Detailed Notes - Topic 1 Atomic Structure and the Periodic Table - Edexcel Chemistry A-level.pdf

Edexcel Chemistry A-Level
Topic 1: Atomic Structure and the
Periodic Table
Detailed Notes

Fundamental Particles
The model for atomic structure has evolved over time as knowledge and scientific
understanding changes.
The current, accepted model of the atom consists
of a small, dense central nucleus surrounded by
orbiting electrons in electron shells . This was
discovered in the Rutherford scattering
experiment in 1911.
The nucleus consists of protons and neutrons
giving it an overall positive charge. It contains
almost the entire mass of the atom.
In a neutral atom, the number of electrons is equal
to the number of protons due to the relative
charges.
Particle Proton Neutron Electron
Relative Charge +1 0 -1
Relative Mass 1 1 1/1840
The maximum number of orbiting electrons that can be held by any single shell, depends on the
number of the shell. This can be calculated using 2 n 2 where n is the number of the shell.
Example:
Electrons in shell 2 = 2(2 2 ) = 8 electrons
Each electron shell must fill before the next one can hold any electrons.
Atomic Number and Mass Number
Mass number is represented using A and can be calculated as the sum of protons and
neutrons in an atom.
Atomic number is represented using Z and is equal to the number of protons in an atom,
hence it can be referred to as proton number.

Using these numbers, the quantity of each fundamental particle in an atom can be calculated.
Example:
Atomic number = 7
Mass number = 14
Proton number = 7
Neutron number = 14 - 7
= 7
Isotopes
Isotopes are atoms of the same element with the same atomic number , but with a different
number of neutrons , resulting in a different mass number .
Neutral atoms of isotopes will react chemically in the same way as their proton number and
electron configuration is the same. The sharing and transfer of electrons is unaffected.
However, the different mass number means they have different physical properties .
Example:
Hydrogen = 1 proton and 0 neutrons
Deuterium = 1 proton and 1 neutron
Tritium = 1 proton and 2 neutrons
Relative Masses
Relative atomic mass ( Ar ) is defined as:
The mean mass of an atom of an element, relative to one twelfth
of the mean mass of an atom of the carbon-12 isotope.
This takes the relative abundances of the different isotopes of an element into account.

Relative isotopic mass is defined as:
The isotopic mass of an isotope relative to one twelfth of the
mean mass of an atom of the carbon-12 isotope.
Relative molecular mass ( Mr ) is defined as:
The mean mass of a molecule of a compound, relative to one twelfth
of the mean mass of an atom of the carbon-12 isotope.
It can be calculated for a molecule by adding together the separate Ar values of the component
elements.
Example:
Relative formula mass is similar to Mr but is used for compounds with giant structures.