Atomic structure

An artist's representation of the orbital model of the atom


Atoms are the building blocks of matter. Atoms are the smallest possible particles into which an element can be subdivided, without changing its properties. 

As building blocks, they can be thought of spheres despite the fact that we now know that they are mostly empty space. 

What's wrong with the image?

The image on the left is an artist's representation of the orbital model of the atom. The centre sphere represents the nucleus while the three outer spheres represent the electrons.  

The three electrons shown appear to be in the same (innermost) shell. This is actually not possible as there can only be two electrons in the innermost shell of an atom. We will see this later on in the course.

Pearson IGCSE

Sub atomic particles 

Sub atomic particles are the particles found inside atoms. These include protons, neutrons and electrons:

  • protons are particles with a positive charge and a mass of 1(amu) . They are part of the nucleus of an atom. The number of protons is given by the atomic number of an element.
  • neutrons are particles with no charge and a mass of 1(amu) they are also part of the nucleus of an atom.
  • electrons are particles with a charge of -1 and a mass of 1/2000 of an amu, they orbit the nucleus in shells.

A more accurate model of the atom 

masses and charges of subatomic particles

Atoms and isotopes 

This image shows an atom of carbon, we know it is carbon because it has six protons shown in the nucleus. The atomic number of carbon is 6. 

The number of protons in the nucleus is given by the atomic number - ( also known as the proton number)

The mass number of an atom is the total numbe rof protons and neutrons. In this case, the nucleus has six neutrons and the atom therefore, has a mass number of 6 + 6  = 12.

If this nucleus had an extra neutron it would still be a carbon atom but it would be an isotope, called carbon-13.

If it had two extra neutrons it would be the isotope carbon 14

Activity 1:  

​Sub atomic particles : Use the game below make sure you know these three sub-atomic particles. Can you beat 3 seconds??

the three isotopes of carbon

Isotopes and relative atomic mass 

All elements can exist as different isotopes, in naturally occuring carbon about 1% of the atoms are carbon 13. Carbon 14 also exists but is unstable. 

In order to calculate the relative mass of the atoms in elements we need to take into account all the masses of the different isotopes. 

We calculate a value called the relative atomic mass.

The relative atomic mass of an element is an average value of the mass of the individual isotopes in a naturally occurring sample of the element.

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Sorting atoms by mass 

 A mass spectrometer is a sophisticated instrument which can separate and sort the particles in a sample of an element .The output from a mass spectrometer is known as a mass spectrum. For a sample of element the mass spectrum tells us the percentage of each isotope which can be found in the element.

This data then allows us to calculate the relative atomic mass for the element under investigation.

The mass spectrum of boron

Calculating relative atomic mass (Ar)

The image shown here is a mass spectrum of the element boron. It shows the relative number of isotopes in a naturally occurring sample of the element. 

We  can calculate Ar values using this isotopic abundance data:

For Boron two isotopes exist, Boron 10 and Boron 11. The mass spectrum shows that 20% of boron atoms are Boron -10 and 80% are Boron -11.

The weighted mean is calculated as follows:

The relative atomic mass of boron is a weighted mean of all the isotopes in a naturally sample of the element.
The mass spectrum of Zinc
The mass spectrum of Magnesium
The mass spectrum of Germanium

 Activity 2

  • ​Look at the mass spectra for Zinc, Magnesium and Germanium and try to estimate the relative atomic mass for each element.
  • Use the data on the mass spectra to calculate the relative atomic masses for those three elements

Activity 3 

Molecules are particles made up of more than one atom


​A molecule is a particle composed of more than one atom joined together by covalent bonds. 

The atoms in a molecule might all be of the same element eg Hydrogen (H2) or Oxygen (O2) or they might be of different elements and the molecule is therefore a molecule of a compound e.g. Water (H2O) or Methane (CH4)

We calculate relative molecular masses by adding up the relative atomic masses of the individual atoms as given in the molecular formula.