3.1.1 - Atomic Structure

1st: Development of the Model of the Atom

1904: J.J. Thompson - proposed the 'plum pudding' model of the atom
A spherical cloud of evenly spread positive charge dotted with electrons

2nd: Development of the Model of the Atom

1909: Ernest Rutherford
Hypothesised evidence for the 'plum pudding' model can be collected through an alpha particle scattering experiment (alpha particles: 2+ positive charge)

Alpha particle scattering experiment

Alpha particles can be fired at a thin piece of gold foil, and then the angles of the scattered particles can be measured

If Thompson's model was correct

Atom scattering should be uniform, as the positive charges were evenly spread around the atom, and couldn't cause much repulsion of the alpha particle
The alpha particles would pass through without much deflection

Results: alpha particle scattering experiment

Most of the alpha particles passed through the gold foil with no deflection, as most of an atom is empty space
Some particles were deflected back on themselves due to the nucleus - a small concentrated positive charge in the atom's centre

Conclusions: alpha particle scattering experiment

Rutherford concluded that the atom was mostly empty space, made form a very dense nucleus of positive charge at the centre of the atom, with the elections orbiting the nucleus in rings

Further development of the model of the atom

The model was further developed by: Neils Bohr, Erwin Schrödinger, James Chadwick

Mass/kg: proton, neutron, electron

Proton: 1.672 * 10^-27
Neutron: 1.674 * 10^-27
Electron: 9.109 * 10^-31

Charge/C: proton, neutron, electron

Proton: +1.602 * 10^-19
Neutron: 0
Electron: -1.602 * 10^-19

Relative mass: proton, neutron, electron

Proton: 1
Neutron: 1
Electron: 1/1840

Relative charge: proton, neutron, electron

Proton: +1
Neutron: 0
Electron: -1

Atomic Number
Symbol
Definition

Z
The number of protons in the nucleus

Mass Number
Symbol
Definition

A
The total number of protons and neutrons in the nucleus

Isotopes

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

ToF Mass Spectrometry

Time of Flight Mass Spectometry

Processes of ToF Mass Spectrometry

Ionisation
Acceleration
Ion drift (in the flight tube)
Detection

Ionisation: two methods

Electron impact
Electrospray

Electron impact: when and why used

used for elements and small molcules (low Mr < 200)
often results in fragmentation - the molecular ion breaks apart, and other peaks relating to the fragment appear in the spectrum

Electron impact: key parts and steps

Sample is vaporised
High energy electrons are fired at it from an 'electron gun'
Gun contains a hot wire filament running through it that emits electrons
Usually knocks off one electron, forming a 1+ ion

Electron impact: equation

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

Electrospray: used for what

used for big molecules (high Mr)

Electrospray: key parts

sample is dissolved in a volatile solvent
Sample is injected through a fine hypodermic needle to give a fine mist
The tip of the needle is attached to the positive terminal of a high-voltage power supply
A proton is added on to give a 1+ ion

Electrospray: final result

the final Mr has been increased by 1 due to H+, so we must subtract one

Electrospray equation

X(g) + H+ -> XH+ (g)

ToF mass spectrometry: acceleration

all ions are accelerated such that they have the same kinetic energy
ions are accelerated by a negatively charged acceleration plate

ToF mass spectrometry: ion drift

Lighter ions travel faster and meet the detector first.
Heavier ions travel slower and take longer to the meet the detector.

ToF mass spectrometry: detection

Ions hit the detector and gain an electron.

Detection equation

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

Detection: abundance and current

The more abundant a particular ion, the greater the current generated - the abundance is proportional to the size of the current generated.

ToF mass spectrometry: detection steps

Ion hits the negative plate and gains an electron
This generates a current
The size of the current is proportional to the abundance of the ion

RAM equation for ToF MS

RAM = Σ(m/z x abundance)/total abundance

ToF MS: kinetic energy equation

KE = 1/2 mv^2

ToF MS: kinetic energy equation units

KE = kinetic energy
mass = kg
velocity = m/s

ToF MS: velocity equation

v = d/t

ToF MS: velocity equation units

v = velocity
d = length of flight tube (m)
t = time (s)

mass on an ion equation

mass of one ion (kg) = (relative isotopic mass * 10^-3) / 6.022*10^23

Avogadro's constant

L
6.022 * 10^23

ToF MS: find velocity equation

maVa^2 = mbVb^2

ToF MS: find time equation

ma/ta^2 = mb/tb^2

ToF mass calculation notes

don't need to convert the masses into kg by dividing by 1,000 or divide by Na when equating 2 KEs

GCSE shells are now…

Energy levels

How are energy levels represented

Principal quantum number - n

What do the energy levels contain

Different maximum numbers of electrons

N (increasing energy)

1 -> 4

what are energy levels made up of?

