Atomic Structure and the Periodic Table

Question 1

What is the mass of an electron?

Answer 1

1/1836 (the mass of a proton)

Question 2

What are isotopes?

Answer 2

Atoms of the same element with the same number of protons but different numbers of neutrons

Question 3

What is the mass number?

Answer 3

The sum of the number of protons and neutrons in the nucleus of an atom of that isotope

Question 4

Properties of isotopes

Answer 4

Chemical properties of isotopes are similar, but physical properties (e.g rate of diffusion, boiling point etc) may differ, especially if their relative atomic masses are very different

Question 5

What is relative isotopic mass?

Answer 5

The mass of an atom of an isotope on the scale on which an atom of the isotope C-12 weighs 12 units exactly

Question 6

What is relative atomic mass?

Answer 6

The weighted average mass of an atom of a element on the scale on which the isotope C-12 is taken to weigh 12 units exactly

Question 7

What is relative molecular mass?

Answer 7

The weighted average mass of a molecule of that element or compound on the scale on which the isotope C-12 is taken to weigh 12 units exactly

Question 8

Main steps of a mass spectrometer

Answer 8

1) Vaporise the sample (if it is not a gas) so it can move through the machine
2) The sample enters the ionisation chamber where it is ionised
3) Positive ions are then accelerated using an electric field of over 1000V and they may travel at up to 100,000 mph
4) Ions pass through a velocity selector, meaning the ions that pass into the rest of the spectrometer are all travelling at the same speed
5) Ions are passed into a magnetic field where they are deflected. The amount they are deflected depends on their charge and mass. The lighter and higher charged the ions, the more they will be deflected. Amount of deflection is proportional to the square root of charge/mass
6) Ions are detected. If the electric and magnetic field remain constant, then ions of 1 particular mass charge ratio will hit the ion detector (ions with a smaller mass charge ration will be deflected too much, and bigger ones too little.

Question 9

Why is the sample ionised? (mass spectrometer)

Answer 9

So it can be accelerated in the electric field and deflected in the magnetic field

Question 10

How is the sample ionised? (mass spectrometer)

Answer 10

Usually by the electron impact method - a stream of high energy electrons bombards the sample of vapour atoms/molecules to remove electron(s). Usually only 1 electron is removed

e^- + M - > M^+ + e^- + e^-
high energy electron + atom - > positive ion + electron knocked out of M + high energy electron retreating

2 electrons may also be knocked out of M

High energy electrons are produced by a heated filament

Question 11

How does the detector in a mass spectrometer work and how does it produce a trace?

Answer 11

Positive ions strike the detector and give a current. The detector is linked through an amplifier to a recorder and the current is measured. The magnetic field is slowly increased and ions of increasing mass are detected. A trace is printed out, giving the masses of the ions and their relative abundances (found from the height of the peaks)

Question 12

How do we know a mass spectrometer is accurate?

Answer 12

It is calibrated by using standard compounds with very accurately known relative atomic masses

Question 13

Why is the mass spectrometer apparatus maintained under high vacuum?

Answer 13

To prevent collisions of ions with air particles

Question 14

Fragmenting (mass spectrometer) (Butane as an example)

Answer 14

Butane has a molecular ion peak/parent ion peak at m/z = 58 - its Mr, but that is not the largest peak. The butane molecule has been fragmented, some of which have a positive charge and so produce a peak in the mass spectrometer. This happens because the beam of electrons used to generate the positive ions may break chemical bonds, causing positively charged fragments to be formed

Question 15

Notes about fragmentation (using butane as an example)

Answer 15

The most common and therefore most stable ion is at the highest peak and is known as the base peak (at m/z = 43 in Butane).
A small peak at m/z = 15 indicates a methyl group (CH3). This peak is smaller because of the lower stability of the ion. CH3 is a free radical, meaning it has unpaired electrons

Question 16

Uses of mass spectrometry

Answer 16

1) Detection of drugs in urine samples
2) In the pharmaceutical industry
3) Radioactive dating

Question 17

What is spectroscopy?

