Atomic structure

Question 1

What is the relative charge of a proton

Answer 1

+1

Question 2

What is the relative charge of a neutron

Answer 2

0

Question 3

What is the relative charge of an electron

Answer 3

-1

Question 4

What is the relative mass of a proton

Answer 4

1

Question 5

What is the relative mass of a neutron

Answer 5

1

Question 6

What is the relative mass of an electron

Answer 6

1/2000

Question 7

What is an isotope

Answer 7

An element with the same number of protons but different number of neutrons

Question 8

State briefly the history of the atom

Answer 8

Dalton - Atoms are spheres
JJ Thompson - Plum pudding model
Rutherford - Gold leaf experiment (alpha particles fired at gold leaf some went through some deflected some reflected)
Bohr - Electrons fixed in energy shells (Experiment to prove this was when EM radiation was absorbed electrons moved between shells and they emit this radiation when electrons move down to lower energy shells)
Today - Electrons have subshells (they don’t have the same energy in shells)

Question 9

What are the 5 stages in TOF mass spectrometry

Answer 9

Vaporisation
Ionisation
Acceleration
Flight tube
Detection

Question 10

Explain how electron impact ionisation works in TOF mass spectrometry

Answer 10

*A vaporised sample is injected at low pressure
*An electron gun fires high energy electrons at the sample
*This knocks out an outer electron
*Forming positive ions with different charges

Question 11

What is the general equation for electron impact ionisation

Answer 11

X(g) —> X⁺(g) + e⁻

Question 12

Explain how electrospray ionisation works in TOF mass spectrometry

Answer 12

• The sample is dissolved in a volatile, polar solvent
• injected through a fine needle giving a fine mist or aerosol
• the tip of needle has high voltage (tip is attached to a high voltage power supply)
• At the tip of the needle the sample molecule, M, gains a proton, H+ from the solvent forming MH+
• The solvent evaporates away while the MH+
  ions move towards a negative plate

Question 13

When is electron impact ionisation more likely to be used over electrospray ionisation

Answer 13

Used for elements and substances with a low formula mass as can cause larger organic molecules to fragment

Question 14

When is electrospray ionisation more likely to be used over electron impact ionisation

Answer 14

Electro spray ionisation is used preferably for larger organic molecules. The ‘softer’ conditions of this technique mean fragmentation does not occur

Question 15

What is the general equation for electrospray ionisation

Answer 15

M(g) + H+ —> MH+(g)

Question 16

How are ions detected in TOF mass spectrometry

Answer 16

The ions reach the detector and generate a small current, which is fed to a computer for analysis. The current is
produced by electrons transferring from the detector to the positive ions. The size of the current is proportional to the
abundance of the species

Question 17

Describe the velocity of ions with different m/z values during the flight tube

Answer 17

The positive ions with smaller m/z values will have the same kinetic energy as those with larger m/z and will move
faster.

Question 18

What is the definition of relative atomic mass (Ar)

Answer 18

The average mass of an atom of an element when measured on a scale on which the mass of an atom of C12 is exactly 12

Question 19

What is the definition of relative molecular mass (Mr)

Answer 19

The average mass of a molecule when measured on a scale on which the mass of an atom of C12 is exactly 12

Question 20

What is the definition of relative isotopic mass

Answer 20

The mass of an atom of an isotope of an element measured on a scale on which the mass of an atom of C12 is exactly 12

Question 21

What is the equation for calculating relative atomic mass from abundance

Answer 21

(%abundance of A x m/z of A) + (%abundance of B x m/z of B)

Question 22

What is the M+1 or molecular ion peak

Answer 22

If a molecule is put through a mass spectrometer with an electron impact ionisation stage it will often break
up and give a series of peaks caused by the
fragments. The peak with the largest m/z will be due to the complete molecule and will be equal to the relative molecular mass , Mr ,of the molecule. This peak is called the parent ion or molecular ion

Question 23

Why does ionisation energy decrease down a group

Answer 23

Atomic radius increases, increase in shielding, therefore the electrostatic force of attraction between the nucleus and outer electron is weaker meaning the energy required to remove the outer electron decreases

Question 24

What is the trend in ionisation energy across a period

Answer 24

Generally increases due to an increase in protons with the same shielding leading to a greater electrostatic force of attraction between outer electron and nucleus

Question 25

State and explain the exceptions to the trend in ionisation energy across period 3

Answer 25

Al - The outer electron in Al sits in a higher energy 3p subshell which is slightly further from the nucleus and therefore has a lower first ionisation energy compared to Mg, whose outer electron sits in the 3s subshell

S - The outer electron in S sits in an orbital with 2 electrons in it. Thus, as opposite charges repel, less energy is required to remove an electron from an orbital with 2 electrons compared to an orbital with 1 electron such as in P

Question 26

What is the definition of first ionisation energy

Answer 26

The enthalpy change when one mole of gaseous
atoms forms one mole of gaseous ions with a single positive charge
or
Enthalpy change for
X(g) —> X+(g) + e-