Simple model of an atom
• An atom is made of a
tiny nucleus with
• The nucleus is made
up of protons and
• Much of an atom is
• Each proton has a +1charge.
• Each proton has a mass of 1 atomic mass
• The number of protons in an atom is called
its atomic number(z).
• The neutrons have no charge.
• Each neutron has a mass of 1 atomic mass
• The total number of protons and neutrons in
an atom is called its mass number (A).
• Each electron has a charge of -1.
• Electrons have negligible mass of 1/1840
that of a proton.
Convention for writing an atom
• Isotopes are atoms of the same element
which have the same number of protons but
different number of neutrons.
Relative Isotopic Mass
• Relative Isotopic Mass= Mass of one
isotope of the element/(1/12) X Mass of one
atom of 126C.
• An atom of 12C has a relative atomic mass
of 12 exactly.
Relative Atomic Mass(Ar)
• Ar = Weighted average of the isotopic
masses/(1/12) X mass of one 126C atom
Calculating the relative atomic
mass of chlorine
• Chlorine has two isotopes 35Cl and 37Cl in
relative proportions of 75% and 25%
• The weighted average mass of a chlorine
atom is 35X(75/100) + 37 X (25/100) =
Relative Molecular Mass(Mr)
• Mr = Mass of one molecule/(1/12)X Mass
of one 126C atom.
• The relative molecular mass can be worked
out by adding the relative atomic masses of
all the atoms present in one molecule.
• A mass spectrometer separates the isotopes
of an element according to their masses and
shows the relative numbers of the different
• Before the isotopes can be separated, they
must be converted to positive ions.
• Evacuation of the instrument
• Vaporisation of liquid or solid samples
• Production of positive ions
• Acceleration of positive ion
• Deflection of positive ions
• Detection of positive ions according to
mass(m)/charge(e). When charge =+1,
mass/charge = mass
The uses of a mass spectrometer
• To find the isotopic composition of an
• To work out the relative atomic mass of an
• To find the relative molecular mass and the
fragmentation pattern of a molecule.
• In forensic science.
The mass spectrum of Cl
• Chlorine has two % Abundance
isotopes: 35Cl and 37Cl
with relative 75
proportions of 75%
and 25% respectively.
35 37 m/ e
First ionisation energy
• It is the energy required to remove one
electron from each of one mole of gaseous
atoms to form one mole of gaseous ions
with single positive charge and one mole of
• The equation for first ionisation energy of
element A is: A(g) A+(g) + e
Successive ionisation energies
• Successive ionisation energies provide
evidence for the existence of quantum shells
or electronic energy levels.
Successive ionisation energies
• If an atom has two electrons, it will have
two ionisation energies, first ionisation
energy and second ionisation energy.
• If an atom has three electrons, it will have
three separate ionisation energies.
• All these ionisation energies for each
element are its successive ionisation
The successive ionisation energy
graph of Be
• The diagram indicates Log IE/kJ mol-1
two electronic energy x
• Electrons 1 and 2 are
at a higher energy x
• Electrons 3 and 4 at a
lower energy level - 1 2 3 4
nearest to the nucleus. Ionisation no
Electron configuration- key
• Each element has a characteristic emission
spectrum which can be used to identify it.
• The electrons in an element can exist only
at certain energy levels - shells and sub-
• The region in which an electron moves for
most of the time is called an orbital.
• An orbital can hold two electrons.
The Line Spectrum
• An electron can absorb sufficient energy
and move to a higher energy level.
• When such an electron drops to a lower
energy level, the energy absorbed is given
• The amount of energy given out appears as
a line in the line spectrum of the element.
First ionisation energies of
successive elements - H toNe
1st IE/kJ mol-1 • These provide
evidence of shells and
x x • The first shell can
have one sub-shell, s
• The second shell can
have two subshells, s
1 2 3 4 5 6 7 8 9 10 z and p
The aufbau principle
• Electrons always occupy the lowest
available energy sub-level or subshell.
• Electrons pair up after a sub-level is half
• Numbers 1, 2, 3 denote the shells. Letters s,
p, d, f denote the subshells. A superscript
indicates the number electrons.
• Sequence of energy levels: 1s2s2p3s3p4s3d
Subshells and orbitals
• An s sub-shell has only one orbital.
• A p sub-shell has three orbitals.
• A d subshell has five orbitals.
Sub-shells and electrons
• An s sub-shell can have a maximum of two
• A p sub-shell can have a maximum of six
• A d sub-shell can have a maximum of ten
Shape of an s orbital
• An s orbital is
spherical in shape
• The sphere is made up
of a cloud of negative
charge from the
Shape of p-orbitals - dumb-bell
• The three p orbitals, py
px,, py and pz. P=x
Electron configuration of
• H 1s1 He 1s2
• Li 1s2 2s1 Be 1s22s2
• B 1s22s22p1 C 1s22s22p2
• N 1s22s22p3 O 1s22s22p4
• F 1s22s22p5 Ne 1s22s22p6
• Na 1s22s22p63s1 Mg 1s22s22p63s2