# Atomic Structure by yurtgc548

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```									Atomic Structure
Simple model of an atom
• An atom is made of a
tiny nucleus with
electrons orbiting
around it.
up of protons and
neutrons.
• Much of an atom is
empty space
The protons
• Each proton has a +1charge.
• Each proton has a mass of 1 atomic mass
unit.
• The number of protons in an atom is called
its atomic number(z).
The neutrons
• The neutrons have no charge.
• Each neutron has a mass of 1 atomic mass
unit.
• The total number of protons and neutrons in
an atom is called its mass number (A).
The Electrons
• Each electron has a charge of -1.
• Electrons have negligible mass of 1/1840
that of a proton.
Convention for writing an atom

A
zX
Isotopes
• 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%
respectively.
• The weighted average mass of a chlorine
atom is 35X(75/100) + 37 X (25/100) =
26.25+9.25=35.50(no unit)
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.
Mass spectrometry
• A mass spectrometer separates the isotopes
of an element according to their masses and
shows the relative numbers of the different
isotopes present.
• Before the isotopes can be separated, they
must be converted to positive ions.
The workings
•   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
element.
• To work out the relative atomic mass of an
element.
• 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.
25

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
electrons.
• 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
energies.
The successive ionisation energy
graph of Be
• The diagram indicates     Log IE/kJ mol-1
x
two electronic energy                   x
levels.
• Electrons 1 and 2 are
at a higher energy                  x
level                           x
• Electrons 3 and 4 at a
lower energy level -           1 2 3 4
nearest to the nucleus.        Ionisation no
Electron configuration- key
points
• 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-
shells
• 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
out.
• 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
subshells.
x                  x
x x           • The first shell can
have one sub-shell, s
x
x                                subshell.
x
x
x
• 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
filled.
• 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
electrons.
• A p sub-shell can have a maximum of six
electrons.
• A d sub-shell can have a maximum of ten
electrons.
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
electrons
Shape of p-orbitals - dumb-bell
shaped
• The three p orbitals,   py
px,, py and pz.              P=x

pz
px
Electron configuration of
elements
•   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

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