Shad Ahmad Chemistry
Unit 1 – Physical Chemistry
1.1 – Atomic Structure
1) What makes up an atom?
• All elements made up of atoms.
• Atoms made up of protons, neutrons, electrons.
• Most mass of atom in nucleus.
• Diameter of nucleus small compared to whole atom.
2) Nuclear symbols.
For neutral atoms, num of electrons = num of protons.
Num of neutrons = Mass num – Atomic num
3) What are ions?
Ions formed when atoms lose or gain electrons.
Negative ions have more electrons than protons; positive ions have more protons than electrons.
4) What are isotopes?
Isotopes of an element = Atoms with the same number of protons but different numbers of
neutrons (so different mass numbers).
Isotopes of the same element have the same chemical properties, because the number and
arrangement of electrons decide the chemical properties of elements.
They have different physical properties, as they depend on the mass of the atom.
,Shad Ahmad Chemistry
5) Models of Atomic Structure changing over time.
1. Dalton – Atoms were solid spheres.
2. Thompson – Plum-pudding model; ball of positive charge with electrons dotted
inside it.
3. Rutherford – Fired positive alpha particles at gold foil.
Neutral plum pudding should get alpha pass through it.
However, some alpha particles were deflected by nucleus.
Atom was concluded to be mostly empty space, most mass
in centre, nucleus is positive, and electrons orbit nucleus.
4. Bohr – Electrons orbit at specific distances; the further an
electron is from the nucleus, the more energy it has. When electron loses
energy, EM radiation is emitted and it jumps down a shell (and vice-versa).
5. Chadwick – Neutrons.
6. Not all electrons in shell have same energy – include sub shells.
6) Definitions of the three relative masses.
Relative Atomic Mass = the average mass of an atom compared to 1/12 mass of an atom of carbon-
12 (12C).
Relative Isotopic Mass = the mass of an atom of an isotope compared to 1/12 mass of an atom of
carbon-12 (12C).
Relative Molecular Mass = the average mass of a molecule compared to 1/12 mass of an atom of
carbon-12 (12C).
7) Electron Impact Ionisation.
• Sample vaporised and fired with high energy electrons from an electron gun, which knocks
off one electron from each particle.
• + ions attracted to a negative plate when they are accelerated.
• Used for elements, and substances made of molecules with a low Mr.
8) Electrospray Ionisation.
• Sample dissolved in a volatile solvent and injected through a needle to give a fine mist.
• Tip of the needle attached to the positive terminal of a high voltage power supply. Particles
are ionised by gaining a proton (H+) from the voltage as they leave the needle.
• Ions formed attracted to a negative plate where they are accelerated.
• Used for substances made of molecules with a high Mr – biological molecules (EG: proteins).
,Shad Ahmad Chemistry
9) Equations for Electron Impact Ionisation and Electrospray Ionisation.
Electron Impact Ionisation = X (g) + e- -> X+ (g) + 2e-
Electrospray Ionisation = X (g) + H+ -> XH+ (g)
10) Measuring relative masses using a TOF (time of flight) Mass Spectrometer.
The ions reach the detector (negative plate) and generate a small
current, which is fed to a computer for analysis. The current is
produced by electrons transferring from the detector to the
positive ions (ions gain an e-). The size of the current is
proportional to the abundance of the species, as current of an ion
can be compared relatively to the current produced by other ions,
causing m/z to be measured.
It needs to be under a vacuum otherwise air particles would ionise and register on the detector.
11) 5th Step – Data analysis – Mass Spectrum created from Electron Impact Ionisation.
Spectrum above produced using electron impact ionisation:
• Detector plate is negative.
• One electron knocked off each particle, turning them to +1 ions – so that the mass/charge
ratio of each peak is the same as the relative mass of that isotope.
,Shad Ahmad Chemistry
• If electrospray ionisation had been used instead, an H+ ion would have been added to each
particle to form +1 ions – so that the mass/charge ratio of each peak would be one greater
than the relative mass of each isotope.
12) Advantages and Disadvantages of Electron Impact Ionisation and Electrospray Ionisation.
Electron Impact Ionisation:
Molecule often fragmented – many peaks. Peak with the highest m/z is the molecule Mr.
Electrospray Ionisation:
Molecules rarely fragmented – one peak, minus the m/z by 1 to get the molecule Mr.
Resulting ions have a Mr one unit higher due to the gain of a proton.
Can’t be used if particles aren’t capable of gaining a proton.
13) Working out Relative Atomic Mass from a mass spectrum (EG: Mg).
14) Identifying elements from a mass spectrum.
,Shad Ahmad Chemistry
15) Identifying molecules using mass spectrometry.
• A molecular ion, M+, is formed in the mass spectrometer when one electron is removed
from the molecule.
• This gives a peak in the spectrum with a mass/charge ratio equal to the relative molecular
mass of the molecule.
• This is used to identify any unknown compounds.
16) Electron shells made up of sub-shells and orbitals.
• Electrons have fixed energies, moving around the nucleus in areas called shells (or energy
levels).
• Each shell is given a principal quantum number.
• The further a shell is from the nucleus, the higher its energy, and the larger its principal
quantum number.
• Experiments show that not all the electrons in a shell have exactly the same energy.
• The atomic model explains why shells are divided up into sub-shells that have slightly
different energies.
• The sub-shells have different numbers of orbitals, which can each hold up to 2 electrons.
• The two electrons in each orbital spin in opposite directions.
,Shad Ahmad Chemistry
17) Working out Electron Configurations.
First ionisation energy of oxygen atoms less than that of nitrogen atoms as two of the electrons in
the outer p subshell of oxygen occupy the same orbital. Therefore, there is repulsion between the
two electrons, lowering the first ionisation energy of oxygen.
18) Behaviour of Transition Metals.
• Chromium and copper donate one of their 4s electrons to the 3d sub-shell, as it’s more
stable with a full or half-full d sub-shell.
• When transition metals become ions, they lose their 4s electrons before their 3d electrons –
when 4s is filled, it becomes higher than 3d, because 3d (more electrons) has a stronger
attraction with the nucleus.
, Shad Ahmad Chemistry
19) Chemical Properties of an element decided by their electronic structure (number of outer shell
electrons).
• The s block elements (Groups 1 and 2) have 1 or 2 outer
shell electrons, which are easily lost to form positive ions
with an inert gas configuration.
• The elements in Groups 5, 6 and 7 (in p block) can gain 1,
2 or 3 electrons to form negative ions with an inert gas
configuration. Groups 4 to 7 can also share electrons
when they form covalent bonds.
• Group 0 (the noble/inert gases) have completely filled s
F block
and p sub-shells – completely inert.
• Transition metals (d block elements) lose s and d electrons to form positive ions.
20) Ionisation.
Ionisation = the removal of one or more electrons.
First ionisation energy = the energy needed to remove 1 electron from each atom in 1 mole of
gaseous atoms to form 1 mole of gaseous 1+ ions.
It is an endothermic process, and you have to put energy in to ionise an atom or molecule.
21) Factors affecting ionisation energy.
Nuclear Charge:
• The more protons there are in the nucleus, the more positively charged the nucleus is and
the stronger the attraction for the electrons.
Distance from Nucleus (Atomic Radius):
• Attraction falls off very rapidly with distance.
• An electron close to the nucleus will be much more strongly attracted than one further
away.
