Atomic Structure

Question 1

Relative Atomic Mass (Ar)

Answer 1

The average mass of an atom of an element on a scale where an atom of carbon-12 is 12

Question 2

Relative Isotopic Mass

Answer 2

The mass of an atom of an isotope of an element on a scale where an atom of carbon-12 is 12

Question 3

Relative Molecular Mass (Mr)

Answer 3

The average mass of a molecule on a scale where an atom of carbon-12 is 12

Question 4

Electrospray Ionisation

Answer 4

The sample is dissolved and pushed through a small nozzle at high pressure.
A high voltage is applied to it, causing each particle to gain an H+ ion.
The sample is turned into a gas made up of positive ions

Question 5

Electron Impact Ionisation

Answer 5

The sample is vaporised and an electron gun is used to fire high energy electrons at it.
This knocks one electron off each particle, so they become +1 ions

Question 6

Describe acceleration of TOFMS

Answer 6

The positively charged ions are accelerated by an electric field so that they all have the same kinetic energy.
This means that the lighter ions will end up moving faster than the heavier ions

Question 7

Describe ion drift of TOFMS

Answer 7

The ions enter a region with no electric field, so they just drift through it.
Lighter ions will drift through faster than heavier ions

Question 8

Describe detection of TOFMS

Answer 8

Because lighter ions travel at higher speeds in the drift region, they reach the detector in less time than heavier ions.
The detectors used in mass spectrometers detect charged particles.
An electrical current is produced in the detector when a charged particle hits it, and this produces a mass spectrometer

Question 9

Relative Formula Mass

How to calculate

Answer 9

Used instead for compounds that are ionic or giant covalent.

To find, just add up the relative atomic masses (Ar) of all the atoms in the formula unit

Question 10

First Ionisation Energy

Answer 10

The energy needed to remove 1 electron from each atom in 1 mole of gaseous atoms to form 1 mole of gaseous 1+ ions

Question 11

Nuclear charge and ionisation energy

Answer 11

The more protons there are in the nucleus, the more positively charged the nucleus is and the stronger the attraction for the electrons

Question 12

Distance from the nucleus and ionisation energy

Answer 12

Attraction falls off very rapidly with distance.

An electron close to the nucleus will be much more strongly attracted than one further away

Question 13

Shielding and ionisation energy

Answer 13

As the number of electrons between the outer electrons and the nucleus increases, the outer electrons feel less attraction towards the nuclear charge.

This lessening of the pull of the nucleus by inner shells of electrons is called shielding (or screening)

Question 14

Second Ionisation Energy

Answer 14

The energy needed to remove 1 electron from each ion in 1 mole of gaseous 1+ ions to form 1 mole of gaseous 2+ ions

Question 15

High Ionisation Energy

Answer 15

Means there’s a high attraction between the electron and the nucleus.

Therefore more energy is needed to remove the electron

Question 16

Trend in Ionisation Energy down Group 2

Explain why

Answer 16

Ionisation energy decreases down group 2

Each element down group 2 has an extra electron shell compared to the one above.

The extra inner shells will shield the outer electrons from the attraction of the nucleus.

The extra shell means that the outer electrons are further away from the nucleus, so the nucleus’s attraction will be greatly reduced.

Both of these factors makes it easier to remove outer electrons resulting in lower ionisation energy

Question 17

Trend in Ionisation Energy across Period (3)

Explain why

Answer 17

Ionisation energy increases across a period

The number of protons is increasing across a period, which means a stronger nuclear attraction.

All the extra electrons are at roughly the same energy level, even if the outer electrons are in different orbital types.

This means there’s generally little extra shielding effect or extra distance to lessen the attraction from the nucleus

Question 18

Explain why aluminium has a lower ionisation than magnesium?

Where is this drop in the periodic table?

Answer 18

Aluminium’s outer electron is in a 3p orbital rather than a 3s.

The 3p orbital has a slightly higher energy than the 3s orbital, so the electron, is on average, to be found further from the nucleus.

The 3p orbital has additional shielding provided by the 3s2 electrons.

Both these factors together are strong enough to override the effect of increased nuclear charge, resulting in the ionisation energy dropping slightly

Between groups 2 and 3, in period 3 (showing sub-shell structure)

Question 19

Explain why sulfur has a lower ionisation energy than phosphorous?

Where is this drop in the periodic table

Answer 19

The shielding is identical in the phosphorous and sulfur atoms, and the electron is being removed from an identical orbital.

In phosphorous, the electron is being removed from a singly-occupied orbital.

But in sulfur, the electron is being removed from an orbital containing two electrons.

The repulsion between two electrons in an orbital means that electrons are easier to remove from shared orbitals.

Between groups 5 and 6, in period 3 (evidence also for the electronic structure model)