Download Atomic structure Atomic masses

Atomic structure
Protons, neutrons & electrons
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Particle
Relative mass
Charge
Protons and neutrons make up the nucleus at the centre
Proton
1.0
1+
of the atom
Neutron
1.0
0
Electrons orbit the nucleus in shells
Electron
1/2000
1The nucleus makes up almost all of an atom’s mass
Most of the volume of an atom consists of empty space between the nucleus and electron
shells
An atom has the same number of protons as electrons, so the atom is electrically neutral
Isotopes
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Isotopes: atoms of the same element but with different numbers of neutrons
Isotopes have different masses, the same number of protons and electrons but different
numbers of neutrons.
Atomic number: The number of protons in the nucleus of an atom
Mass number: The number of particles in the nucleus of an atom (protons + neutrons)
The atomic number is below the element symbol and the mass number is above it
Different isotopes of the same element react in the same way because chemical reactions
involve electrons, and neutrons make no difference to chemical reactivity.
Atomic structure of Ions
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Ion: a positively or negatively charged atom or (covalently bonded) group of atoms (a
molecular ion)
Ions are charged because they have different numbers of protons and electrons
Atomic masses
Instead of finding the mass of atoms directly, we compare the masses of different atoms using the
idea of relative mass. The carbon-12 isotope has been chosen as the international standard for the
measurement of relative mass.
Relative isotopic mass
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Relative isotopic mass: the mass of an atom of an isotope compared with 1/12 the mass of
an atom of carbon-12
For an isotope, the relative atomic mass is the same as the mass number
Relative atomic mass, Ar
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Relative atomic mass, Ar: the weighted mean mass of an atom of an element compared with
1/12 of the the mass of an atom of carbon-12
The term ‘weighted mean mass’ is used to account for the contribution made by each
isotope to the overall mass of an element.
The contribution made by an isotope to the overall mass depends on, the percentage
abundance of the isotope and the relative mass of the isotope
To find the relative atomic mass, find the contribution of each isotope and add them
together. To find the contribution of an isotope: % abundance of isotope/100 x mass
number of isotope
Relative molecular mass, Mr
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Relative molecular mass, Mr: the weighted mean mass of a molecule compared with 1/12 of
the mass of an atom of carbon-12
The relative molecular mass can be found by adding together the relative atomic masses of
the each atom making up a molecule.
Relative formula mass
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Relative formula mass: The weighted mean mass of a formula unit compared with 1/12 of
the mass of an atom of carbon-12.
Compounds with giant structures do not exist as simple molecules. These include ionic
compounds and covalent structures.
Relative molecular mass can be used, but relative formula mass is a better term.
It is calculated by adding together the relative atomic masses of each atom making up the
formula unit.