3.1.1 Atomic structure

Question 1

What is atomic number & what is atomic mass?

Answer 1

Atomic number - number of protons.
Mass number - number of protons + number of neutrons.

Question 2

What is the relative charge & relative mass of each subatomic particle?

Answer 2

Proton - relative charge = +1, relative mass = 1
Neutron - relative charge = 0, relative mass = 1
Electron - relative charge = -1, relative mass = 1/1840

Question 3

What are isotopes and why do isotopes of the same elements have similar chemical properties?

Answer 3

Isotopes - atoms of the same elements with the same number of protons but a different number of neutrons.
They have similar chemical properties because they have the same electronic structure (which determines its chemical properties). Their physical properties may vary slightly due to differing atomic masses.

Question 4

What is relative atomic mass?

Answer 4

The average mass of one atom of an element relative to 1/12th of the mass of an atom of carbon-12.

Question 5

What is a mass spectrometer?

Answer 5

An accurate scientific instrument used to determine the relative isotopic mass (mass of each isotope relative to carbon-12) & isotopic abundance. These are then used to calculate the elements relative atomic mass.

Question 6

What are the essential 4 steps in a mass spectrometer?

Answer 6

• Ionisation
• Acceleration
• Ion drift in flight tube
• Detection

Question 7

What are the two methods of ionisation?

Answer 7

Electrospray:
- The sample is dissolved in a volatile, polar solvent
- It is injected through a fine needle giving a fine mist or aerosol
- The tip of needle has high voltage
- At the tip of the needle the sample molecule, M, gains a proton, H^+, from the solvent forming MH^+
- M(g) + H^+ → MH^+(g)
- The solvent evaporates away while the MH+ ions move towards a negative plate
(Used preferably for larger organic molecules - the ‘softer’ conditions of this technique mean fragmentation does not occur)

Electron impact:
- 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
X(g) → X^+(g)+ e^–
(Used for elements & substances with low formula mass - can cause larger organic molecules to fragment)

Question 8

What happens in step 2 (acceleration) of a mass spectrometer?

Answer 8

The positive ions are accelerated using an electric field so that they all have the same constant kinetic energy.

KE = 1/2mv^2 or v = root(2KE/m)

(Given that all the particles have the same kinetic energy, the velocity of each particle depends on its mass. Lighter particles have a faster velocity, & heavier particles have a slower velocity)

Question 9

What happens in step 3 (ion drift in a flight tube) of a mass spectrometer?
What are the equations for time of flight (TOF)?

Answer 9

The positive ions with smaller m/z values will have the same kinetic energy as those with larger m/z and will move faster. The heavier particles take longer to move through the drift area. The ions are distinguished by different flight times.

ToF = d/v
ToF = d(root(m/2KE))
(Used to calculate relative isotopic mass)

Question 10

What happens in step 4 (detection) of a mass spectrometer?

Answer 10

The ions reach the detector & generate a small current (which is fed to computer for analysis). The current is produced by electrons transferring from the detector to the positive ions. The size of the current is proportional to the abundance of the species.

Question 11

For each isotope, what can the mass spectrometer measure?

Answer 11

A m/z (mass to charge ratio) & an abundance.

Question 12

How do you calculate relative atomic mass (a weighted atomic mass of all the isotopes)?

Answer 12

RAM = Σ(isotopic mass x relative abundance) / total relative abundance

Question 13

Blank

Question 14

How do you measure the Mr of a molecule using a mass spectrum?

Answer 14

If a molecule is put through a mass spectrometer with an Electron impact ionisation stage it will often break up and give a series of peaks caused by the fragments. The peak with the largest m/z, however, will be due to the complete molecule and will be equal to the relative molecular mass, Mr, of the molecule. This peak is called the parent ion or molecular ion.

If a molecule is put through a mass spectrometer with electrospray ionisation then fragmentation will not occur. There will be one peak that will equal the mass of the MH^+ ion. It will therefore be necessary to subtract 1 to get the Mr of the molecule. So if a peak at 521.1 is for MH^+, the relative molecular mass of the molecule is 520.1.

Question 15

How are electrons arranged in an atom?

