3.1.1 Atomic Structure

Position, relative charge and mass of proton, neutron, electron

Changing understanding of atomic structure over time

1. Dalton
   → atoms are solid spheres with no internal structure
   
   

2. JJ Thomson
   → discovered electrons
   → plum pudding model: ball of positive charge with negative electrons embedded in it
   
   

3. Rutherford gold foil experiment
   → alpha particles fired at a thin sheet of gold foil
   → most went straight through: atom is mostly empty space
   → some were deflected and some went straight back: mass concentrated at centre, centre is positively charged
   
   

4. Nuclear model
   → positively charged nucleus
   → mass concentrated at nucleus
   
   

5. Bohr Model
   → electrons orbit nucleus at specific distances: energy levels

Define and give the letter which represents atomic number

number of protons

(Z)

Define and give the letter which represents mass number

number of neutrons + number of protons
(A)

What are nucleons?

protons and neutrons

How are positive and negative ions formed?

positive: atom loses electron(s)

negative: atom gains electron(s)

Define isotope

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

Define relative atomic mass

the average mass of an atom of an element compared to 1/12^th the mass of an atom of carbon-12

or

weighted average mass of all isotopes of an element compared to 1/12^th the mass of an atom of carbon-12

Elemental Mass spectrum

Horizontal position of a peak gives its mass-to-charge ratio (m/z)

Peak height shows the relative abundance of that ion

For an isotope ion carrying a 1+ charge, the numerical m/z value gives its…

relative isotopic mass

Mass spectrum for Mg⁺

How do we identify the mass of ions

using a simple time of flight (TOF) mass spectrometer

Function of TOF Mass Spectrometry

to identify elements by determining all of the isotopes present in a sample

What are the 5 stages of TOF mass spectrometry?

1. ionisation
   → sample particles are changed into positive ions

2. acceleration
   → an electric field gives the ions equal kinetic energy

3. ion drift
   → the ions cross the flight tube and become separated by different flight times

4. ion detection
   → arriving ions create signals that are measured electronically

5. data analysis
   → computer records flight times to construct mass spectrum

What condition must be maintained through the TOF instrument and why?

a high vacuum so that collisions with air molecules do not interfere with the motion of the ions

TOF Step 1: What are the two methods of ionising particles from the sample?

electron impact

electrospray ionisation

How do we carry out (ionisation by) electron impact?

1. High energy electrons

2. From hot cathode/electron gun

3. Are fired at sample

4. This knocks off an outer electron from each particle

5. Forming a 1+ ion

X_(g) → X⁺_(g) + e^-

What is the equation for electron impact?

Electron impact is suitable for which types of samples?

elemental samples and compounds of lower formula mass

Why is electron impact less/not suitable for elemental samples and compounds of larger formula mass?

the energetic ionisation process can break larger molecules into smaller fragment ions which can negatively impact the spectra analysis

What type of ionisation is used for larger molecules?

electrospray ionisation

Steps of electrospray ionisation

1. Sample is dissolved in a volatile, polar solvent

2. It is then injected through a fine hypodermic needle giving a fine mist

3. The tip of the needle has a high voltage

4. This causes the sample molecule to gain a proton as it leaves the needle

Equation for electrospray ionisation

X_(g) + H⁺ → XH⁺_(g)

TOF Step 2: Acceleration

• The electric field pulls and accelerates the positive ions towards a negatively charged plate

• The particles are accelerated so that every ion enters the drift region at the same kinetic energy

Acceleration Stage: At constant kinetic energy, what factor causes the differing velocities of the ions?

