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Physical chemistry 1
Atomic structure
Fundamental particles
Developing ideas of the atom
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Greek philosophers had a model where matter was made up of a continuous substance making 4
elements – air, earth, fire and water
...


•

Alchemists built up a lot of evidence about how substances change and combine – trying to turn
metals into gold
...


•

1803 – Henri Becquerel discovered radioactivity, showing particles could come from inside the
atom, so it wasn’t indivisible
...
J Thompson discovered the electron, the first sub-atomic particle to be discovered
...


•

As electrons were negative but atoms were neutral, there had to be a source of positive charge
...


•

J
...


•

1911 – Ernest Rutherford and his team found that most of the mass and all the positive charge
was in a tiny central nucleus
...
E
...
, at CERN
...


•

Protons and neutrons form the nucleus and are nucleons
...
673x10-27
1
...
9x10-30
-19
Charge (C)
+1
...
602x10-19
The numbers are very small, so in practise relative masses and charges are used:

•

Property
Proton p
Neutron n
Relative mass
1
0
Relative charge
+1
0
In a neutral atom, the number of protons and electrons are equal
...
It acts over short distances
...


Mass number, atomic number and isotopes
Atomic number z
•

The number of protons in the nucleus is called the atomic number or proton number z
...


•

The atomic number defines a chemical identity because the number of electrons in the outer
shell determines the chemical properties
...
Atoms of different elements have
different atomic numbers
...


•

Electrons’ weight is negligible, so they don’t contribute to the mass number
...


•

Atoms with the same number of protons but different numbers of neutrons are called isotopes
...


•

Isotopes very in the mass number
...

o

The 13 is the mass number
...


Carbon dating
•

Sometimes isotopes are unstable, and the nucleus breaks down giving off bits of energetic rays –
this is radioactivity
...


•

Each radioactive isotope decays at a rate measured by its half-life – the time taken for half its
radioactivity to decay
...
It’s used to date organic matter up to 60,000 years but
most accurately up to 2000 years
...
Once it dies, this stops as the
radioactive carbon breaks down and the level of radioactivity slowly falls
...


The arrangement of electrons pt
...
The movement of electrons from 1 shell to
the next explains how atoms absorb and give out light – the beginning of quantum theory
...
This led to a theory called quantum mechanics – which
can be used to predict the behaviour of sub-atomic particles
...


•

At the same time, chemists were developing ideas about how electrons allowed atoms to bond
...


•

The inertness of noble gases is related to their full outer shell of electrons
...


•

Atoms could also bond by sharing electrons to form full outer shells
...
Later theories suggest they’re clouds
of charge, you can never tell where exactly the electron is – only the probability that they’re in a
certain space
...


•

Darwin’s model can be used to explain geometrics of crystals
...


•

Charge cloud idea is used for a more sophisticated explanation of bonding and shapes of
molecules
...


Electron shells
•

The shell closest to the nucleus (1st) fills first, then the 2nd etc
...


•

The number of electrons in each shell = 2n2 where n is the number of shell
...


•

You can draw a diagram or write the number of electrons in each shell, starting from the inner
and working out, separating each with a comma
...
g
...


•

It’s an extremely successful idea that underlies electronic gadgets such as computers
...


o

Find the relative molecular mass (Mr) of substances made of molecules
...
The time taken to travel a fixed distance is used to calculate the mass
...
This
would interfere with the results
...
This is a hot wire filament with a current running through it that emits
electrons (because metals lose electrons when given enough energy)
...


o

X(g) + e- → X+(g) + 2 e-

o

OR X(g) → X+(g) + e-

o

This technique is used for elements and substances with a low formula mass
...


o

This molecular ion often breaks down into smaller fragments (because adding electrons
messes up the bonds) some which are detected in the mass spectrum
...


Electron spray
o

The sample is dissolved in a volatile solvent (e
...
water or methanol)
...


o

The sample is then injected through a fine hypodermic needle to give a fine mist
(aerosol)
...


Physical chemistry 1
Atomic structure

•

•

•

•

o

The particles are ionised and a proton (H+ ion) from the solvent as they leave the needle
producing XH+ ions
...
g
...


o

It’s known as “soft” ionisation technique and fragmentation usually takes place
...


o

Proton is datively bonded to the molecule so doesn’t change the element
...


Acceleration
o

The positive ions are accelerated using an electric field as they are attracted to a
negatively charged plate
...


o

KE = ½ mv2

o

V = √(2KE/m)

o

The velocity of each particle depends on its mass as KE is the same
...


Flight tube/ion drift
o

The ions pass through a hole in the negatively charged plate, forming a beam and
travelling along a tube – the flight tube
...


o

The time of flight depends on the velocity which depends on the mass of the particles
...
Therefore lighter ions travel faster and reach the
detector in less time
...
The flight
times are recorded
...


o

When they hit the plate, they are discharged by gaining electrons from the plate,
generating a movement of electrons and therefore a current
...


