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Atomic Structure
Details of the three Sub-atomic (fundamental) Particles
Particle

Position

Relative Mass

Relative Charge

Proton
Neutron
Electron

Nucleus
Nucleus
Orbit

1
1
1/1800

+1
0
-1

There are various
models for atomic
structure

: s w oll of s a d et n e s e r p e r e b n a c )i L ( m ui hti L f o m ot a n A
Mass Number
Atomic Number

7
3

Li

Atomic Symbol

The atomic number, Z, is the number of protons in the nucleus
...


Number of neutrons = A - Z

Isotopes

Isotopes are atoms with the same number of protons, but different numbers of neutrons
...
They may have slightly varying physical properties because they have
different masses
...


magnetic field
defects the ions

Vacuum
pump
Vaporised
sample
The following are the essential 4 steps
in a mass spectrometer
...
Ionisation
•A Vaporised sample is injected at low pressure
•An electron gun fires high energy electrons at the sample
•This Knocks out an (outer) electron
•Forming positive ions with different charges E
...
Ti Ti+ + e–

If the sample is not vaporized
then vaporizing it would be
the first step
...
Acceleration
•A negative electric field accelerates the positive ions and makes them into a beam
3
...

•The degree of deflection depends on the mass-to-charge ratio, m/z
...

•By varying the magnetic field ratio, ions of different m/z ratios pass through the centre
...
Detection
•The ions reach the detector and generate a small current, which is fed to a computer for analysis

Copyright N Goalby Bancroft's School

1

The relative atomic mass quoted on the periodic table is a weighted average of all the isotopes
Fig: spectra for
Magnesium from mass
spectrometer

100
80
%

78
...
13%
24

25

11
...
A
...
A
...
Mg24 with a 2+ charge would
have a m/z of 12

For above example of Mg
R
...
M = [(78
...
13 x 25) + (11
...
3

Mass spectra for Cl2 and Br2
Cl has two isotopes Cl35 (75%) and Cl37(25%)

Br has two isotopes Br79 (50%) and Br81(50%)

These lead to the following spectra caused by the diatomic molecules
Br79Br81
Br81Br79
Cl35Cl35
relative
abundance

relative
abundance

Cl35Cl37

Br79Br79

Br81Br81

Cl37Cl37
70

72

74

m/z

158

160

m/z

162

Mass spectrometers have been included in planetary space probes so that elements on other planets can
be identified
...
The peak with the largest m/z, however, will be
due to the complete molecule and will be equal to the Mr of
the molecule
...
) with
electrons in spherical orbits
...


The A-level model
Electrons are arranged on:

Sub energy levels labelled s ,
p, d and f
s holds up to 2 electrons
p holds up to 6 electrons
d holds up to 10 electrons
f holds up to 14 electrons

Split
into

Principle energy levels
numbered 1,2,3,4
...

Each orbital has its own
approximate, three
dimensional shape
...


Writing electronic structure using letters and numbers

4

2

l e v el - b u s
f o e p yt
fo e ma N

negyxo roF
l e v el y g r e n e
ni a m f o r e b m u N

2p

sll e b b m u d e kil
d e p a h s e r a sl e v el b u s p •

1

n o rt c el e e n o si w o r r a n A
l ati b r o
e n o st n e s er p er x o B

1s

l e v el - b u s ni
s n o rt c el e f o r e b m u N

For fluorine

2s

l a ci r e h p s
e r a sl e v el b u s s •

Using spin diagrams

1s 2s 2p

The arrows going in the
opposite direction represents
the different spins of the
electrons in the orbital

When filling up sub levels with several
orbitals, fill each orbital singly before
starting to pair up the electrons

The periodic table is split into
blocks
...

It does not matter if the atom does
not normally form a +1 ion or is not
gaseous

Definition :Second ionisation energy

e g r a h c e viti s o p el b u o d a hti w s n oi
s u o e s a g f o el o m e n o s m r of e g r a h c e viti s o p el g ni s a hti w s n oi s u o e s a g
f o el o m e n o n e h w e g n a h c y pl a ht n e e ht si y g r e n e n oit a si n oi d n o c e s e h T

Ti+ (g)

Ti2+(g) + e-

: n oit a u q e e ht y b d et n e s e r p e r si si h T
Factors that effect Ionisation energy

There are three main factors
1
...
The distance of the electrons from the nucleus
(The bigger the atom the further the outer electrons are from the nucleus and the
weaker the attraction to the nucleus)
3
...

Why is the successive ionisation energies always larger?
The second ionisation energy of an element is always bigger than the first ionisation energy
...

The ion increases the attraction on the remaining electrons and so the energy required to
remove the next electron is larger
...

1

2
3
4
5
No of electrons removed

6

Explanation
The fifth electron is in a inner
shell closer to the nucleus and
therefore attracted much more
strongly by the nucleus than the
fourth electron
...
A repeating pattern across a
period is called periodicity
...
Why has Helium the largest first ionisation energy?
A
...
He has a bigger first ionisation
energy than H as it has one more proton
Q
...
As one goes down a group, the outer electrons are found in shells
further from the nucleus and are more shielded so the attraction of
the nucleus becomes smaller
Q
...
As one goes across a period the electrons are being added to the same
shell which has the same distance from the nucleus and shame shielding
effect
...

Q
...
So Na’s outer electron is easier to
remove and has a lower ionisation energy
...
Why is there a small drop from Mg to Al?
Al is starting to fill a 3p sub shell, whereas Mg has its outer electrons in the 3s
sub shell
...
Why is there a small drop from P to S?
With sulphur there are 4 electrons in the 3p sub shell and the 4th is starting to doubly
fill the first 3p orbital
...


3p

Learn carefully the
explanations for
these two small
drops as they are
different to the
usual factors

3p

3s

3s
Two electrons of opposite spin in
same orbital
4p32s36p2 2s2 1s1

Copyright N Goalby Bancroft's School

3p32s36p2 2s2 1 s1
r u h pl u s

s ur o h p s o h p



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