Mass
→ lighter ions travel faster

because v depends on mass

TOF Step 3: Ion Drift

• After passing through an opening in the plate, the ions enter the flight tube

• They drift towards the ion detector

• The interval between entering the flight tube and reaching the detector is the ion’s time of flight

Why do different ions have different flight times?

equal kinetic energy
different mass = different velocity

= different flight times

TOF Step 3 Continued: Ion Detection

• at the end of the flight path, each positive ion reaches the detector (a negatively charged plate)

• the positive ions gain an electron

• the electron transfer from detector to ion produces a small current

• the magnitude of the current increases with the number of ions of that type arriving at the detector
  → this provides a measure of species abundance/ relative abundance

TOF Step 4: Data Analysis

• two pieces of information are analysed in combination
  → current which is produced
  → time of flight

• this produces a mass spectra with the relative abundance produced by ions with varying m/z ratios

Which two equations are used for TOF calculations?

v=d/t

KE = 0.5mv²

How do we calculate the value of m (mass of one ion) for TOF calculations?

Divide the mass number by the avogadro constant, 6.022×10²³

This gives the mass in grams, we need it in kilograms so divide by 1000

The arrangements of electrons in an atom is called…

the electron configuration

Electrons are arranged around the nucleus in…

energy levels

Each energy level is split into…

sub-shells

Names of the sub-shells

s, p and d (and f)

The energy of the electrons in the sub-shells increases in the order < <

s < p < d

How many electrons does each sub shell hold?

s: 2

p: 6

d: 10

f: 14

Each sub energy level/ sub shell is split into…

orbitals

What does each orbital hold?

2 electrons of opposite spin

The number of orbitals in each sub-shell is as follows:

• s : one orbital (1 x 2 = total of 2 electrons)

• p : three orbitals ( 3 x 2 = total of 6 electrons)

• d : five orbitals (5 x 2 = total of 10 electrons)

• f : seven orbitals (7 x 2 = total of 14 electrons)
  
  

Shape of s orbital vs shape of p orbital

s: spherical

p: dumbbell shape

Electron arrangement summary

Writing electronic structure using letters and numbers

Using spin diagrams

Blocks of the periodic table

Electronic structure for ions

(electrons are lost from the 4s electrons before the 3d electrons even though the 3d are filled first)

Exceptions to normal electronic structure

Copper and chromium have a single electron in 4s¹ before 3d instead of a 4s² as this gives a more stable structure

Define ionisation energy

the amount of energy required to remove one mole of electrons from one mole of gaseous atoms of an element to form one mole of gaseous ions

Define 1st ionisation energy

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

1st ionisation energy equation

X_(g) → X⁺_(g) + e^-

2nd ionisation energy equation

X⁺_(g) → X²⁺_(g) + e^-

Which trends are there for ionisation energy?

• across a period

• down a group

• successive ionisation

Factors that affect ionisation energy

• atomic radius (distance)
  → the further the distance from the nucleus, the easier to remove an electron due to a weaker attraction

• shielding
  → the more shells between nucleus and outer electron, the easier to remove an electron so the lower the ionisation energy

• nuclear charge
  → the more protons there are in a nucleus, the greater the nuclear charge
  → the greater the nuclear charge, the stronger the attraction of the outer electron to the nucleus
  → a stronger nuclear charge also pulls the outer shell closer to the nucleus
  → this results in a smaller distance
  so more difficult to remove an electron so greater ionisation energy

I.E down a group

I.E decreases down a group

Why does I.E decrease down a group?

• atoms get bigger

• increase atomic radius so greater distance between nucleus and outer electron

• more shells so more shielding

• therefore weaker attraction between outer electron and nucleus

• so lower ionisation energy

I.E trend across a period (left to right)

I.E increases across a period

Why does I.E increase across a period?

• more protons so greater nuclear charge

• same shielding across the period

• smaller atomic radius so shorter distance between outer electron and nucleus

• so stronger attraction between nucleus and outer shell

Deviations in first ionisation energy trend across a period

I.E drops between group 2-3 and group 5-6

Why does first I.E drop from group 2 to 3?

in group 2 the outer electron is in an s sub shell whereas in group 3 the outer electron is in a p sub shell

p sub shells are more shielded, so there is a weaker attraction between nucleus and outer electron, so ionisation energy drops

Why does first I.E drop from group 5 to 6?

the outermost electron in group 6 is paired in a p orbital

paired electrons repel

so less energy is needed to remove the outer electron