Answer 17

The study of radiation emitted or absorbed by bodies is called spectroscopy. A substance is excited by the absorption of energy and it emits energy as it returns to its normal or ground state

Question 18

Emission spectroscopy

Answer 18

Emission spectra are produced by excited bodies emitting energy and returning to their ground state, Gas atoms/ions produce line spectra, which is a series of lines of different wavelengths. Every element has a specific spectrum and so substances can be analysed to find what substances are present and at what concentration

Question 19

How are emission spectra produced?

Answer 19

1) It can be vaporised in a Bunsen burner flame. This happens in flame tests of alkali metals and alkaline earth metals in particular
2) Gases may be excited by applying a high voltage to gases at a low pressure in a discharge tube
3) If 2 metal rods in contact are connected to a DC supply and are then drawn a few mm apart then a continuous spark is obtained across the gap, called an electric arc. If a sample of solid is placed in the gap it vaporises and gives an emission spectrum

Question 20

What is first ionisation energy?

Answer 20

This is the energy required to remove one mole of electrons from one mole of free gaseous atoms if an element, giving 1 mole of unipositive ions

Ne (g) - e^- -> Ne^+ (g)

Question 21

What is second ionisation energy?

Answer 21

This is the energy required to remove 1 mole of electrons from 1 mole of free gaseous unipositive ions of an element, giving 1 mole of dipositive ions

Question 22

Why are logarithms of ionisation energies used on graph axis rather than just the ionisation energy?

Answer 22

There is a large range of values, and so plotting log values allows for a meaningful scale

Question 23

Which group of elements have the highest ionisation energies?

Answer 23

Noble gases

Question 24

Which group of elements have the lowest ionisation energies?

Answer 24

Alkali metals

Question 25

What happens to the ionisation energies as you go down the group?

Answer 25

Ionisation energy decreases. Going down the group, nuclear charge increases, but the distance between the outer electron and the nucleus increases as the number of shells increases.
There are also more shells of electrons between the nucleus and the outer electron, resulting in an increased shielding effect, which shield the outer electron from the effect of the nucleus, making it easier to escape. More complete shells = more shielding, so a lower ionisation energy

Going down the group, the increase in the nuclear charge is outweighed by the increased distance of the outer electron and increased shielding effect, so the 1st ionisation energy decreases

Question 26

What happens to the ionisation energy as you move across a period? (GENERALLY)

Answer 26

Going across the period, an extra electron is added to the outer shell and an extra proton to the nucleus. The shielding effect hardly changes, but the extra proton can pull the electrons in more closely and make it more difficult for an electron to be lost, therefore ionisation energy increases

Distance - slight decrease
Shielding effect - stays the same
Nuclear charge - stays the same

Question 27

What are the subshells?

Answer 27

1s, 2s, 2p, 3s, 3p, 3d,

Then continues: 4s, 4p, 4d, 4f etc, but don’t need to know

Question 28

What is the order of energy levels in the subshells?

Answer 28

1s, 2s, 2p, 3s, 3p, 4s, 3d, 4p, 4d,4f

Question 29

What is an orbital?

Answer 29

An orbital is a 3-dimensional volume of space surrounding the nucleus in which there is a high probability of finding an electron. Each orbital has a fixed energy level and can hold up to 2 electrons

Question 30

What is the Pauli Exclusion Principle?

Answer 30

Each electron in an atom must be distinguishable and therefore no two electrons can have the same values for all 4 quantum numbers. If 2 electrons are in the same orbital (have the same values of n, l and m) then they must have opposite spin.

Question 31

What is the Aufbau Principle / Hund’s rules of filling up orbitals?

