3.1.1 - Atomic Structure Flashcards
What holds protons and neutrons close together in the nucleus of an atom?
The ‘strong nuclear force’
What attracts electrons to the nucleus of an atom?
Electrostatic forces of attraction between negative electrons and positive protons
Which is stronger, the force that holds protons and neutrons close together in the nucleus of an atom, or what attracts electrons to the nucleus of an atom?
The ‘strong nuclear force’ between protons and neutrons in the nucleus
Why do protons not leave the nucleus towards the electrons?
The ‘strong nuclear force’ is stronger than the electrostatic forces of attraction between negative electrons and positive protons
The force that holds protons and neutrons close together in the nucleus of an atom only works over ______ distances (e.g. ?)
The ‘strong nuclear force’ only works over short distances, e.g. within the nucleus
What is the relative mass of an electron?
1/1840
What is the relative mass of a proton and neutron?
1
Define atomic number, Z
The number of protons in the nucleus of an atom
Define mass number, A
The number of protons AND neutrons in the nucleus of an atom
Define ‘isotope’
The same number of protons but different number of neutrons in the nucleus
Do isotopes of an element react in the same way?
Yes (same electron configuration and number)
Do isotopes of an element have the same mass numbers?
No - mass no. is number of protons AND neutrons in the nucleus of an atom, and no. of neutrons changes between isotopes
Why do two paired electrons in the same orbital repel each other?
Due to opposing spins
What does an s orbital look like?
(Spherical)
What does a p orbital look like?
(Paired)
What does a d orbital look like?
(Double)
Rules of electron configurations? (x2)
1 - Build up from the lowest to highest energy levels
2 - Fill orbitals singly before pairing (as to avoid repulsion)
Define ‘ionisation energy’
The amount of energy required to remove one electron from each atom of one mole of gaseous atoms, to form one mole of gaseous +1 ions
Unit of ionisation energy?
kJ mol^-1
Trend in ionisation energy across a period?
INCREASES
- Atomic radii decrease
- Same energy level/same shielding effectively
- Nuclear charge INCREASES
Trend in ionisation energy down a group?
DECREASES
- Atomic radii increase
- More energy levels/greater shielding
- Nuclear charge stays same
Why does the IE decrease between Mg and Al, going across period 3 from left to right?
Mg: 1s2, 2s2, 2p6, 3s2
-> Al: 1s2, 2s2, 2p6, 3s2, 3p1
So the electron from aluminium is removed from the 3p sub-shell, which has a higher energy level and thus requires less energy to remove
Why does the IE decrease between phosphorous (P) and sulphur (S), going across period 3 from left to right?
P: 1s2, 2s2, 2p6, 3s2, 3p3
S: 1s2, 2s2, 2p6, 3s2, 3p4
In S, there is electron pairing in the 3p orbital, so the electrons have opposite spin and repel each other, thus the electron in S requires less energy to remove than that in P - despite increased nuclear charge
What evidences the existence of p,s,d sub-shells?
The IE decrease between phosphorous (P) and sulphur (S), going across period 3 from left to right, due to electron pairing in sub-shells and repulsion
What is mass spectrometry useful for?
Determination of an atom’s relative atomic mass (Ar)/a molecule’s relative molecular mass (Mr)
Define Ar (relative atomic mass)
Ar = mean mass of 1 atom/ (1/12 mass of 1 atom of 12C)
Define Mr (relative molecular mass)
Mr = mean mass of 1 molecule/ (1/12 mass of 1 atom of 12C)
What are the five stages of time of flight mass spectrometry?
Ionisation Acceleration Deflection Drift Detection
Why does ToF MS occur in a vacuum?
So charged particles do not collide with other molecules in the air (impacting flight time)
What happens in acceleration in ToF MS?
All ions are accelerated by an electrical field to have equal kinetic energy. Ions are accelerated into a finely focused beam
What happens in deflection in ToF MS?
Ions are deflected by a magnetic field, dependent on their m/z ratio
Deflection in ToF MS
- Lighter ion?
- More charge?
- Lighter ion = greater deflection
- More charge = greater deflection
The higher m/z ratio, the greater the deflection
If an ion has a mass of 56 and a charge of +2, what is its m/z ratio?
m = 56
z = 2
so 56/2 = 28 = m/z
What happens at the ion detector?
Ion hits the detector and picks up an electron (becoming neutrally charged again), causing an electrical current to flow
A small m/z leads to lots of deflection. How can you reduce this?
Use a smaller magnetic field
A high m/z leads to little deflection. How can you increase this?
Use a larger magnetic field
Using a mass spectrum, how do you calculate the Ar of an atom?
Ar = sum of abundance x mass/ total abundance
What is the molecular ion peak and what is it equal to?
M+ formed when one electron is removed from the molecule. Equal to the Mr of the molecule
What are the isotopes of chlorine and their ratio to each other?
Chlorine- 35 and chlorine- 37
3:1
Why does the mass spectrum of a chlorine MOLECULE have three peaks?
3 possible types of the chlorine molecule can be formed:
35 + 35 = 70 (m/z)
35 + 37 = 72
37 + 37 = 74
What is the abundance on the mass spectrum of each of the 3 possible types of the chlorine molecule that can be formed?
35 + 35 = 70
3/4 x 3/4 = 9/16 = 56% abundance
35 + 37 = 72
3/4 x 1/4 = 3/16 = 19% abundance
37 + 37 = 74
1/4 x 1/4 = 1/16 = 6.25% abundance
What are the two types of ionisation in ToF MS?
Electrospray ionisation and electron impact ionisation
Describe electron impact ionisation x4
- Sample is vaporised
- High energy electrons are fired at it, from an ‘electron gun’ (a hot wire filament with a current running through it that emits electrons).
- This knocks off one electron from each particle forming a 1+ ion.
- The 1+ ions are then attracted towards a negative electric plate where they are accelerated.
Describe electrospray ionisation x4
- Sample is dissolved in a volatile solvent (eg water or methanol)
- Sample injected through a fine hypodermic needle to give a fine mist (aerosol).
- The tip of the needle is attached to the positive terminal of a high-voltage power supply, so the particles are ionised by gaining a proton
- The solvent evaporates away while the XH ions are attracted towards a negative plate where
they are accelerated.
General equation in electrospray ionisation?
X(g) + H+ -> XH+(g)
General equation in electron impact ionisation?
X(g) -> X+(g) + e–
When do we generally use electron impact ionisation?
For element/ substances with low formula mass
When do we generally use electrospray ionisation?
Substances with higher molecular mass including many biological molecules such as proteins
With which ionisation technique is fragmentation more likely to occur?
Electron impact ionisation
Detection: the size of the current gives a measure of what?
The number of ions hitting the plate.
