Physical 1: Atomic Structure

Question 1

What are the relative masses and charges of each fundamental particle?

Answer 1

Question 2

What are the symbols for atomic number and mass number?

Answer 2

Atomic number = Z
Mass number = A

Question 3

Define relative atomic mass.

Answer 3

The mean mass of an atom relative to 1/12th of the mass of one atom carbon-12.

Question 4

Define relative molecular mass.

Answer 4

The mean mass of a molecule relative to 1/12th of the mass of one atom of carbon-12.

Question 5

Define isoelectronic.

Answer 5

Atoms with the same number of electrons

Question 6

Define isotope.

Answer 6

Atoms with the same number of protons but different numbers of neutrons (same proton/atomic number, different mass number)

Question 7

How do isotopes of the same element react chemically?

Answer 7

They react the same because they have the same electron configuration

Question 8

How do isotopes relate to radioactivity?

Answer 8

Some isotopes are unstable, so the nucleus can break down, giving off energetic rays

Question 9

What is carbon-14 and what is it used for?

Answer 9

Radioactive isotope produced by cosmic-ray activity in the atmosphere
Used to carbon-date organic matter

Question 10

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

Answer 10

1) Ionisation
2) Acceleration
3) Ion drift
4) Detection

Question 11

Why does a TOF mass spectrometer need to be in a vacuum?

Answer 11

Because air particles would ionise and show up on the spectra

Question 12

Describe how electron impact ionisation works.

Answer 12

Used for low Mr compounds

Gaseous sample sent in at low pressure
Electron gun fires high-energy electrons at the sample
To knock off one outer electron, forming a 1+ ion
X(g) –> X+(g) + e-

Question 13

Describe how electrospray ionisation works.

Answer 13

Used for higher Mr compounds

Sample is dissolved into a volatile solvent
Sent through a thin, hypodermic needle
The tip of the needle has a high positive voltage
So the particles gain a proton

X(g) + H+ –> XH+(g)

Question 14

Why do you need to ionise samples before putting them in a mass spectrometer?

Answer 14

So that they will be accelerated by the electric field
So that they will generate a current when they reach the detector

Question 15

What happens during acceleration?

Answer 15

Positive ions are accelerated by an electric field to a constant kinetic energy
Towards a negative plate

Question 16

What happens during ion drift?

Answer 16

Positive ions with smaller m/z values will move faster because they’re lighter
But they each have the same kinetic energy

Question 17

What happens during detection?

Answer 17

When ions reach the detector, they generate a small current produced by electrons transferring from the detector to the ions
The size of the current is proportional to the abundance of ions

Question 18

What is Avogadro’s Constant?

Answer 18

6.022x10^23
This is used to describe how many atoms are in one mole of a substance

Question 19

How do you calculate mass of an atom, e.g. magnesium?

Answer 19

Mass number of magnesium = 24
This means that there are 24g per mole of atoms
24 divided by 6.022x10^23 = 3.99x10^23
So the mass of one atom is 3.99x10^23

Question 20

How do you calculate the speed of one ion?

Answer 20

v = Square root of (2 x kinetic energy / mass )

Question 21

How do you calculate time of flight?

Answer 21

Time = distance/velocity

Question 22

How do you calculate time of flight when comparing two isotopes?

Answer 22

m/t^2 = m/t^2

Question 23

What is Pauli’s principle?

Answer 23

No more than two electrons can occupy one orbital
They must have opposite spins

Question 24

What is the Aufbau principle?

Answer 24

Electrons must fill the lowest energy orbitals first then build up

Question 25

What is Hund’s rule?

Answer 25

Every orbital in a subshell must be filled singly before pairing them
All electrons in singly occupied orbitals must have the same spin

Question 26

Which electrons are removed first when transition metals become ions and why?

Answer 26

They lose the 4s electrons first, because the 4s orbital has a slightly higher energy than the 3d orbital

Question 27

What is Rutherford’s model of the atom?

