3.1.1 Atomic structure

Isotopes

Same number of protons but a different number of neutrons

Behave similarly because they have the same number of protons and electrons, and so they have almost identical chemical properties that react in the same way

Neurons give isotopes different masses

TOF Mass spec:

Separates ions based on their m/Z ratio by measuring how long it takes an ion to travel a fixed distance

Done under a vacuum otherwise air particles would ionise and would also register in the detector

Four stages:

• Ionisation

• Acceleration

• Ion drift

• Detection

Stage one mass spec: Ionisation

Electron impact:

-Vaporised sample is injected at low pressure

-High energy electrons fired using an electron gun

-This knocks out an outer electron

-Forms an ion with a plus one charge

-Used for compounds with a low formula mass

X (g) —> X+ (g) + e-

Electrospray ionisation:

• Sample is dissolved in polar, volatile solvent

• Volatile so can evaporate

• Polar to supply H plus ions

• Injected through a needle at high voltage

• For substances with a higher molecular mass

M (g) —> MH+ (g)

Stage two: Acceleration

Samples are ionised to be accelerated by attraction to electric field, to hit detector to gain an electron to generate a current

Positive ions are accelerated by attraction towards a negatively charged plate

To a constant kinetic energy

Light ions have a faster velocity, heavier ions have a slower velocity

Stage three: ion drift

Ions hit the detector and each ion gains an electron which generates a current

Abundance is proportional to the size of the current produced

This is fed into a computer for analysis

X (g) + e- —> X (g)

• Determining molecular mass

Electron impact:

Fragmentation due to being an energetic ionisation technique

Fragmentations are smaller parts of the original molecule

Relative Mr is the greatest m/Z value

Smaller peaks due to the presence of isotopes

Electrospray:

Only one peak

Ion does not fragment or break up

Subtract one from the Mr of the molecule for the hydrogen bonded to it

Mass spectrum graph for chlorine and bromine

Calculating relative atomic mass

TOF mass spec calculations formulas

Electronic configuration

Sub shells have orbitals, (s,p,d,f)- use atomic number

S holds up to so electrons, p holds up to six electrons, d holds up to ten electrons

The block the element is in is the sub shell of the last electron it is in

Special cases in electron configuration

Crypton an copper only have one electron in their 4s sub shell and an extra electron in 3d, as 3d is not stable

Transition metals- 4s orbital is always before the 3d orbital as it has less energy

When forming ions, transition electrons lose electrons in their 4s subshell before 3d subshell

Examples for electron configuration

Ionisation energy

The first ionisation energy: the energy needed to remove one mole of electron from one mole of gaseous atom to form one mole of gaseous ion with a plus one charge

Second ionisation energy: energy needed to remove one mole of electron from one mole of gaseous ion with a plus one charge to form one mole of gaseous ion with a plus two charge

X (g) —> X + (g) + e-

X+ (g) —> X 2+ (g) + e-

An endothermic process as energy is needed to overcome the attraction between the negative electron and the positive nucleus

On a table this is shown as a large jump in energy, showing that the electron has been removed from a different shell which is closer to the nucleus, as the attraction is stronger

Factors affecting ionisation energy

• Nuclear charge/ proton number: greater for of attraction between outer electrons and nucleus which requires more energy to remove an electron

• Distance between the nucleus and outer electron: weaker attraction as distance increases to less energy needed

• Shielding: more electron shells means more repulsion experienced by the outer shell by the inner shells, so less energy required to remove an electron

• Electron pairing: paired electrons in an orbital repel each other, making it easier to remove

Ionisation energy trends

Down a group: IE decreases

• Shells increase so more shielding

• More electron repulsion by inner shells

• Nuclear radius increases so distance increases

• Less energy required remove outer shell electrons

• Even though the proton number does increase, the effect of shielding and the increase in radius is greater

A cross a period: IE increases

• More protons so stronger nuclear charge

• No effect of sheilding as all have the same number of electron shells

• Stronger attraction between nucleus and outer shell electron

Ionisation energy patterns that defy the general trend

• Elements group two and five have higher ionisation energy than in three and six in the same period

Group two greater than group three:

• In group two outer electron is in the s orbital

• In group three the outer electron is in the p orbital

• The p orbital is further from the nucleus than the s orbital

• So the distance is greater, and so the attraction between the outer electron and the positive nucleus is weaker

Group five is greater than group six:

• Group five has an odd number of electrons in its outer electron shell

• Group six has an even number of electron pair in its outer subshell

• The electron pairs repel each other, asking it easier for an electron to be lost

Why is the second ionisation energy higher than the first.

After the first ionisation energy, there are more protons than electrons as an electron has been removed

So the electron in the outer shell is held more strongly by the positive nucleus because it is closer to the nucleus

This requires more energy to overcome