Atomic structure

Question 1

How do you calculate relative atomic mass?

Answer 1

(mass number of isotope 1 x abundance) + (mass number of isotope 2 x abundance) / 100 (or total abundance, usually 100)

Question 2

How did John Dalton describe the atom?

Answer 2

Tiny solid spheres that couldn’t be divided

Question 3

How did JJ Thompson describe the atom?

Answer 3

The plum pudding model
A ball of positive charge with negatively charged electrons studded throughout

Question 4

How did Ernest Rutherford describe the atom?

Answer 4

The nuclear model
The atom is mostly empty space in which negative electrons surround the positive nucleus (where most of the mass is concentrated) in a ‘cloud’

Question 5

What was the alpha particle scattering experiment and what did it determine?

Answer 5

Alpha particles were fired at a thin sheet of gold foil - most of the particles passed through or were deflected from their path, proving that the atom is mostly empty space and has a nucleus where most of the mass is concentrated

Question 6

How did Neils Bohr describe the atom?

Answer 6

Bohr’s nuclear model
The electrons orbit the nucleus at a fixed distance in shells / energy levels

Question 7

What did James Chadwick discover?

Answer 7

The neutron - the nucleus is made up of both protons and neutrons

Question 8

What kind of samples is electron impact used for?

Answer 8

Samples of a smaller mass

Question 9

What kind of samples is electrospray used for?

Answer 9

Samples of a larger mass

Question 10

Why does a mass spectrometer use a vacuum pump?

Answer 10

To remove air from the apparatus so the ions don’t collide with air particles

Question 11

What is mass spectrometry used for?

Answer 11

To find the abundance and mass of each isotope (therefore determining relative atomic mass) and finding the relative molecular mass of substances

Question 12

What is the process of electron impact (also called electronic ionisation) ?

Answer 12

The sample is vaporised and bombarded by a stream of high energy electrons from an electron gun which can knock off an electron from an atom, producing a positively charged 1+ ion to be attracted towards the negatively charged plate

Question 13

What is the general formula of electron impact ionisation?

Answer 13

X(g) –> X+(g) + e-

Question 14

What is the process of electrosray ionisation?

Answer 14

The sample is dissolved in a volatile solvent (e.g. water / methanol) and injected through a fine hypodermic needle to give a fine mist (aerosol)
The tip of the needle is then attached to a high voltage power supply and the particles are ionised by gaining a proton (usually from a H+ ion) from the solvent as they leave the needle so they are then attracted towards the negatively charged plate

Question 15

What is the general equation for electrospray ionisation?

Answer 15

X(g) + H+ –> XH+(g)

Question 16

What are the four stages in a time of flight mass spectrometer?

Answer 16

Ionisation
Acceleration
Ion drift
Detection

Question 17

What happens during acceleration?

Answer 17

The positive ions are accelerated using an electric field strength so that they all have the same kinetic energy

Question 18

What does the velocity of each particle depend on?

Answer 18

It’s mass because all the particles have the same kinetic energy; lighter particles have a faster velocity, heavier particles have a slower velocity

Question 19

What happens during ion drift?

Answer 19

The ions travel down a long tube at varying velocity due to their masses

Question 20

What happens during detection?

Answer 20

The ions are distinguished by the time it takes for them to hit the detector

Question 21

How is the mass (and therefore Mr) of the ions determined by the detector?

Answer 21

When the positive ions are discharged by gaining electrons from the plate, it generates an electric current that is measured; the more ions that hit the plate the bigger the current
A mass to charge ratio (m/z) is detected, but given the charge of each ion is +1, the m/z is effectively the mass of each ion

Question 22

Why might some other higher peaks be present on the mass spectrometer?

Answer 22

Due to the presence of ²H or ¹³C atoms

Question 23

Why is it necessary to ionise the sample?

Answer 23

• To allow particles to be accelerated
• To generate a current (electrical impulse) in the detector

Question 24

What is an electron orbital?

Answer 24

A region of space in which the electron spends most of its time

Question 25

What does each orbital hold?

Answer 25

It can hold up to 2 electrons with opposite spins (one going clockwise and the other anticlockwise)

Question 26

What is the Aufbau principle?

Answer 26

The idea that electrons enter the lowest energy orbital available first

Question 27

What is the order of all of the sub levels?

Answer 27

1s² 2s² 2p⁶ 3s² 3p⁶ 4s² 3d¹⁰ 4p⁶

Question 28

What are the two exceptions of the expected pattern of electron sub level arrangements and why?

Answer 28

Cr - 1s² 2s² 2p⁶ 3s² 3p⁶ 4s¹ 3d⁵ (because then the 3d level is half full which is a more stable arrangement)
Cu - 1s² 2s² 2p⁶ 3s² 3p⁶ 4s¹ 3d¹⁰ (because then the 3d level is full which is a more stable arrangement)

Question 29

What is 1st ionisation energy?

Answer 29

The energy required to remove one electron from each atom in a mole of gaseous atoms producing one mole of 1+ gaseous ions

Question 30

How does atomic radius affect ionisation energy?

Answer 30

The smaller the radius, the stronger the attraction between the nucleus and the electron

Question 31

How does the number of protons affect ionisation energy?

Answer 31

The more protons, the stronger the attraction between the nucleus and the electron

Question 32

What is shielding?

Answer 32

The repulsion by electrons in shells between the outer electron and the nucleus

Question 33

How does shielding affect ionisation energy?

Answer 33

The more shielding, the weaker attraction between the nucleus and the electron

Question 34

What is the trend of ionisation energy down a group?

Answer 34

Ionisation energy decreases as you go down the group because:
- there is going to be more shielding
- and the atomic radius increases
therefore weaker attraction between the nucleus and electron

Question 35

What are the trends in ionisation energy down a group evidence for?

Answer 35

The electrons of atoms being organised in energy levels / shells

Question 36

What is the trend in ionisation energy across a period?

Answer 36

Across a period, the ionisation energy increases because the nuclear charge (number of protons) increases

Question 37

What are the two blips in the trend across period 3 and why?

Answer 37

Al - the P sub level is higher in energy so the electron is more easily lost
S - The electrons in the 3p sub level are in a pair and repel each other so the electron is more easily lost

Question 38

What is the variation across period 3 regarded as evidence for?

Answer 38

The existence of atomic sub levels