1.1 Atomic structure

Question 1

Define Isotopes

Answer 1

Isotopes are atoms with the same number of protons, but different number of nutrons.

Question 2

What properties do isotopes share and why?

Answer 2

Isotopes have similar chemical properties because the have the smae electron structure.
They may have slightly varying physical properties because the have different masses.

Question 3

What is a ToF mass spectrometer used for?

Answer 3

They can be used to determine all the isotopes present in a sample of an element and to identify elements.
They must be in a vaccum otherwise air particles would ionise and register on the detector.

Question 4

What sort of ionisation is used for substances with low formula mass?

Answer 4

Electron impact. It can cause larger orgainc molecules to fragment

Question 5

Describe the process of electron impact ionisation

Answer 5

-A vaporised sample is injected at low pressure.
-An electron gun fires high energy electrons at the sample.
-This knocks out an outer electron
-Forming positive ions with different charges

Question 6

Give an example equation for electron impact ionisation

Answer 6

Ti (g) > Ti^+ (g)+ e-

Question 7

What sort of ionisation is used for larger organic compounds?

Answer 7

Electro spray ionisation. The ‘softer conditions’ of this technique mean fragmentation does not occur.

Question 8

Describe the procces of electrospray ionisation

Answer 8

-The sample is dissolvers in a volatile, polar solvent
-It is injected through a fine needle giving a fine mist or aerosol
-The tip of the needle has a high voltage
-At the tip the sample molecule gains a proton, H+, from the solvent forming XH+
-The solvent eveaporates away while the XH+ ions move towards the negative plate

Question 9

Give an example equation for electronspray ioniasation

Answer 9

X(g) + e– > X+(g) + 2e–

or

X(g) > X+(g) + e–

Question 10

Describe how acceleration takes place in ToF mass spectrometry

Answer 10

-Positive ions are accelerated by an electric field to a constant kinetic energy.
-Given all particles have the same kinetic energy, the velocity of each particle depends on its mass. Lighter particles have a faster velocity, and heavier particles have a slower velocity.

KE=1/2mv^2

Question 11

What happens in the flight tube during ToF mass spectrometry?

Answer 11

-It is a vacum so all particles travel with a constant speed
-The positive ions with smaller m/z values will have the same kinetic energy as thoes with higher m/z and will move faster
-The heavier particles will take longer to move through the drift area
-The ions are distinguished by differnt flight times

Question 12

What is the equation used to calculate the speed of an ion in the drift area

Answer 12

t=dv

t=time of flight(s)
d=length of flight tube(m)
v=velocity of the particle(ms^-1)

Question 13

What happens at the detector during ToF mass spectrometry?

Answer 13

-The ions reach the detector and generate a small current, which is fed to the computer for analysis
-The current is produced by electrons transferring from the detector to positive ions
- The size of the current is proportional to the abundance of the species.
-For each isotope the mass spectrometer can messure a m/z(mass/charge ratio) and an abundance

Question 14

What can take place changing the m/z value in ToF?

Answer 14

Sometimes two electrons may be removed froma particle forming a 2+ ion. This halves the m/z value

Question 15

Give the equation for calculating relative atomic mass

Answer 15

R.A.M=sum (isotopic mass x % abundance) / 100

or

R.A.M=sum(isotopic mass x relative abundance) / total relative abundance

Question 16

How is the Mr of a molecule mesured?

Answer 16

If electron impact ionisation is used fragmentation will often occur causing lots of peaks to be formed. The peak with the largest m/z value is the compleate molecule and is equal to the Mr. This peak is called the parent ion.
If electrospray ionisation is used fragmentation won’t occur giving one peak of the MH+ ion. To get the Mr simply subtract 1 from the m/z value.

Question 17

What is the Alevel model for electrons?

Answer 17

Principal energy levels are labeled 1,2,3…
These are split into sub energy levels s, p, d and f
S holds up to 2
P holds up to 6
D holds up to 10
F holds up to 14
These are split into orbitals which hold up to 2 electrons in opporsite spin

Question 18

How is the electronic structure affected when ions are formed?

Answer 18

When a positive ion is formed an electron(s) is lost from the outermost shell.
E.g Mg is 1s2 2s2 2p6 3s2 becomes Mg2+ is 1s2 2s2 2p6

When a negative ion is formed an electron(s) is gained.
E.g O is 1s2 2s2 2p4 becomes O2- is 1s2 2s2 2p6

Question 19

What about the electron structure of d block elelements is special?

Answer 19

Both chromium and copper have half filled 4s subshells
When d block elements form ions the lose the 4s electrons first.

Question 20

Give the electron structure for Chromium Cr

Answer 20

1s2 2s2 2p6 3s2 3p6 4s1 3d5

Question 21

Give the electron structure for copper Cu

Answer 21

1s2 2s2 2p6 3s2 3p6 4s1 3d10

Question 22

Give the definition for first ionisation energy

Answer 22

The first ionisation energy is the enthalpy change when one mole of gaseous atoms forms one mole of gasepus ions with a single positive charge.

Question 23

Give an example equation for first ionisation energy

Answer 23

X(g) > X+(g) + e-

Question 24

Give the definition for second ionidation energy

Answer 24

The second ionisation energy is the enthalpy change when one mole of gaseous ions with a single positive charge forms one mole of gaseous ions witha double positive charge.

Question 25

Give the definition for second ionidation energy

Answer 25

The second ionisation energy is the enthalpy change when one mole of gaseous ions with a single positive charge forms one mole of gaseous ions witha double positive charge.

Question 26

Give an example equation for second ionisation energy

Answer 26

X+(g) > X2+(g) + e-

Question 27

Give the main factors that affect ionisation energy

Answer 27

• The attraction of the nucleus (the more protons in the nucleus the stronger the attraction)
• The distance of the electrons from the nucleous (The bigger the atom the further away the outer electrons are from the nucleus and the weaker the attraction of the nucleus)
  -Shielding of the attraction of the nucleus (An electron in an outer shell is repelled by electrons in complete inner shells, weakening the attrrraction of the nucleus)

Question 28

Why are successive ionisation energys always larger?

Answer 28

The second ionisation energy of an elelement is always greater as an ion is formed by removing the first electron. This increases attraction on the remaning electrons so the energy required to remove the next electron is greater.

Question 29

Why does helium have the largest first ionisation energy?

Answer 29

Its first electron is in the first shell closest to the nucleus and has no shielding effects from inner shells. He has a bigger first ionisation energy than H as it has one more proton.

Question 30

Why do first ionisation energies decrease down a group?

Answer 30

As one goes down a group, the outer electrons are found in shells further from the nucleus and are more shielded so the attraction of the nucleus becomes smaller

Question 31

Why is there a general increase in ionisation energies across a period?

Answer 31

As one goes across a period the electrons are being added to the same shell which has the same distance from the nucleus and same shielding effect. The number of protons increases, however, making the effective attraction of the nucleus greater.