Atomic structure (1)

Question 1

Define relative isotopic mass and relative atomic mass based on 12C (2)

Answer 1

1. The relative isotopic mass of an isotope is the mass of the isotope relative to 1/12 of the mass of a carbon-12 atom
2. The relative atomic mass of an element is the weighted average of the masses of its isotopes relative to 1/12 of the mass of a carbon-12 atom.

Question 2

Define 1st Ionisation Energy (1)

Answer 2

1. Energy required to remove one mole of electrons from one mole of gaseous atoms to form one mole of gaseous ions

Question 3

Explain why there is a general increase in first ionisation energy across a period (3)

Answer 3

1. Across the period nuclear charge increases, increasing the force of electrostatic attraction between the nucleus and outer electron making it harder to lose the outer electron
2. However increase in electrons, increases electron shielding which would make it easier to remove the outer electron on its own
3. Increase in nuclear charge is more significant than increase in shielding so more energy is needed to remove the outer electron and so 1st IE increases

Question 4

Explain why there is a decrease in 1st ionisation energy down a group (4)

Answer 4

1. Increase in atomic radius (additional shell) reduces the force of attraction between nucleus and outer electron
2. Increase in shielding repels outer electrons, making it easier to be lost
3. However, increase in nuclear charge opposes this force by increasing the force of attraction between the nucleus and outer electron
4. Overall, increase in atomic radius and shielding is more significant than increase in nuclear charge so less energy is needed to remove the outer electron and 1st IE decreases

Question 5

How does atomic emission spectra provide evidence for the existence of quantum shells (4)

Answer 5

1. When gaseous atoms are given energy, the electrons move to higher energy levels
2. When they return to lower energy levels, they emit electromagnetic radiation that can be analysed
3. Only certain frequencies of radiation are emitted (rather than a continuous spectrum) and atoms of a particular element radiate the same set of frequencies
4. This suggest that energy of electrons in atoms can only have certain fixed values (not continuous range of values)

Question 6

Explain how successive ionisation energies provide evidence for the existence of quantum shells and the group an element belongs to (5)

Answer 6

1. Big jumps in values of successive ionisation energies suggest that the next electron is being removed from a lower quantum shell
2. The electron in the lower quantum shell is significantly closer to the nucleus and so experiences a stronger electrostatic attraction
3. Shielding is also reduced but this is less significant than the shift in quantum shells
4. Therefore, more energy is needed to remove the outer electron and so the ionisation energy suddenly increases
5. The no. Of electrons removed before a big shift in ionisation energies can suggest the no. Of electrons in each quantum shell, and thus the group of the element can be predicted