Atomic Structure Flashcards
What is mass spectrometry?
A method to determine the mass of atoms or molecules
What can mass spectrometry be used for?
- finding relative isotopic abundance
- molecular mass
- helping determine the structure of a compound
What is stage 1 of mass spectrometry if the sample is NOT a large molecule?
Ionisation
- fire high-energy electrons from an electron gun
- this knocks off electrons of each particle to make it a 1+ ion
X(g) + e- —-> X(g)+ + 2e- or X(g) —-> X(g)+ + e-
What is stage 1 of mass spectrometry if the sample IS a large molecule?
Electrospray ionisation
- The sample is dissolved in a volatile solvent (like water) and injected through a fine hypodermic needle to give a fine mist (aerosol)
- The tip of the needle is attached to a positive terminal of a high-voltage power supply
- Particles are ionised by gaining a proton (H+) from the solvent
M + H+ —> MH+
What is the equation in electrospray ionisation and what does MH+ stand for?
M + H+ —> MH+
MH+ is whats detected, the mass of this is recorded and will be the Mr of the substance + 1
What’s the 2nd stage of mass spectrometry?
Acceleration
- Positive ions are accelerated with an electric field (a negatively charged plate)
- Regardless of mass, all particles will have the same amount of kinetic energy
What’s the 3rd stage of mass spectrometry?
Flight Tube
- All the particles will have different velocities (due to their different masses) so we can use this to differentiate between them
- This is because velocity affects the time of flight of each particle
What’s the 4th stage of mass spectrometry?
Detection
- The positively charged ions hit a negatively charged plate
- They gain an electron, causing an electric current
- The electric current is measured, and the size gives a relative measure of the number of specific ions hitting the plate.
What’s the equation for Kinetic energy?
Ke=1/2mv^2
In summary, what are the 4 stages of Mass spectrometry?
- Ionisation
- Acceleration
- Flight Tube
- Detection
What must we do to the sample before starting mass spectrometry?
Vapourise it
What was the first model of the atom?
Dalton 1803
- Atoms are tiny solid spheres which cant be divided
What was the second model of the atom?
Thompson 1897; Plum pudding model
- Electron discovered
- Positive ball of charge with atoms embedded
What was the third model of the atom?
Rutherford 1911; Nuclear Model
- Alpha particles (He2+) fired at gold atoms ( some deflected and bounced back, most passed through)
- Small positively charged nucleus, most of the mass found here
- Electrons orbit the nucleus, mostly empty space though
What was the fourth model of the atom?
Bohr 1913
- Electrons found in shells at certain distances surrounding the nucleus
When was the proton discovered and by who?
Rutherford 1917
What was the last model of the atom?
Chadwick 1932
- Neutrons discovered and located in the nucleus
Things to look out for when looking at a mass spectrometer graph?
- Don’t assume that m/z is mass all the time, there could be unexpected peaks due to 2+ charges (will be 1/2 the mass)
- Relative abundance isn’t always a %
Bromine has 2 isotopes at m/z = 79 and 81, how many peaks would you expect and why? What about their heights?
5, for 5 different combinations.
Since Bromine is diatomic, we can add them tg asw to make Br2.
Br: 79, 81
Br2: (79+81=160), (79+79=158), (81+81=162)
The 160 peak would be double in height since there are 2 ways of making 160 (79+81) and (81+79). because the ratio of abundance of 158, 160 and 162 is 1:2:1.
How would you label the peak for an isotope of Br2, when Br has peaks at m/z=79 and 81 (the species)
You would label at 160, [79Br 81Br], you would label at 158 [79Br2], at 162 it would be [81Br2]
What is the first ionisation energy trend along a period?
Energy increases
- Decreasing atomic radius (electrons are added to the same shell but more protons in the nucleus pull electrons in the shells closer)
- Increasing nuclear charge (more protons)
- Sheilding satys the same
These factors increase the electrostatic forces of attraction
How would you calculate the mass of oxygen, knowing’s it’s Ar is 16?
16/6.022x10^23
What do isoelectronic species have in common?
Their electron configurations
What is the first ionisation energy?
The energy required to remove one electron from each atom in a mole of gaseous atoms, producing one mole of 1+ gaseous ions
Where does the evidence for how electrons are arranged in atoms come from?
Ionisation energies
What is ionisation energy measured in?
kJmol-1
What’s the equation for the 1st ionisation energy of Na?
Na(g) —> Na+(g) + e-
What’s the equation for the 2nd ionising energy of Na?
Na+(g) —> Na2+(g) + e-
What is the first ionisation energy trend down a group?
First ionisation energy decreases
- Atomic radius increases
- Shielding increases (electrons in shells cause repulsion between electron and nucleus)
Weakens the electrostatic forces of attraction
Why do group 3 elements have a lower first ionisation energy than group 2?
Group 2 loses an electron from the s orbital whereas group 3 loses an electron from the p orbital. Since the p orbital has more energy than s, it’s easier to lose an electron from it.
Why do group 6 elements have a lower first ionisation energy than group 5?
In group 6, this is the first element in the period that there are paired electrons in the _P orbital. These repel each other, meaning less energy is required to remove one of them.
What is the electronic configuration of Cr?
Cr:1s2 2s2 2p6 3s2 3p6 4s1 3d5
What is the electronic configuration of Cu?
Cr:1s2 2s2 2p6 3s2 3p6 4s1 3d10
Why is the first ionisation energy of neon lower than the third ionisation energy of magnesium?
Mg2+ and Ne have the same amount of electrons but
- Mg2+ has more protons (bigger nuclear charge)
- This means the IONIC radius decreases
