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 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.
Sub atomic particles
- 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
Atoms and isotopes
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
Sub atomic particles : Use the game below make sure you know these three sub-atomic particles. Can you beat 3 seconds??
Isotopes and relative atomic mass
We calculate a value called the relative atomic mass.
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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.
Calculating relative atomic mass (Ar)
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:
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.