Atomic structure and the Periodic Table
Structure of an atom
Proton, p, has relative mass 1, relative charge +1, and is found in nucleus.
Neutron, n, has relative mass 1, relative charge 0, and is found in nucleus.
Electron, e-, has relative mass , relative charge -1, and is found in
energy levels / quantum shells surrounding nucleus.
Terminology
Atomic (proton) number is number of protons in nucleus of atom.
Mass number is total number of protons and neutrons in nucleus of atom.
Isotopes are atoms of same element with different mass numbers; they
have same number of protons but different number of neutrons.
Relative isotopic mass is average mass of an atom of an isotope, relative
to 1/12th of mass of an atom of carbon-12.
Relative atomic mass is average mass of an atom of an element, relative
to 1/12th of mass of an atom of carbon-12.
Relative molecular mass and relative formula mass are both sum of
relative atomic masses in molecule/formula. Relative formula mass is
used for compounds with giant structures.
Mass spectrometry
Mass spectrometer measures masses of atoms or molecules. It does this
by producing positive ions from these, which are then deflected by
magnetic field according to their mass-to-charge ratio (m/z). It then
calculates relative abundance of each positive ion and displays this as a
percentage.
You can determine relative atomic mass from mass spectrum of element
and relative abundance of its isotopes.
Mass spectrum of Br2:
◊ Br has two isotopes: 50% abundance of 79Br and 50% abundance
of 81Br
, relative [Br79-Br81]+
intensity/
abundance
[Br79-Br79]+ [Br81-Br81]+
79
Br+ 81
Br+
m/z
Mass spectrum of Cl2:
◊ Cl has two isotopes: 75% abundance of Cl35 and 25% abundance of
Cl37
9
35
[Cl35-Cl35]+
Cl+
6
[Cl35-Cl37]+
37
Cl+
1
[Cl37-Cl37]+
To determine relative molecular mass of polyatomic molecule, look for
molecular ion peaks in sample. There might be small peak to right of each
molecular ion peak, caused by small percentage of carbon-13 isotope
present in organic molecule. This small peak is referred to as M+1 peak.
It is more often seen in molecules with large masses where percentage of
carbon-13 becomes significant.
Ionisation energies
First ionisation energy is energy required to remove outer electron from
each atom in one mole of gaseous atoms. A(g) → A+(g) + e-
Successive ionisation energies are amounts of energies required to
remove 2nd, 3rd, 4th…electron from cations after removal of 1st electron
from each atom in one mole of gaseous atoms. A+(g) → A2+(g) + e-
Structure of an atom
Proton, p, has relative mass 1, relative charge +1, and is found in nucleus.
Neutron, n, has relative mass 1, relative charge 0, and is found in nucleus.
Electron, e-, has relative mass , relative charge -1, and is found in
energy levels / quantum shells surrounding nucleus.
Terminology
Atomic (proton) number is number of protons in nucleus of atom.
Mass number is total number of protons and neutrons in nucleus of atom.
Isotopes are atoms of same element with different mass numbers; they
have same number of protons but different number of neutrons.
Relative isotopic mass is average mass of an atom of an isotope, relative
to 1/12th of mass of an atom of carbon-12.
Relative atomic mass is average mass of an atom of an element, relative
to 1/12th of mass of an atom of carbon-12.
Relative molecular mass and relative formula mass are both sum of
relative atomic masses in molecule/formula. Relative formula mass is
used for compounds with giant structures.
Mass spectrometry
Mass spectrometer measures masses of atoms or molecules. It does this
by producing positive ions from these, which are then deflected by
magnetic field according to their mass-to-charge ratio (m/z). It then
calculates relative abundance of each positive ion and displays this as a
percentage.
You can determine relative atomic mass from mass spectrum of element
and relative abundance of its isotopes.
Mass spectrum of Br2:
◊ Br has two isotopes: 50% abundance of 79Br and 50% abundance
of 81Br
, relative [Br79-Br81]+
intensity/
abundance
[Br79-Br79]+ [Br81-Br81]+
79
Br+ 81
Br+
m/z
Mass spectrum of Cl2:
◊ Cl has two isotopes: 75% abundance of Cl35 and 25% abundance of
Cl37
9
35
[Cl35-Cl35]+
Cl+
6
[Cl35-Cl37]+
37
Cl+
1
[Cl37-Cl37]+
To determine relative molecular mass of polyatomic molecule, look for
molecular ion peaks in sample. There might be small peak to right of each
molecular ion peak, caused by small percentage of carbon-13 isotope
present in organic molecule. This small peak is referred to as M+1 peak.
It is more often seen in molecules with large masses where percentage of
carbon-13 becomes significant.
Ionisation energies
First ionisation energy is energy required to remove outer electron from
each atom in one mole of gaseous atoms. A(g) → A+(g) + e-
Successive ionisation energies are amounts of energies required to
remove 2nd, 3rd, 4th…electron from cations after removal of 1st electron
from each atom in one mole of gaseous atoms. A+(g) → A2+(g) + e-
