UNIT 1: SECTION 1 - ATOMIC STRUCTURE

Question 1

Define Relative Isotopic Mass

Answer 1

Mass of an atom of an isotope compared to 1/12th of the mass of C-12.

Question 2

Define Relative Atomic Mass (Ar)

Answer 2

Mass of an average atom compared to 1/12th of the mass of C-12.

Question 3

Define Relative Molecular Mass (Mr)

Answer 3

Average mass of a molecule on a scale where an atom of carbon-12 is 12.

Question 4

3 uses of Mass Spectrometry?

Answer 4

• Provides structural information.
• Identification of unknown compounds.
• Can determine relative abundance of each isotope in an element.

Question 5

Why is a TOF Spectrometer under vacuum?

Answer 5

It prevents the ions from colliding with air.

Question 6

Two types of ionisation and how they occur?

Answer 6

1) Electrospray Ionisation:
Sample dissolved in volatile solvent (H2O).
Injected through hypodermic needle and high + voltage applied.
Sample vaporises and left with +ly charged ions.

2) Electron Impact:
Electron gun shoots beam of high charged electrons at sample; electron knocked off sample atoms; left with positive ions and fractionation may occur.

Question 7

How does TOF MS work?

Answer 7

1) Ionisation

2) Acceleration: +ions -> -ly charged plate.
All ions same Ek (mass dependent now)

3) Ion drift: pass through hole in -ly charged plate; forms ion beam and travels through flight tube.
4) Detection: Arrive at detector, +ions pick up electron and cause current to flow. Flight times recorded.

Question 8

Formula to calculate Relative Atomic Mass?

Answer 8

(relative abundance x atomic number)n /

sum of relative abundance .

Question 9

Why do sub-shells exist in shells?

Answer 9

Because not all electrons in a shell have the same energy.

Question 10

What does the principle quantum number refer to?

Answer 10

Energy level of an electron;

Further the shell is, larger the PQN.

Question 11

How do Chromium and Copper differentiate from the general electronic structure?

Answer 11

Chromium and copper both donate an electron from the 4s shell to the 3d shell.

Question 12

Define 1st ionisation energy.

Answer 12

Energy required to remove an electron from each atom in 1 mole of gaseous atoms to form 1 mole of +1 gaseous ions.

Question 13

True or false:

Ionisation energy is always an endothermic process.

Answer 13

TRUE

Question 14

What factors affect ionisation energy?

3

Answer 14

• nuclear charge: more protons, stronger attraction to outer electrons.
• distance from nucleus: closer, stronger attraction
• shielding: shells between outer electron and nucleus lessen attraction.

Question 15

What do successive ionisation energies show?

Answer 15

They show electronic structure;

1) Ei increases successively - electron closer to nucleus, less shielding, increasingly positive ion.
2) Bigger jumps in Ei - new shell, big jump closer to nucleus.

Question 16

How to calculate which group unknown ion is in from the graph?

Answer 16

Count the number of electrons removed before the first big jump in Ei.

Question 17

Two common trends in Ei?

Answer 17

Down a group, Ei decreases.

Across a period, Ei increases.

Question 18

How does Ei decrease down G2?

Answer 18

Each element down group 2 has an extra shell -
more shielding,
distance from nucleus increases,
EASIER TO REMOVE!

Question 19

How does Ei increase across P3?

Answer 19

More protons -
Stronger nuclear attraction,
Extra shielding (slight)

Question 20

What are the exceptions to the trend in P3?

Answer 20

Aluminium - Ei drops;

• Outer electron in 3p not 3s.
• 3p higher up than 3s,
• extra shielding and further away from nucleus.
  
  • requires less energy to remove.

Sulfur - Ei drops; 
NOT BECAUSE OF SHIELDING!
- Electron removed from occupied orbital,
- repulsion between 2 electrons,
   - easier to remove. 

^Sulfur’s ionisation abnormality is evidence for sub-shells.