3.1.1 - Atomic Structure

Question 1

What holds protons and neutrons close together in the nucleus of an atom?

Answer 1

The ‘strong nuclear force’

Question 2

What attracts electrons to the nucleus of an atom?

Answer 2

Electrostatic forces of attraction between negative electrons and positive protons

Question 3

Which is stronger, the force that holds protons and neutrons close together in the nucleus of an atom, or what attracts electrons to the nucleus of an atom?

Answer 3

The ‘strong nuclear force’ between protons and neutrons in the nucleus

Question 4

Why do protons not leave the nucleus towards the electrons?

Answer 4

The ‘strong nuclear force’ is stronger than the electrostatic forces of attraction between negative electrons and positive protons

Question 5

The force that holds protons and neutrons close together in the nucleus of an atom only works over ______ distances (e.g. ?)

Answer 5

The ‘strong nuclear force’ only works over short distances, e.g. within the nucleus

Question 6

What is the relative mass of an electron?

Answer 6

1/1840

Question 7

What is the relative mass of a proton and neutron?

Answer 7

1

Question 8

Define atomic number, Z

Answer 8

The number of protons in the nucleus of an atom

Question 9

Define mass number, A

Answer 9

The number of protons AND neutrons in the nucleus of an atom

Question 10

Define ‘isotope’

Answer 10

The same number of protons but different number of neutrons in the nucleus

Question 11

Do isotopes of an element react in the same way?

Answer 11

Yes (same electron configuration and number)

Question 12

Do isotopes of an element have the same mass numbers?

Answer 12

No - mass no. is number of protons AND neutrons in the nucleus of an atom, and no. of neutrons changes between isotopes

Question 13

Why do two paired electrons in the same orbital repel each other?

Answer 13

Due to opposing spins

Question 14

What does an s orbital look like?

Answer 14

(Spherical)

Question 15

What does a p orbital look like?

Answer 15

(Paired)

Question 16

What does a d orbital look like?

Answer 16

(Double)

Question 17

Rules of electron configurations? (x2)

Answer 17

1 - Build up from the lowest to highest energy levels

2 - Fill orbitals singly before pairing (as to avoid repulsion)

Question 18

Define ‘ionisation energy’

Answer 18

The amount of energy required to remove one electron from each atom of one mole of gaseous atoms, to form one mole of gaseous +1 ions

Question 19

Unit of ionisation energy?

Answer 19

kJ mol^-1

Question 20

Trend in ionisation energy across a period?

Answer 20

INCREASES

• Atomic radii decrease
• Same energy level/same shielding effectively
• Nuclear charge INCREASES

Question 21

Trend in ionisation energy down a group?

Answer 21

DECREASES

• Atomic radii increase
• More energy levels/greater shielding
• Nuclear charge stays same

Question 22

Why does the IE decrease between Mg and Al, going across period 3 from left to right?

Answer 22

Mg: 1s2, 2s2, 2p6, 3s2
-> Al: 1s2, 2s2, 2p6, 3s2, 3p1
So the electron from aluminium is removed from the 3p sub-shell, which has a higher energy level and thus requires less energy to remove

Question 23

Why does the IE decrease between phosphorous (P) and sulphur (S), going across period 3 from left to right?

Answer 23

P: 1s2, 2s2, 2p6, 3s2, 3p3
S: 1s2, 2s2, 2p6, 3s2, 3p4
In S, there is electron pairing in the 3p orbital, so the electrons have opposite spin and repel each other, thus the electron in S requires less energy to remove than that in P - despite increased nuclear charge

Question 24

What evidences the existence of p,s,d sub-shells?

Answer 24

The IE decrease between phosphorous (P) and sulphur (S), going across period 3 from left to right, due to electron pairing in sub-shells and repulsion

Question 25

What is mass spectrometry useful for?

Answer 25

Determination of an atom's relative atomic mass (Ar)/a molecule's relative molecular mass (Mr)

Question 26

Define Ar (relative atomic mass)

Answer 26

Ar = mean mass of 1 atom/ (1/12 mass of 1 atom of 12C)

Question 27

Define Mr (relative molecular mass)

Answer 27

Mr = mean mass of 1 molecule/ (1/12 mass of 1 atom of 12C)

Question 28

What are the five stages of time of flight mass spectrometry?

