3.1.1 Atomic Structure

Question 1

what is first ionisation energy?

Answer 1

energy needed to remove one mole of electrons from one mole of atoms in their gaseous states

Question 2

what is second ionisation energy?

Answer 2

energy required to remove one mole of electrons from one mole of +1 ions in their gaseous states

Question 3

what are factors that affect ionisation energy?

Answer 3

1. atomic radius
2. no. of protons
3. shielding

Question 4

what is atomic radius?

Answer 4

distance between nucleus and outermost electrons

Question 5

how does atomic radius affect ionisation energy?

Answer 5

as the atomic radius increases, the force of attraction between nucleus and outermost electrons decreases, so first ionisation energy decreases

Question 6

how does no. of protons affect ionisation energy?

Answer 6

greater no. of protons = greater force of attraction between outer electrons and the nucleus therefore higher ionisation energy

Question 7

how does shielding affect ionisation energy?

Answer 7

higher shielding = electrons in outer shells are repelled by electrons in inner shells which reduces attraction between outer electrons and the nucleus so lower ionisation energy

Question 8

what causes gradual increase in ionisation energy in a graph?

Answer 8

• one outer electron removed
• rest of outer electrons pulled slightly closer to nucleus
• slightly greater attraction between outer electrons and nucleus
• ionisation energy gradually increases

Question 9

what causes a rapid sudden increase in ionisation energy in a graph?

Answer 9

• all outer electrons have been removed
• no more outer shell
• first shell has much less shielding and less atomic radius so force of attraction between electrons and nucleus increases
• ionisation energy increases

Question 10

what is the trend of first ionisation energy down a group?

Answer 10

as you go down a group, the first ionisation energy decreases
• atomic radius increases
• more shielding

Question 11

what is the trend of the first ionisation energy across a period?

Answer 11

as you go across a period the first ionisation energy generally increases
• no. of protons increase
• atomic radius decreases
• shielding stays the same

BUT there are exceptions :
boron (B)
oxygen (O)

Question 12

why are oxygen and boron exceptions to the trend of first ionisation energy across a period?

Answer 12

oxygen —
[¡!] [¡ ] [¡ ] —> electronic configuration
(pretend ! is an arrow)

it has a pair of electrons in first subshell which creates higher shielding, less ionisation energy needed compared to nitrogen

boron:
electronic configuration = 2s(2) 2p(1)

2p subshell has higher energy than 2s subshell which takes less energy to remove outer electron of boron, so ionisation energy decreases

Question 13

what is relative atomic mass?

Answer 13

average mass of an atom of an element compared to 1/12 mass of one atom of carbon 12

Question 14

what is relative molecular mass?

Answer 14

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

Question 15

what is relative isotopic mass?

Answer 15

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

Question 16

what happens in a time of flight (TOF) spectrometer?

Answer 16

1. atoms go through ionisation and become positive ions
2. positive ions are now attracted to negatively charged plate
3. negative charge causes ions to accelerate, which increases their KE
4. ions stop accelerating after negative plate
5. ions drift down chamber towards detector
6. ions reach the detector and ions gain an electron which causes a current to flow

Question 17

why are atoms ionised in TOF?

Answer 17

• to accelerate them
• to deflect the beam

Question 18

why are the atomic radius of different isotope of the same element the same?

Answer 18

each isotope have the same number of protons and same number of electrons

Question 19

how is the current generated at the detector in a mass spectrometer?

Answer 19

when electrons are transferred to the ion

Question 20

how is abundance measured in TOF spectrometer?

Answer 20

• by the current that flows after ions being detected
• current depends on no. of ions hitting detector

Question 21

why is TOF done in a vacuum?

Answer 21

molecules of air would cause beam of ions to collide with them

Question 22

describe electrospray induction

Answer 22

• H+ ions added to each molecule
• sample is dissolved and high voltage applied to it

Question 23

how does electron impact ionisation work

Answer 23

• sample is bombarded with high energy electrons (electron gun)
• sample loses an electron to become + ion

Question 24

how do ions accelerate in TOF?

Answer 24

by an electric field

Question 25

how can TOF separate two species?

Answer 25

- positive **ions accelerated** by **electric field** - to a **constant KE** - ions with **lower m/z** have **same KE as higher m/z** but **move faster** - **lower m/z arrives** at detector **first**