AS Chemistry unit 1

• Electrons are arranged in energy levels (1, 2, 3, 4 etc.), which are divided into sub-levels (s, p, d, f). 
• Each orbital can hold a max. of 2eˉ. When two electrons are placed in an orbital they must be spin paired.
• The lowest energy levels are filled first (the ones closest to the nucleus).
• The sub levels are always written in this order: 1s 2s 2p 3s 3p 3d 4s
• NOTE: the 3d sub-level is igher in energy than the 4s sub-level so the 4s sub-level is filled before the 3d,
• Groups 1 and 2: Outer electrons in an s sub-level - known as s block elements
• Groups 3 to 9: Outer electrons in a p sub-level - known as p block elements
• Transistion metals: d block elements

EXCEPTIONS:

• Chromium: 1s2 2s2 2p6 3s2 3p5 3d5 4s1 
• Copper: 1s2 2s2 2p6 3s2 3p5 3d10 4s1 

1. Vapourisation: the sample must be in a gaseous state. If it isn't, it must be ionised first

2. Ionisation: a metal wire is heated (the electron gun) until it emits a beam of high energy electrons. These knock electrons off the gaseous atoms/molecules, ionising them to 1+ ions

3. Acceleration: an electric field is created using negative plates, which attract the positive ions as they pass through the slit in the plate. This creates a beam of accelerated positive ions

4. Deflection: the positive ions are deflected using a magnetic field. The amount of deflection depends on the mass/charge ratio (m/z). Lighter/more charged particles deflect the most

5. Detection: as the ions land on the detection plate they create an electric current.  The larger the current, the greater the abundance of the isotope

Notes

• the minimum possible energy is used to ionise the sample otherwise 2+ ions are produced and fragmentation can occur 
• the peak with the highest m/z ratio is known as the molecular or parent ion
• if 2+ ions are produced the peaks are shifted to the left to half their original m/z value. Their relative abundance stays the same

Use of spectrum

• number of peaks = number of isotopes
• height of peaks = relative abundance of each isotope
• use of this information enables the relative atomic mass to be calculated

To calculate relative mass: multiply the m/z by the relative abundance for each isotope, add all the values up, and divide by the total abundance

(First) Ionisation Energy

The enthalpy change when 1 MOLE of elctrons is removed from 1 MOLE of gaseous atoms under standard conditions (298K and 100kPa) to form 1 MOLE of 1+ ions

Second Ionisation Energy

The enthalpy change with the removal of 1 mole of electrons from 1 mole of gaseous 1+ ions.

Trends

The trend in first ionisation energies across a period and down a group depend upon:

• number of filled energy levels (SHIELDING)
• number of outer shell electrons
• charge of nucleus (number of protons)

Across a period

1st I.E.'s increase because:

• nuclear charge is increasing and pulls in electrons
• electrons added to the same shell (no extra shielding)
• atom is smaller, outer electrons closer to nucleus, more strongly held
• therefore more energy is needed to remove electrons

Down a group

1st I.E.'s decrease because:

• although nuclear charge is increasing, whole shells of electrons are added
• more shielding
• atom is bigger, outer electrons are further away, less strongly held
• therefore less energy needed ti remove electrons

Li to Be - Rise in I.E. as eˉ added to same shell. No increase in shielding. Extra proton increases nuclear charge. Pulls in eˉ so harder to remove

Be to B - Drop in I.E. as sub orbital (2p) added. Outer eˉ further from nucleus. Full 3s orbital shields extra nuclear charge. eˉ easier to remove due to extra distance and shielding

B to N - Rise in I.E. (as for Li to Be)

N to O - Drop in I.E. as eˉ in 3p being paired. Paired eˉ repel, making it easier to remove

O to NE - Rise in I.E. (as for Li to Be)

Ne to Na - Large drop in I.E. New shell added. Eˉ much further away from nucleus. Extra nuclear charge shielded by full 2s2p shell

The trend in successive ionisation energies of an element shows the number of electrons in the outer shell and hence which group it belongs to. There is always a large jump in I.E. as you move from the outer shell to the next shell. 

 This is because the next shell is:

• closer to the nucleus
• less shielded than the outer shell was
• therefor a lot more energy is required to remove an Ionisation Energies  from this shell