Answer 15

They are arranged into
- principle energy levels numbered 1, 2, 3, 4, etc.
Which are split into
- sub energy levels labelled s, p, d & f (s holds max 2 electrons, p holds max 6 electrons, d holds max 10 electrons, f holds max 14 electrons).
Which are split into
- orbitals which hold up to 2 electrons of opposite spin (orbitals represent the mathematical probabilities of finding an electron at any point within certain spatial distributions around the nucleus)

An atom fills up the sub shells in order of increasing energy (note 3d is higher in energy than 4s & so gets filled after the 4s).
Each orbital is filled singly before electrons are paired up.

Question 16

How is the periodic split relating to electronic structure?

Answer 16

The periodic table is split into blocks.
A s block element is one whose outer electron is filling a s-sub shell, etc.

Question 17

How does the electronic structure for ions work?

Answer 17

When a positive ion is formed electrons are lost from the outermost shell.
When a negative ion is formed electrons are gained to the outermost shell.

Question 18

How are chromium & copper anomalies in reference to their electronic structure?

Question 19

What is first ionisation energy?

Answer 19

The enthalpy change when one mole of gaseous atoms forms one mole of gaseous ions with a single positive charge.
H(g) → H^+(g) + e^-

Question 20

How do you measure successive (2nd, 3rd, etc) ionisation energies?

Answer 20

Remove electrons one by one.

Question 21

What is second ionisation energy?

Answer 21

The enthalpy change when one mole of gaseous ions with a single positive charge forms one mole of gaseous ions with a double positive charge.
Ti^+(g) → Ti^2+(g) + e^-

Question 22

What are the factors that affect ionisation energies?

Answer 22

1. Nuclear charge (the more protons in the nucleus the greater the attraction) (the more electrons removed, the more positive the charge of the ion)
2. Nuclear radius - distance of the outer electrons from the nucleus (the bigger the atom, the further the outer electrons are from the nucleus & the weaker the attraction to the nucleus)
3. Shielding of the attraction of the nucleus (an electron in an outer shell is repelled by electrons in complete inner shells, weakening the attraction of the nucleus)

Question 23

Why are successive ionisation energies always larger?

Answer 23

• When the first electron is removed a positive ion is formed.
• The ion increases the attraction on the remaining electrons & so the energy required to remove the next electron is larger.

Question 24

Why is there a gradual increase in ionisation energy required to remove each successive electron from the same principle energy level?
Why is there a big increase in ionisation energy required to remove an electron from 2p compared to 3s (different principle energy level)?

Answer 24

More IE is needed to remove each successive electron in the same principle energy level as the charge of the ion is increasing with each electron removed - ion becoming more positive, increasing the attraction between the outer electron and the nucleus, making it harder to remove (requiring more energy).

Big increase in IE required to remove electron from 2p compared to 3s due to 2p being a lower energy level - closer to the nucleus so therefore attracted more strongly & much harder to remove. There is also less shielding as there are less inner complete shells of electrons repelling the outer electron.

Question 25

What is the general trend in ionisation energies down a group and across a period?

Answer 25

Down a group:
General decrease in ionisation energy.
As you go 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.
(- nuclear charge increases - shielding increases - atomic size increases)

Across a period:
General increase in ionisation energy.
As you go across a period, the electrons are being added to the same shell which has the same distance from the nucleus & same shielding effect.
The number of protons increases, however, increasing the nuclear charge, making the effective attraction of the nucleus greater.
(- nuclear charge increases - no increase in shielding - atomic size slightly decreases (due to the more +ive nucleus pulling outer shell further in))

Question 26

What are the anomalies to the trend in ionisation energy across period 3?

Answer 26

Easier to remove an electron from Al than Mg
(Al is starting to fill a 3p sub shell, whereas Mg has its outer electrons in the 3s sub shell. The electrons in the 3p subshell are slightly easier to remove because the 3p electrons are higher in energy and are also slightly shielded by the 3s electrons.)

Easier to remove an electron from S than P
(With sulfur there are 4 electrons in the 3p sub shell and the 4th is starting to doubly fill the first 3p orbital. When the second electron is added to a 3p orbital there is a slight repulsion between the two negatively charged electrons which makes the second electron easier to remove.)