Chemistry unit 1- section 1: atomic structure

Relative atomic mass (Ar)

The average mass of an atom of an element when measured on a. Scale on which the mass of an atom of c-12 is exactly 12

Relative isotopic mass

The mass of an atom of an isotope of an element measured on a scale on which the mass of an atom of c-12

Relative molecular mass (Mr)

The average mass of a molecule when measure on a scale on which the mass of an atom of c-12 is exactly 12

John Dalton-1803

Atoms are spheres and each element is made up of different spheres

1897-J.J Thompson

Discovered the electron

Said atoms wasn’t solid and was made up of other particles

Plum pudding model developed—> positive sphere with negative particles embedded

1909-Ernest Rutherford

Discovered the nucleus which was very small and positive. Concluded the atom was mainly empty space, made up of a negative cloud

Ernest Rutherford- gold leaf experiment

(+) alpha-particles fired at thin sheet of gold leaf.

Most went through the gold leaf = most of atom is empty space

Small number of them deflected back = hit the small (+) nucleus.

1913 Niels Bohr

Discovered a problem with Rutherfords model. The cloud of electrons could collapse into the (+) nucleus but it doesn’t

Why= proposed e- were in fixed energy shells

1913 Niels Bohr

Wondered why the negative cloud doesn’t collapse into the (+) nucleus

Proposed the idea that

e- only exist in fixed orbits(shells)

Each shell has a fixed energy

When an electron moves between shells em radiation is emitted or absorbed

Because energy of shells is fixed, frequency of radiation is also fixed

Atomic model today - Quantum model

Electrons don’t have the same energy shells - we have sub-shells, this explains ionisation trends

Mass spectrometer :

A machine used to analyse elements or compounds

Time of flight mass spectrometer steps

1.) ionisation

2.) acceleration

3.) ion drift

4.) detection

1.) ionistion

First Sample is vaporised so it can move trough TOF mass spectrometer

A.) electrospray ionisation:

Solvent is dissolved and pushed through a small nozzle at high pressure

Then a high voltage is passed through, causing the loss of 1e-

And so a gaseous positively charged sample is produced

B.) electron-impact ionisation:

Sample is vaporised and an ‘electron gun’ is used to fire high energy e- at it

This knocks off an e- off each particle so they become +1 ions

2.) acceleration

(+) ions are passed through an electric field, this gives off the same kinetic energy to all the ions

The lighter ions experience a greater acceleration than the heavier ones despite being given the same k.e

3.) ion drift

Next ions enter a region with no electric field.

They drift through it at the same speed they left the electric field

This means lighter ion drift at higher speeds

4.) detection

Ions detected as the electrical current is made.

When particles hit the plate, those with lower m/z reach the detector first as they travel faster.

Why do we need to need to ionise the sample first ?

To make it accelerate

To deflect/ bend the beam

What is adjusted in the mass spectrometer to enable ions formed to be directed on to the detector ?

The electric field/electromagnet used

When would you use the different types of ionisation in a mass spec?

Electron impact used for organic or inorganic molecules with a low formula mass.

Electrospray used for substances with a higher molecular mass including biological molecules, e.g. proteins.

What are two features of the current model that are not shown in Rutherfords’ ?

Current model includes neutrons and protons in the nucleus

Current model shows electrons in different energy levels/ orbits

What do the peaks on the mass spectra show?

Peaks show fragments of original molecule

The last peak = m+1, this is the same as the relative molecular mass of the molecule

What are the 4 subshells?

S— has 1 orbital and can hold 2e-

P— has 3 orbitals and can hold 6e-

D— has 5 orbitals and can hold 10e-

F— has 7 orbitals and can hold 14e-

Shortened electron configurations

Noble gases in square brackets are sometimes used as short hand in electron configuration

E.g calcium which is 1s² 2s² 2p⁶ 3s² 3p⁶ 4s² can Be written as

[Ar] 4s² where [Ar]= 1s² 2s² 2p⁶ 3s² 3p⁶

Electron configurations of transition metals

Chromium and copper behave differently. For example they donate one of their 4s e- to the 3d sub shell. This is because they prefer to have a more stable full or half- full d-sub shell

e.g) Cr E.C = 1s² 2s² 2p⁶ 3s² 3p⁶ 3d⁵ 4s¹ instead of 1s² 2s² 2p⁶ 3s² 3p⁶ 3d⁴ 4s²

Explain why chromium doesn’t fit the trend of E.C

It only has one electron in its 4s orbital before filling the 3d

1s…3p, 4s¹, 3d⁵

Explain why copper doesn’t fit the trend E.C

It only has one electron in its 4s orbital before filling the 3d

1s²……. 3p⁶ 4s¹ 3d¹⁰

What is ionisation energy?

The energy required to remove 1 electron from each atom in 1mol of gaseous atoms to form 1 mol of gaseous 1+ ions.

What is the general formula for ionisation energy

X^(n-1) (g) ———>X⁽ⁿ⁾ (g) + e^-

^{N= number of ionisations}

What are the three factors that affect ionisation

1) nuclear charge

2) distance from nucleus

3) shielding

How does nuclear charge affects I.E?

The more protons there are in the nucleus, the more positively charged the nucleus is.

This means the electrostatic forces of attraction between the nucleus and the outer electrons are stronger.

So more energy is needed to break these bonds .

How does the distance from the nucleus affect the I.E?

As distance increases the electrostatic forces of attraction decreases.

An electron close to the nucleus will be more strongly attached than one further away.

This means if it ids further away less I.E is needed to remove the electron

How does shielding affect the I.E?

As the number of electrons between the outer e^- and the nucleus increases, the outer electron feels less attraction to the nucleus.