Analysis

Physical chemistry 1
Atomic structure
o

The signal from the detector is passed to a computer which generates a mass spectrum
...


•

It shows the mass to charge (m/z) ratio and relative abundance of each ion
...


•

For molecules ionised by electron bombardment, the signal with the greatest m/z value is from
the molecular ion (giving its Mr)
...


•

When using electron bombardment, there might be peaks at lower m/z values due to fragments
caused by the breakup of molecular ions
...


•

They are used to identify elements in rock samples
...


•

The unit, including rechargeable batteries, weighs 10kg, light enough to be carried by scene of
crime officers looking for drugs, explosives, or chemical weapons
...


The arrangement of electrons pt
...


Energy levels
•

Electrons in different shells have different energy and can be represented on an energy level
diagram
...
and can each hold a maximum number of
electrons
...


Physical chemistry 1
Atomic structure
•

Level 2 has s and p sublevels
...


Quantum mechanics
•

For a more complete description of the electrons in an atom, the theory quantum mechanics is
used which was developed in the 1920s
...


Atomic orbitals
•

An electron is a cloud of negative charge which fills a volume of space called its atomic orbital
...


•

Different atomic orbitals have different energies, each with a number corresponding with a main
energy level
...


Main energy level
1
2
2
3
3
3
4
4
4
4
Sublevel
s
s
p
s
p
d
s
p
d
F
No
...
of electrons in sublevel 2e 2e 6e
2e
6e
10e
2e 6e 10e
14e
Total no
...


•

Any single atomic orbital can hold a maximum a maximum of 2 electrons
...


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P orbital – 3 orbitals – 6 electrons
...


•

This can be shown in an energy level diagram
...


•

4s has less energy than 3d but this changes when ions are formed
...


•

2 electrons in the same orbital have different spin
...


Putting electrons in orbitals
•

The label of atomic orbitals tells us about the energy and shape of an electron cloud
...


o

Atomic orbitals of the same energy fill singly before pairing starts because electrons
repel each other
...


Writing electron structures
•

A shorthand way of writing electronic structure is:

•

E
...
Calcium has 20 electrons
o

1s2 2s22p6 3s23p64s2

•

Notice how the 4s orbital is filled before the 3d orbital
...
The 4s is then written at the end, because when an
electron is lost, it is lost from this orbital
...
g
...


•

Sometimes it’s easier to use the previous noble gas symbol
...
g
...


•

E
...
Na+
o

1s2 2s22p6

o

This has 1 less electron than Na
...


Background
•

Electrons give out specific amount of energy
...
This makes
them unstable, meaning they drop back down, releasing energy as light
...


Ionisation energy
•

Patterns in the 1st ionisation energy across a period provide evidence for electron energy
sublevels
...


•

Ionisation energy (IE) is the energy required to remove 1 mole of electrons from 1 mole of atoms
in the gaseous state and is measured in kJmol-1
...


•

The 1st electron needs the least amount of energy because it’s being removed from a neutral
atom
...


•

The 2nd electron needs more energy than the 1st because it’s being removed from a +1 ion,
meaning it’s attracted more strongly
...


•

The 3rd needs even more and so on
...


•

1st IE = X(g) → X+(g) + e-

•

2nd IE = X+(g) → X2+(g) + e-

•

3rd IE = X2+(g) → X3+(g) + e-

•

E
...
Sodium

Electron removed
Ionisation energy
(kJmol-1)

1
496

2
4,563

3
4
5
6
7
8
9
10
11
6,913 9,544 13,352 16,611 20,115 25,491 28,934 141,367 159,079

•

Notice that the 2nd IE isn’t Na(g) → Na2+(g) + 2e-
...


•

1 electron is relatively easy to remove then comes a group of 8 that are more difficult to remove
and then 2 which are very difficult to remove
...


o

8 electrons nearer to the positive nucleus (harder to remove)
...


•

A graph of this tells you about the number of electrons in each main level and, with a good scale,
the sublevels too
...


Physical chemistry 1
Atomic structure

Trends in the IE across a period
•

Also gives information about number of electrons in the main shells and the sublevels
...


•

The element that has an electron in a new orbital will have slightly less IE despite the increase in
nucleic charge (e
...
Magnesium to Aluminium) because the new orbital has slightly higher
energy, needing less energy to remove the electron
...


•

These are evidence of sublevels, predicted by quantum theory and the Schrodinger’s equation
...


•

The increase in nucleic charge doesn’t make it harder to remove the electron because electron
shielding causes the positive charge ‘felt’ by the electron to be less than the nuclear charge
...




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