Answer 28

``` Ionisation Acceleration Deflection Drift Detection ```

Question 29

Why does ToF MS occur in a vacuum?

Answer 29

So charged particles do not collide with other molecules in the air (impacting flight time)

Question 30

What happens in acceleration in ToF MS?

Answer 30

All ions are accelerated by an electrical field to have equal kinetic energy. Ions are accelerated into a finely focused beam

Question 31

What happens in deflection in ToF MS?

Answer 31

Ions are deflected by a magnetic field, dependent on their m/z ratio

Question 32

Deflection in ToF MS - Lighter ion? - More charge?

Answer 32

- Lighter ion = greater deflection - More charge = greater deflection The higher m/z ratio, the greater the deflection

Question 33

If an ion has a mass of 56 and a charge of +2, what is its m/z ratio?

Answer 33

m = 56 z = 2 so 56/2 = 28 = m/z

Question 34

What happens at the ion detector?

Answer 34

Ion hits the detector and picks up an electron (becoming neutrally charged again), causing an electrical current to flow

Question 35

A small m/z leads to lots of deflection. How can you reduce this?

Answer 35

Use a smaller magnetic field

Question 36

A high m/z leads to little deflection. How can you increase this?

Answer 36

Use a larger magnetic field

Question 37

Using a mass spectrum, how do you calculate the Ar of an atom?

Answer 37

Ar = sum of abundance x mass/ total abundance

Question 38

What is the molecular ion peak and what is it equal to?

Answer 38

M+ formed when one electron is removed from the molecule. Equal to the Mr of the molecule

Question 39

What are the isotopes of chlorine and their ratio to each other?

Answer 39

Chlorine- 35 and chlorine- 37 | 3:1

Question 40

Why does the mass spectrum of a chlorine MOLECULE have three peaks?

Answer 40

3 possible types of the chlorine molecule can be formed: 35 + 35 = 70 (m/z) 35 + 37 = 72 37 + 37 = 74

Question 41

What is the abundance on the mass spectrum of each of the 3 possible types of the chlorine molecule that can be formed?

Answer 41

35 + 35 = 70 3/4 x 3/4 = 9/16 = 56% abundance 35 + 37 = 72 3/4 x 1/4 = 3/16 = 19% abundance 37 + 37 = 74 1/4 x 1/4 = 1/16 = 6.25% abundance

Question 42

What are the two types of ionisation in ToF MS?

Answer 42

Electrospray ionisation and electron impact ionisation

Question 43

Describe electron impact ionisation x4

Answer 43

- Sample is vaporised - High energy electrons are fired at it, from an ‘electron gun’ (a hot wire filament with a current running through it that emits electrons). - This knocks off one electron from each particle forming a 1+ ion. - The 1+ ions are then attracted towards a negative electric plate where they are accelerated.

Question 44

Describe electrospray ionisation x4

Answer 44

- Sample is dissolved in a volatile solvent (eg water or methanol) - Sample injected through a fine hypodermic needle to give a fine mist (aerosol). - The tip of the needle is attached to the positive terminal of a high-voltage power supply, so the particles are ionised by gaining a proton - The solvent evaporates away while the XH ions are attracted towards a negative plate where they are accelerated.

Question 45

General equation in electrospray ionisation?

Answer 45

X(g) + H+ -> XH+(g)

Question 46

General equation in electron impact ionisation?

Answer 46

X(g) -> X+(g) + e–

Question 47

When do we generally use electron impact ionisation?

Answer 47

For element/ substances with low formula mass

Question 48

When do we generally use electrospray ionisation?

Answer 48

Substances with higher molecular mass including many biological molecules such as proteins

Question 49

With which ionisation technique is fragmentation more likely to occur?

Answer 49

Electron impact ionisation

Question 50

Detection: the size of the current gives a measure of what?

Answer 50

The number of ions hitting the plate.