Answer 27

Most of the mass of an atom is located in the central nucleus, along with all of its positive charge

Question 28

What is the current model of the atom?

Answer 28

The protons and neutrons are held in the centre of the atom by a strong nuclear force
Nucleus is surrounded by electrons found in shells

Question 29

How do you calculate the relative atomic mass of an atom given the m/z values and percentage abundance from a mass spectra?

Answer 29

Sum of m/z x percentage abundance / 100

Question 30

What is the ‘plum pudding’ model?

Answer 30

Suggested by J.J Thomson
Electrons located in circular arrays within a sphere of positive charge

Question 31

Why are positive ions smaller than atoms?

Answer 31

Because they have lost an outer electron, so the force of attraction between the outermost electron and the central positive nucleus increases
So the atomic radius of the ion decreases

Question 32

Define first ionisation energy.

Answer 32

The energy needed to remove one mole of electrons from one mole of atoms in their gaseous state to form one mole of gaseous 1+ ions
X(g) –> X+(g) + e-

Question 33

What three factors affect ionisation energy?

Answer 33

1) Atomic radius (smaller = more energy needed)
2) Number of protons (more = stronger charge = more energy needed)
3) Shielding (more shells = more shielding = less energy needed)

Question 34

Why does ionisation energy decrease down a group?

Answer 34

Because the atomic radius increases due to more electron shells
So there is a weaker force of attraction between the nucleus and the outermost electron due to shielding
So less energy is needed to remove it

Question 35

How does ionisation energy change across a period?

Answer 35

Number of protons increases, so atomic radius decreases due to greater force of attraction

Question 36

Why are there decreases in ionisation energy at group 3 and group 6?

Answer 36

Group 3:
There is an unpaired electron in the 3p orbital, which is a higher energy level than 3s so is easier to remove

Group 6:
The outer electron is paired in a p orbital
Electron pair repulsion makes it easier to remove one of them

Question 37

Define second ionisation energy.

Answer 37

The energy needed to remove one mole of electrons from one mole of gaseous 1+ ions to produce one moe of gaseous 2+ ions
X+ (g) –> X2+(g) + e-

Question 38

Why is there a general increase in successive ionisation energy?

Answer 38

There is an increasing effective charge on the nucleus as more electrons are removed

Question 39

What causes the jumps in successive ionisation energies?

Answer 39

The jumps occur when moving between energy levels
It takes much more energy to remove the outer electron from a lower energy level because there is a greater FOA between the outer electron and the nucleus due to decreased shielding

Question 40

What is the ground state of an atom?

Answer 40

Its most stable configuration
The lowest allowed energy state

Question 41

In a TOF mass spectrometer, ions are accelerated to the same kinetic energy.
KE = 1/2mv^2
KE = kinetic energy (J)
m = mass (kg)
v = speed (ms-1)
Each 84Kr+ ion is accelerated to a kinetic energy of 4.83x10-16, and the time of flight is 1.72x10-5s
Calculate the length of the flight tube in metres.

Answer 41

1. Calculate the mass of the ion
   84 divided by 6.022x1023 = 1.39x10-22 (grams)
2. Convert to kilograms
   1.39x10-22 divided by 1000 = 1.39 x 10-25
3. Calculate velocity
   v = Root (2 x KE / m)
   v = 83364.502
4. Calculate length of tube
   d = v x t
   d = 83364.502 x 1.72 x 10-5
   d = 1.43m

Question 42

Why does helium have a higher first ionisation energy than hydrogen?

Answer 42

Because helium has a higher nuclear charge, so the force of attraction between the nucleus and the outer electron is greater
So more energy is needed to remove the outer electron

Question 43

How would a TOF mass spectrometer separate two species produced in a reaction to give two separate peaks on a mass spectra?

Answer 43

Positive ions are accelerated by an electric field
To a constant kinetic energy
The positive ions with lower m/z value will have the same kinetic energy but higher speed as they are lighter
So will reach the detector first
And produce an individual peak
Then the higher m/z ions will reach the detector and produce their own peak