Chemistry - 3.1.1: Atomic Structure

5 Stages of the development of the model of the atom

1. John Dalton: all elements composed of indivisible atoms
2. JJ Thomson's Plum Pudding Model: atoms are balls of positive charge, with negative charges (electrons) embedded within them.
3. Ernest Rutherford's Alpha Particle Scattering experiment: fired alpha particles at thin gold sheet, found that mass of atom is concentrated in nucleus, nucleus is positively charged and most of an atom is empty space.
4. Niels Bohr: electrons orbit the nucleus on shells of fixed size. The movement of electrons from one shell to the next explained how atoms absorbed and gave out light.
5. James Chadwick: neutrons are subatomic particles in the nucleus that have no overall charge.

In a mass spectrometer, what ion deflects the most?

The one with the lowest mass

Nucleons

The subatomic particles found in the nucleus of an atom - protons and neutrons.

Relative mass of 3 subatomic particles

P: 1
E: 0 / negligible
N: 1

Relative charge of 3 subatomic particles

P: +1
E: -1
N: 0

Mass number and symbol

The sum of the number of neutrons and protons in an atom (symbol: A)

Atomic number and symbol

The number of protons in an atom (symbol: Z)

Isotopes

Atoms with the same number of protons (of the same element) with different numbers of neutrons

Do different isotopes of the same element react in the same way chemically and why?

Yes - they have the same electronic configuration and atomic number.

How are protons and neutrons held together in the centre of the atom?

By a force called the strong nuclear force

What determines the chemical properties of an element?

The number of electrons in the outer shell

Formula for the number of electrons on each shell

2n^2 (where n = the number of the shell)

When can atoms fill the next shell?

Once they have filled their current shell

Relative atomic mass

The mean mass of an atom of an element compared to one twelfth of the mean mass of an atom of C-12.

Ions

A charged particle that forms after an atom loses or gains electrons.

How does the mass number of an atom affect its properties?

It affects its physical properties.

5 stages of TOF Mass Spectrometry

1. Ionisation: electrospray OR electron impact
2. Acceleration: the ions are accelerated to have the same kinetic energy due to their attraction to the negatively charged plate in the electric field.
3. Ion Drift: the ions travel to the end of the chamber through a hole in the negatively charged plate into a tube, forming a beam - the flight tube. there is no electric field - the ions do not deflect. lighter ions drift faster.
4. Detection: when the ions collide with the negatively charged plate/detector, they gain an electron. the greater the abundance of each isotope, the greater the current generated.
5. Analysis: the flight times + current values are used to produce mass spectrum

What does the radius of the curve of an ion (TOF MS) depend on?

Its charge and its mass

What can you learn from a mass spectra?

Isotopes present in sample (their mass) and their abundance

Why do you need to ionise each sample (TOF MS)? (2)

1. So the ions are attracted to the negatively charged plate and accelerate towards it.
2. So the ions can be detected by gaining an electron once they reach the plate.

If a 2+ ion forms, what's the effect on the ion's radius and m/z?

Radius will be smaller (due to higher charge) and will be half m/z of 1+ ion.

Why is the apparatus (in TOF MS) kept in a high vacuum?

To stop the ions from colliding with molecules from the air.

What is the effect of an ion's mass on its speed (TOF MS)?

The lighter the mass, the faster its speed

Why is a volatile solvent used (TOF MS)?

So that it evaporates and only leaves the sample's droplets

Formula for electron impact ionisation (TOF MS)

X(g) + e- -> X+(g) + 2e-

What happens during electrospray ionisation?

In vacuum, sample is dissolved in highly volatile solvent, vaporised then injected into chamber using a needle connected to positive terminal of a high voltage power supply. it gains a proton (H+ ion) to form a +1 ion.

What happens during electron impact ionisation?

Sample is vaporised. 'electron gun' (hot wire filament with charge running through it) is used to fire high energy electrons, knocking one off the sample to form a +1 ion.

How does the type of ionisation used impact the results of the mass spectra?

If electron impact used, no impact on mass - electron knocked off.
If electrospray used, mass increased by 1 - proton (H+) gained.

On a mass spectra, what does each peak represent?

A particle of a different mass

Why are there smaller peaks after the main peak (M+1 peak) on a mass spectra?

