Topic 1 - Atomic Structure

Question 1

Define the term isotope

Answer 1

An atom of an element with the same number of protons but a different number of neutrons

Question 2

Define relative atomic mass

Answer 2

The weighted mean mass of an atom of an element, compared to 1/12th the mass of an atom of carbon 12

Question 3

Define relative isotopic mass

Answer 3

The mass of an isotope, compared to 1/12th of the mass of an atom of carbon 12

Question 4

What is the formula for relative atomic mass?

Answer 4

(m/z A x abundance of A) + (m/z B x abundance of B) divided by total abundance

Question 5

Why do arrows showing electrons in an orbital point in opposite directions?

Answer 5

Spin pairing. When 2 electrons occupy the same orbital, they must have opposite ‘spin’

Question 6

Why do electrons fill orbitals in the same sub-shell singly first and then pair up?

Answer 6

To minimise electron repulsion (Hund’s rule)

Question 7

Why does vanadium (4s2 3d3) form a 2+ ion with electron configuration 3d3 rather than 4s2 3d1?

Answer 7

Lose from the 4s orbital first then from 3d as 4s has a higher energy when filled but lower energy when empty

Question 8

Define atomic (proton) number

Answer 8

The number of protons in the nucleus

Question 9

Define first ionisation energy

Answer 9

The energy required to remove one mole of electrons from one mole of gaseous atoms to form one mole of gaseous positive ions

Question 10

Define mass number

Answer 10

The sum of the protons and neutrons in the nucleus of an atom

Question 11

Define an orbital

Answer 11

The region of space around a nucleus in which there is a high probability of finding an electron. Each orbital contains up to two electrons.

Question 12

Define periodicity

Answer 12

A repeating trend or pattern of properties shown across different periods of the Periodic Table

Question 13

What is m/z?

Answer 13

The mass to charge ratio of an isotope/ fragment

Question 14

What is the M+ peak in mass spectrometry? Where is it found?

Answer 14

It is the molecular ion peak which tell you the Mr of the whole compound. It is the peak furthest right with the highest m/z value.

Question 15

Why does a Cl2 molecule have 3 molecular ion peaks?

Answer 15

Cl exists as two isotopes Cl-35 and Cl-37.
A Cl2 molecule could contain
- 2x Cl-35 (m/z = 70)
- 2x Cl-37 (m/z = 72)
- 1 of each isotope Cl-35 and Cl-37 (m/z = 71)
so there are 3 possible combinations, hence 3 peaks

Question 16

Write the equation for the 3rd ionisation energy of an atom

Answer 16

X^2+ (g) –> X^3+ (g) + e-

Question 17

Why do first ionisation energies increase across a period?

Answer 17

• nuclear charge increases
• electrons are added to the same shell so shielding stays about the same
• attraction between nucleus and valance electrons increases so more energy required to remove a valance electron

Question 18

Why do first ionisation energies decrease down a group?

Answer 18

down the group
- distance between the nucleus and valance shell electrons increases as they are added to a new shell each time
- shielding increases because there are now more inner shells
- although nuclear charge increases, this is outweighed by the increased shielding and distance so less energy is required to remove a valance electron

Question 19

Why is the first ionisation energy of Magnesium higher than that of Aluminium?

Answer 19

Mg valance electron is in the 3s orbital, Al valance electron is in the 3p orbital.
3p have a higher energy than 3s and so require less energy to remove. They are also shielded by the 3s electrons reducing the force of attraction from the nucleus so easier to remove.

Question 20

Why is there a drop in 1st IE between nitrogen and oxygen?

Answer 20

N 1s2 2s2 2p3
O 1s2 2s2 3p4

In oxygen the electron being removed is from an orbital containing a pair of electrons. The repulsion between the two electrons makes it easier to remove