AQA AS Chemistry Revision Notes 2: Periodicity & Mass Spectrometry

The following are the typical questions that you'll find in many of the past papers. Each question also comes along with the generic answer that the examiners are looking for, so if you learn these then you should be able to answer these questions pretty easily when they come up. If you think that I've missed out any important questions/answers or you'd like to correct any mistakes then feel free to mention them in the comments and I'll make adjustments as soon as possible.

Typical structure: Question (Marks Available) [Paper]: Answer. Extra notes/comments.

NB: Underlined words/phrases are the key terms needed to get the marks. Different points are highlighted in different colours.

AQA AS CHEMISTRY REVISION NOTES 2: PERIODICITY

Which atom in Period 2 (C to Fl) has the largest atomic radius, and why? (3) [May 2013]

Carbon. Has the lowest nuclear charge/weakest nuclear attraction to electrons, but has similar/same shielding.
 
Similarly,

Why does the atomic radius decrease as you go across the Period 2 elements? (2)

Nuclear charge increases/stronger attraction between the outer shell and nucleus; similar/same shielding.

The main points to remember are that the atomic radius decreases as you go from left to right, the nuclear charge always increases, causing the attraction between the nucleus and outer electron shell to increase, and the shielding (no of shells between the outer shell and nucleus) is similar/the same.

What's the general trend in first ionisation energies from carbon to neon? Which element deviates from this trend and why? (4) [May 2013]

Increase. Oxygen. Paired electrons. Paired electrons repel.

Not as complicated as it seems, eh? Just remember that from group 3 to 8/0, the ionisation energies increases as the nuclear charge increases, which means the outer electrons become more attracted to the nucleus. But at Group 6, the 4th electron in the p-orbital pairs with the 1st electron in said sub-shell, and as they're both negative, they repel each other, making it easier to ionise.

Why is the second ionisation energy of carbon higher than the first ionisation energy? (1)

More energy is required to remove the electron from the positive ion.

Remember, an electron has already been removed, meaning that there is now more protons than electrons, making it a positive ion.

Why does lithium have the highest second ionisation energy in Period 2? (1)

Electron removed from the 1p orbital.

The electron removed is closer to the nucleus, as it's in a different shell, so it requires more energy to remove.

Why is more energy required to remove an electron from Al³⁺ than an Na⁺ ion? (3)

More protons/stronger nuclear charge; Al³⁺ is smaller than Na⁺/electrons are close to the nucleus; electron gets more attraction/pulled more strongly by Al³⁺.


AQA AS CHEMISTRY REVISION NOTES 2.1: MASS SPECTROMETRY

How are the atoms ionised in a mass spectrometer? (2)

Electron gun fires high speed electrons; which knock off electrons from the gaseous atom.

How is current detected at the detector? (2)

Electrons are transferred from the detector to the (+) ion.

How is the relative abundance of an ion measured? (2)

Ions hit detector and accept electrons/cause an electron flow; a bigger current means more of that isotope/current is proportional to abundance.

Why do we only use the minimum amount of energy required to ionise the sample? (1)

So only one electron gets knocked out.

Whenever you talk about the atoms/substance/sample, remember that it's always been vapourised, and is therefore in a gaseous form, and never forget to put the state symbol when you're writing an equation eg. C_(g), Na_(g), O_(g) etc.


AKBIT-HOE, 23:45 15/05/2015