There is a molecule containing one 13C or one 2H.

What are the causes of an M+2 peak on a mass spectra?

The main molecular ion is chlorine (if M+2 peak is a third of main peak height - 3:1 ratio) or bromine (if M+2 peak is as tall as the main peak - both same abundance).

First ionisation energy

The energy required to remove one mole of electrons from one mole of gaseous atoms to form 1+ gaseous ions.

3 factors that affect ionisation energy and how?

1. Atomic radius (as it increases, ionisation energy decreases due to weaker electrostatic forces of attraction)
2. Charge (as it increases, ionisation energy increases due to stronger electrostatic forces of attraction)
3. Shielding (as it increases, ionisation energy decreases due to weaker electrostatic forces of attraction)

First ionisation energy equation

X (g) -> X+ (g) + e-

Second ionisation energy equation

X+ (g) -> X2+ (g) + e-

4s and 3d subshells - which one fills and empties first?

4s fills first
3d empties first

Why do ionisation energies decrease as you go down a group (group 2)?

- Atomic radius and shielding increase
- Electrostatic forces of attraction weaken
- Less energy is needed to remove one electron

Trend in ionisation energies across period 3

Generally increases due to increase in nuclear charge and decrease in atomic radii, all in same shell so similar shielding
Dips between group 2 and group 3 + group 5 and group 6

4 sub-levels and the number of electrons they can hold

s - 2
p - 6
d - 10
f - 14

Atomic orbital

a region of space in which there is a high probability of finding an electron

In energy level diagrams, why are the electrons represented by arrows pointing in opposite directions?

Two electrons in the same orbital have opposite spins

3 rules of allocating electrons to atomic orbitals

- Lower energy atomic orbitals fill first (Aufbau principle)
- Atomic orbitals of the same energy fill singly before pairing due to repulsion (Hund's rules)
- No atomic orbital can hold more than two electrons (Pauli Exclusion principle)

Unit of ionisation energy

kJ mol-1

Why does removing an electron from a 1+ ion have a higher IE than removing an electron from an atom?

- When removing first electron, it is being removed from a neutral atom that's further away from the nucleus. - When removing second electron, it is being removed from a positive ion and is closer to the nucleus (stronger electrostatic forces of attraction)

2 exceptions to electron configuration rules and why?

1. Copper: [Ar] 3d10 4s1 not [Ar] 3d9 4s2 (3d fills first)
2. Chromium: [Ar] 3d5 4s1 not [Ar] 3d4 4s2
- both 4s1. Why?: These configurations are energetically stable - a half / fully-filled d sublevel results in lower energy.

What is special about the ions from Sc to Zn?

They lose electrons from the 4s sublevel first, rather than the 3d.

Why is there a dip in first ionisation energies between group 2 and 3 on period 3?

- In Al, valence electron is on 3p sublevel - higher energy orbital; further away from nucleus than valence electrons on 3s sublevel in Mg + experience more shielding.
- Weaker electrostatic forces of attraction between the nucleus and the valence electron in Al so less energy is needed to lose.

Why is there a dip in first ionisation energies between group 5 and 6 on period 3?

- In 3p sublevel of S, there is a pair of electrons (whereas in P, they are all unpaired).
- The paired electrons repel each other more, so less energy is needed to lose one.

Successive ionisation energies

The ionisation energies that come after the first ionisation energy.

How to identify group of an element from successive ionisation energies?

It will be in the group before its highest energy increase. (big increase as it starts to remove an electron from a new full shell)

Why do successive ionisation energies increase?

The electrons are being removed from an increasingly positive ion, which requires more energy.

How do the graphs of successive ionisation energy provide evidence for shells?

The big jumps provide evidence - show that the electron is being removed from an inner shell.

How do the graphs of successive ionisation energy provide evidence for sublevels?

Dip between group 2 and 3 on period 3 - expect ionisation energy to increase (as higher nuclear charge) but doesn't.

How do the graphs of successive ionisation energy prove that electrons in the same orbital repel more than electrons in different orbitals?

Dip in ionisation energy between group 5 and 6.

H vs He - which has the higher first ionisation energy? Why?

He - has 2 protons in nucleus, while H only has one, so the He nucleus more strongly attracts the electron