Atomic Structure

Question 1

What is first ionisation energy

Answer 1

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

Question 2

What is second ionisation energy

Answer 2

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 with a double positive charge.

Question 3

What is electron impact ionisation

Answer 3

1. A sample is vaporised and injected at a low pressure
2. An electron gun fires high energy electrons at the sample
3. This knocks off an electron from the outer shell, forming a positive ion

Question 4

What is electro spray ionisation

Answer 4

1. A sample is dissolved in a volatile, polar solvent
2. It is injected through a fine needle, which gives a fine mist/ aerosol
3. The tip of the needle has a high voltage
4. At the tip of the needle, the sample molecules gain a proton from the solvent
5. The solvent evaporates while the positive ions move towards the negative plate

Question 5

Why has helium the largest first ionisation energy

Answer 5

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 6

Why do first ionisation energies decrease down a group

Answer 6

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 7

Why is there a general increase in first ionisation energy across a period

Answer 7

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.

Question 8

Why has Na a much lower first ionisation energy than neon

Answer 8

This is because Na will have its outer electron in a 3s shell further from the nucleus and is more shielded. So Na’s outer electron is easier to remove and has a lower ionisation energy.

Question 9

Why is there a small drop from Mg to Al

Answer 9

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

Question 10

Why is there a small drop from P to S

Answer 10

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.

Question 11

Define relative atomic mass

Answer 11

The average mass of one atom of an element divided by 1/12 the mass of one atom of carbon12