Sub-shells

4 different types of sub-shells (in order)

s
p
d
f

s subshell orbital number

1

p subshell orbital number

3

d subshell orbital number

5

f subshell orbital number

7

what is an atomic orbital

an atomic orbital is a region of space where there is a high probability of finding an electron

how many electrons does an orbital contain?

each orbital contains a maximum of 2 electrons

how do orbital electrons 'spin'

In opposite directions to reduce repulsions between each other

s orbital shape

spherical

p orbital shape

dumbell

• and x, y, or z axis

max number of electrons on principal energy level 1

2

max number of electrons on principal energy level 2

8

max number of electrons on principal energy level 3

18

max number of electrons on principal energy level 4

32

Max number of electrons in s subshell

2

Max number of electrons in p subshell

6

Max number of electrons in d subshell

10

Max number of electrons in f subshell

14

how to find the max number of electrons on a subshell

number of orbitals on that subshell * 2

When writing electronic configurations, where is the highest energy electron found?

Furthest to the right

What is the s subshell orbital called

s

What are the p subshell orbitals called

px, py, pz

Three rules when filling atomic orbitals

The Pauli exclusion principle
The Aufbau principle
Hund's rule

The Pauli Exclusion Principle

When 2 electrons occupy the same atomic orbital, they do so with different spins

Why do electrons spin in opposite directions?

To reduce the repulsions between them

The Aufbau principle

Electrons fill lower energy atomic orbitals before higher energy orbitals are filled
filling Christmas tree image

Hund's rule

When electrons are filled into orbitals in the same shell, the occupy the orbitals singularly first with parallel spins, before pairing occurs
You fill the boxes per orbital and energy electron singularly, before double filling

Notes when filling atomic orbitals

Orbitals are filled singularly first before pairing occurs. When electrons are paired in the same orbital their spins must be opposite.
Always draw out all available orbitals in a subshell before adding electrons.

enthalpy change of ionisation def

the energy required to remove 1 mole of electrons from 1 mole of gaseous atoms to form 1 mole of gaseous +1 ions

enthalpy change of ionisation equation

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

key factors affecting ionisation energy

nuclear charge
atomic radius
shielding

nuclear charge affection ionisation energy

the number of protons in the nucleus
the higher the nuclear charge, the stonger the electrostatic force of attraction between the nucleus and the electrons

atomic radius affecting ionisation energy

the greater the atomic radius, the further the outermost electron is from the nucleus, the weaker the electrostatic force of attraction between them

shielding affecting ionisation energy

this is an electronic screening effect. the more filled energy levels there are between the outmost electrons and the nucleus, the greater the shield, so the weaker the electrostatic force of attraction between the nucleus and the outermost electron

how does nuclear charge change down the group
and how does it affect ionisation energy

nuclear charge increases down the group
this increases the ionisation energy required, due to a stronger ESFA between valence electrons and the nucleus

how does nuclear charge change across the period
and how does it affect ionisation energy

nuclear charge increases across the period
this increases the ionisation energy required, due to a stronger ESFA between valence electrons and the nucleus

how does atomic radius change down the group
and how does it affect ionisation energy

atomic radius increases down the group
this decreases the ionisation energy required due to a weaker ESFA between valence electrons and the nucleus

how does atomic radius change across the period
and how does it affect ionisation energy

atomic radius decreases across the period
this increases the required ionisation energy. this is due to a stronger ESFA between valence electrons and the nucleus -> the valence electrons aren't further away, but are pulled stronger

how does shielding change down the group
and how does it affect ionisation energy

shielding increases down the group
this decreases the required ionisation energy, and there is more shielding, and thus a weaker ESFA between valence electrons and nucleus

how does shielding change across the period
and how does it affect ionisation energy

stays the same so ionisation energy stays the same

what is successive ionisation energy

removing each of the electrons in sequence, starting with the outermost electron (the electron that is highest in energy)

why are all ionisation values endothermic

it required energy to overcome the ESFA between the valence electron and the nucleus

first ionisation of magnesium

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

second ionisation of magnesium

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

third ionisation of magnesium

X 2+(g) -> X 3+(g) + e-

how will the value of the second ionisation energy of magnesium compare to the first

more energy is required due to a stronger ESFA between the nucleus and valence electrons
the electron is also being removed from a positive ion

what are big jumps in ionisation energies in ionisation charts

new energy level

what does a big increase in energy mean in ionisation charts

a big increase in energy means the electron is removed from a new (lower) energy level

Nuclear charge, atomic radius and shielding down the group

Down the group:
Nuclear charge increases
Atomic radius increases
Shielding increases

So IE decreases

IE Trend:Down the group

Despite the increase in nuclear charge, the increase in atomic radius and shielding leads to a greater ESFA between the valence electron (or electrons for not 1st IE) and nucleus, so IE decreases

Nuclear charge, atomic radius and shielding across the period

Nuclear charge increases
Atomic radius decreases
Shielding stays the same

IE increases

IE trend: across the period

Nuclear charge increases, atomic radius decreases and constant shielding leads to a stronger ESFA between nucleus and valence electrons, so IE increases

Electron shells: different energy levels

3p at a higher energy level, and further from the nucleus than 3s

1st ionisation energy trend (U&D)

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1st ionisation energy trend (U&D)

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