a level chem: electronic structure, mass spectrometry and ionisation energy

what is mass spectrometry used for?

determining the mass of separate atoms/molecules/molecule fragments, which can be used to find the relative atomic/molecular mass

how is mass spectrometry used to measure the mass of atoms/molecules?

the atoms/molecules are vaporised, ionised (electrons are removed), accelerated so that they have the same kinetic energy, deflected by a magnetic field (the more deflection they have, the lighter they are) and then the mass spectrometer measures the charge. thus, the charge to mass ratio (m/z) is found

A sample of water containing ¹H, ²H and ¹⁶O was analysed in a mass spectrometer. The trace showed peaks at mass numbers 1, 2, 3, 4, 17, 18, 19 and 20. Suggest which ions are responsible for these peaks.

H⁺ is responsible for the peak at 1, ²H⁺ and/or H₂⁺ are responsible for the peak at 2, [¹H-²H]⁺ is responsible for the peak at 3, [²H₂]+ is responsible for the peak at 4, [O-¹H]⁺ is responsible for the peak at 17, [O-¹H²H]⁺ is responsible for the peak at 18, [O-²H₂]⁺ is responsible for the peak at 19

what is the s block (elements where the highest-energy electron is in the s subshell)?

the left side of the periodic table (first two groups)

what is the d block?

the transition metals

what is the p block?

right side of the periodic table

what is the f block?

elements on the bottom of the periodic table in their own two rows

what is the shape of the s subshell?

spherical (NOT circular)

what is the shape of the p orbital?

dumb-bell (it can be in different orientations depending on the orbital; these variations are at 90 degrees to each other)

what is the maximum number of subshells an energy level can have?

4 (s, p, d, f)

what is an orbital?

an area in an atom where an electron is very likely to be

how many orbitals are in the s subshell?

1

how many orbitals are in the p subshell?

3

how many orbitals are in the d subshell?

5

how many orbitals are in the f subshell?

7

how many electrons (of opposite spin) can fill each orbital?

2

what is the Aufbau principle?

electrons enter the lowest available energy level

what is Hund’s rule?

when in orbitals of equal energy, electrons will try to remain unpaired

what is the Pauli exclusion principle?

two electrons can go in each orbital when they are of opposite spin

what is the Heisenberg Principle of Uncertainty?

you can never know exactly where an electron will be

which subshell is filled first: 3d or 4s?

4s–it is of slightly lower energy than 3d

which subshell’s electron will be removed first: 3d or 4s?

4s (again)

how do you write the short electronic structure of an element or ion?

pick the noble gas in the period one above it and then add on whatever electrons it needs

what is the electronic configuration of Chromium (it’s an exception)?

1s² 2s² 2p⁶ 3s² 3p⁶ 4s¹ 3d⁵ (after one electron has gone into 4s, the others fill up 3d, because there will be less repulsion if the electrons go into 3d instead of pairing up with the one in 4s, so this will be a lower energy arrangement)

what is the electronic configuration of copper (it’s an exception)?

1s² 2s² 2p⁶ 3s² 3p⁶ 4s¹ 3d¹⁰ (after one electron has gone into 4s, the others fill up 3d, because there will be less repulsion if the electrons go into 3d instead of pairing up with the one in 4s, so this will be a lower energy arrangement)

what is ionisation?

the loss of electrons to form ions

in which order are electrons removed from atoms?

in the opposite order that they were filled (i.e. higher energy electrons are removed first as they are farther to the nucleus and so less attracted to it)

what is the first ionisation energy?

the energy required to remove one mole of electrons from one mole of gaseous atoms to form one mole of gaseous ions with a 1+ charge. measured in kJ/mol

what are the factors affecting ionisation energy (they all affect nuclear attraction)?

1. nuclear charge

2. atomic radius

3. shielding

how does nuclear charge affect ionisation energy?

• increases it

• because: as proton number increases, nuclear charge increases, so the force of attraction experienced by the electrons increases

• this means a higher energy is needed to overcome these forces

how does atomic radius affect ionisation energy?

• decreases it

• the farther an electron is to the nucleus, the weaker the attraction between them

• this means less energy is required to overcome the forces of attraction

how does shielding affect ionisation energy?

• decreases it

• the repulsion from inner electrons against outer electrons means that the nuclear charge is shielded from the outer electrons

• this means that nuclear attraction to the outer electrons is lower and less energy is required to overcome the forces of attraction

what are the trends in ionisation energy as you go along the period and why?

• general trend: increases

• reason: as you go across the period, nuclear charge increases, so the force of attraction experienced by the electrons increases and a higher energy is needed to overcome these forces. since nuclear charge is greater, atomic radius decreases (electrons are pulled in to the centre), while shielding stays constant (as the electrons are in the same energy level), so the outer electrons are closer to the nucleus and so the nuclear attraction to the outer electrons is greater, meaning more energy is needed to overcome the forces

• specific trends: peak at group 2, dip at group 3, peak at group 5, dip at group 6, then rises afterwards

• reason: at group 3, the highest-energy electron is now in a higher-energy subshell (3s instead of 3p) so less energy is needed to overcome its nuclear attraction. at group 6, the highest-energy electron is now paired, so there is higher electron repulsion and therefore lower nuclear attraction, and less energy is required to overcome this force of attraction