Answer 31

1) You fill orbitals with the lowest value of n first - shells
2) You fill s orbitals before p orbitals before d orbitals before f orbitals - subshells
3) electrons occupy each m level singly before pairing takes place - orbitals

Question 32

What is the electronic structure of Cu (one of the exceptions to the rules of filling orbitals)?
Or anything with 29e-

Answer 32

1s2 2s2 2p6 3s2 3p6 4s1 3d10

Has extra stability of half filled subshell

Question 33

What is the electronic structure of Cr (one of the exceptions to the rules of filling orbitals)?
Or anything with 24e-?

Answer 33

1s2 2s2 2p6 3s2 3p6 4s1 3d5

Extra stability of half filled subshell

Question 34

Why is the first ionisation energy of beryllium more than that of boron? / Why does moving from group 3 - 4 cause a decrease in ionisation energy?

Answer 34

Be - 1s2 2s2
B - 1s2 2s2 2p1
The 2p electron is higher in energy than the 2s electron, and so less work is needed to remove the 2p electron, which outweighs the increase in nuclear charge

Question 35

Why is the first ionisation energy of nitrogen greater than that of oxygen? / Why does moving from group 5 - 6 cause a decrease in ionisation energy?

Answer 35

N - 1s2 2s2 2p3
O - 1s2 2s2 2p4
Both losing a 2p electron / one from same subshell, however, the paired electron in O is easier to remove due to repulsion between the 2 electrons in the same orbital, which outweighs the increase in nuclear charge

Question 36

What is first electron affinity? (enthalpy of electron attachment)

Answer 36

The enthalpy change when one mole of free gaseous atoms of an element gains one mole of electrons, forming one mole of uninegative ions

I (g) + e^- -> I^- (g)

Question 37

What is second electron affinity?

Answer 37

This is the enthalpy change when one mole of free gaseous uninegative ions of an element gains one mole of electrons, forming one mole of dinegative ions

I^- (g) + e^- -> I^2-

Question 38

Facts about first electron affinities

Answer 38

First electron affinities are negative for most elements, because the process of gaining an electron is exothermic and therefore favourable.
Theoretically, electron orbitals extend to infinity and so nuclear charge is never quite balanced by electronic charge, to restore the balance, the atom tends to gain an electron
Electron affinites become more exothermic across a period, but they vary irregularly like ionisation energies. The minima are in group VII (where electron gain completes a p subshell), group IV (where electron gain half completes the p subshell), and group I (where gain of an electron complete the s subshell)

Question 39

Why are second electron affinites positive?

Answer 39

There is repulsion between the negatively charged anion and the negatively charged, incoming electron

Question 40

If the second electron affinity of oxygen is positive, why does oxygen form solid oxides containing O^2- ions?

Answer 40

Energy is given out when ionic lattice is formed. Ions with higher charges have a greater attraction for each other and therefore a more exothermic lattice energy. The extra energy given out compensates for tje second ionisation energy

Question 41

s-block

Answer 41

Group 1 and 2. Called the s block because their outer electrons are in an s subshell.
Metals with lower densities, melting and boiling points than most other metals. Form stable, involatile, ionic compounds. High in the electrochemical series

Question 42

p-block

Answer 42

Groups 3- 8. Outer electrons are in a p subshell. Most are non-metals or semi-metals. Those at the bottom of the group are sometimes termed ‘poor metals’

Question 43

d-block

Answer 43

Block of metals in the centre of the periodic table. Less reactive than s-block metals and show horizontal and vertical likeness of each other. Contain between 1 and 10 electrons in d orbitals but no electons in (n+1)p orbitals. Most d-block elecments are transition elements and have properties like high melting and boiling points, variable valency, formation of coloured compounds, complex formation and catalytic action

Question 44

What is periodicity?

Answer 44

A pattern or trend in properties of the elements according to their position on the periodic table

Question 45

What is meant by a periodic property?

Answer 45

A property of the elements that changes in a regular way according to their position on the periodic table.