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Title: AS Chemistry Class notes - Physical chemistry - Atomic structure
Description: This is class notes for the first year/AS chemistry. It covers the first chapter of physical chemistry - atomic structure including: Fundamental physics Mass number, atomic number and isotopes The arrangement of electrons The mass spectrometer Ionisation energy
Description: This is class notes for the first year/AS chemistry. It covers the first chapter of physical chemistry - atomic structure including: Fundamental physics Mass number, atomic number and isotopes The arrangement of electrons The mass spectrometer Ionisation energy
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Physical chemistry 1
Atomic structure
Fundamental particles
Developing ideas of the atom
•
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 office
Title: AS Chemistry Class notes - Physical chemistry - Atomic structure
Description: This is class notes for the first year/AS chemistry. It covers the first chapter of physical chemistry - atomic structure including: Fundamental physics Mass number, atomic number and isotopes The arrangement of electrons The mass spectrometer Ionisation energy
Description: This is class notes for the first year/AS chemistry. It covers the first chapter of physical chemistry - atomic structure including: Fundamental physics Mass number, atomic number and isotopes The arrangement of electrons The mass spectrometer Ionisation energy