2.1.1 Atomic Structure
Details of the three Sub-atomic (fundamental) Particles
Particle Position Relative Mass Relative Charge
Proton Nucleus 1 +1
Neutron Nucleus 1 0 There are various
Electron Orbitals 1/1800 -1 models for atomic
structure
An atom of Lithium (Li) can be represented as follows:
Mass Number 7 Li Atomic Symbol
Atomic Number 3
The atomic number, Z, is the number of protons in the nucleus.
The mass number ,A, is the total number of protons and neutrons in the atom.
Number of neutrons = A - Z
Isotopes Isotopes are atoms of same element with the same number of protons, but different numbers of neutrons.
Isotopes have similar chemical properties because they have the same electronic
structure. They may have slightly varying physical properties because they have
different masses.
DEFINITION: Relative Isotopic mass is the mass of one isotope compared to
one twelfth of the mass of one atom of carbon-12
DEFINITION: Relative atomic mass is the weighted mean mass of one atom
compared to one twelfth of the mass of one atom of carbon-12
DEFINITION: Relative molecular mass is the average mass of a molecule
compared to one twelfth of the mass of one atom of carbon-12
Calculating the Relative Atomic Mass of an Element
The relative atomic mass quoted on the periodic table is a weighted mean mass of all the
isotopes- taking into account the relative abundances of all the isotopes.
Percentage Abundance 78.7 10.13 11.17
R.A.M = (isotopic mass x % abundance)
Relative Isotopic mass 24.00 25.00 26.00
100
Isotope Mg24 Mg25 Mg26
For above example of Mg
R.A.M = [(78.7 x 24) + (10.13 x 25) + (11.17 x 26)] /100 = 24.3
R.A.M = (isotopic mass x relative abundance)
If relative abundance is used instead of
total relative abundance percentage abundance use this equation
N Goalby chemrevise.org 1
Details of the three Sub-atomic (fundamental) Particles
Particle Position Relative Mass Relative Charge
Proton Nucleus 1 +1
Neutron Nucleus 1 0 There are various
Electron Orbitals 1/1800 -1 models for atomic
structure
An atom of Lithium (Li) can be represented as follows:
Mass Number 7 Li Atomic Symbol
Atomic Number 3
The atomic number, Z, is the number of protons in the nucleus.
The mass number ,A, is the total number of protons and neutrons in the atom.
Number of neutrons = A - Z
Isotopes Isotopes are atoms of same element with the same number of protons, but different numbers of neutrons.
Isotopes have similar chemical properties because they have the same electronic
structure. They may have slightly varying physical properties because they have
different masses.
DEFINITION: Relative Isotopic mass is the mass of one isotope compared to
one twelfth of the mass of one atom of carbon-12
DEFINITION: Relative atomic mass is the weighted mean mass of one atom
compared to one twelfth of the mass of one atom of carbon-12
DEFINITION: Relative molecular mass is the average mass of a molecule
compared to one twelfth of the mass of one atom of carbon-12
Calculating the Relative Atomic Mass of an Element
The relative atomic mass quoted on the periodic table is a weighted mean mass of all the
isotopes- taking into account the relative abundances of all the isotopes.
Percentage Abundance 78.7 10.13 11.17
R.A.M = (isotopic mass x % abundance)
Relative Isotopic mass 24.00 25.00 26.00
100
Isotope Mg24 Mg25 Mg26
For above example of Mg
R.A.M = [(78.7 x 24) + (10.13 x 25) + (11.17 x 26)] /100 = 24.3
R.A.M = (isotopic mass x relative abundance)
If relative abundance is used instead of
total relative abundance percentage abundance use this equation
N Goalby chemrevise.org 1
