which are C612 and C614

Chlorine has two isotopes, C1735l and C1737l

Hydrogen has three isotopes,H11, H12 and H13

Electronic configuration

  • It is the arrangement of electrons in the orbital of an atom or a molecule.
  • The number of electrons in an atom is equal to the atomic number of the atom.
  • The first atomic shell or energy level is closer to the nucleus and is small, it can only accommodate 2 electrons.
  • The second shell can only accommodate 8 electrons.
  • The third shell can accommodate 18 electrons.
  • However for elements 1-20 in the periodic table, the maximum number of electrons that occupy the third shell is 8; the additional 1 or 2 electrons go into the fourth shell.

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Take note: When arranging electrons in an energy level you have to put four on separate positions then you have to pair the remaining electrons. For example, carbon has four electrons in the outer shell.  Therefore it does not have a pair of electrons in the outer shell. Nitrogen has one more electron in the outer shell than carbon therefore after arranging four separately; the extra electron is then paired. The unpaired electrons represent the valency of the element (capacity to take in or lose electrons). Carbon has a valency of 4 and nitrogen has a valency of 3.

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Element Symbol Proton number Electronic configuration Valency number
hydrogen H 1 1 1
Helium He 2 2 0
Lithium Li 3 2. 1 1
Beryllium Be 4 2. 2 2
Boron B 5 2. 3 3
Carbon C 6 2. 4 4
Nitrogen N 7 2. 5 3
Oxygen O 8 2. 6 2
Flourine F 9 2. 7 1
Neon Ne 10 2. 8 0
Sodium Na 11 2. 8. 1 1
Magnesium Mg 12 2. 8. 2 2
Aluminium Al 13 2. 8. 3 3
Silicon Si 14 2. 8. 4 4
Phosphorus P 15 2. 8. 5 3
Sulphur S 16 2. 8. 6 2
Chlorine Cl 17 2. 8. 7 1
Argon Ar 18 2. 8. 8 0
Potassium K 19 2. 8. 8. 1 1
Calcium Ca 20 2. 8. 8. 2 2

Chemical significance of electronic structure

  • Atoms of the same elements have the same number of electrons.
  • In a chemical reaction electrons move from one atom to another or electrons are shared between two atoms. 
  • The electrons in the outer energy level or shell are the ones which take part in a chemical reaction.
  • Electrons are lost or gained by the outermost shell only.
  • The electrons in the outer shell are called valence electrons.
  • Not all of the valence electrons take part in the chemical reaction.
  • Valency is the number of electrons that an atom needs to fill the outer most shell or the number of electrons that an atom can lose from the outer most shell to achieve stable state.
  • Simply put, it is the number of electrons that an atom will gain, lose or share to achieve stable configuration.
  • Atoms of elements are not stable unless they attain a stable structure called the noble gas configuration.
  • Atoms have to lose or gain electrons in order to attain a noble gas configuration.
  • The noble gas configuration has 8 or`18 electrons in the outer shell.
  • Electrons need energy for them to move from their shells.
  • Since atoms of the same element have the same number of electrons, they behave the same way in the chemical reaction.