atomic structure

Question 1

Why does the first ionisation energy of atoms generally increase across a period?

Answer 1

Number of protons increases, shielding stays the same, so attraction of outer electrons to nucleus increases

Question 2

Why is the first ionisation energy of boron less than that of beryllium?

Answer 2

Outermost electron in B is 2p, outermost electron in Be is 2s, 2p electron in B better shielded than 2s electron in Be, so it is less attracted to nucleus

Question 3

Why is the first ionisation energy of oxygen less than that of nitrogen?

Answer 3

2p electron is paired in O but unpaired in N, so in O there is more repulsion in the orbital which makes the electron easier to remove

Question 4

Why does helium have the highest first ionisation energy of all the elements?

Answer 4

No shielding in 1st period so electrons closely held than in other periods, and more protons than hydrogen so greater attraction to nucleus

Question 5

Why is the second ionisation energy of an atom always greater than the first?

Answer 5

Less electrons, so less electron repulsion

Question 6

Why is the second ionisation energy of sodium much greater than the first?

Answer 6

1st electron removed from 3s, second electron removed from 2p so much less shielding

Question 7

Why does atomic size decrease across a period?

Why does atomic size increase down a group?

Answer 7

Number of protons increases, shielding stays the same, so attraction of outer electrons to nucleus increases and they move closer
More shells, so more shielding, so attraction of outer electrons to the nucleus decreases and they are pushed further away

Question 8

Why are cations always smaller than the corresponding atoms?

Why are anions always larger than the corresponding atoms?

Answer 8

Less electrons, so less repulsion, so electrons can get closer to the nucleus
More electrons, so more repulsion, so electrons are pushed further away

Question 9

define relative atomic mass (2)

Answer 9

• avg mass of an atom of an element
• compared to 1/12th the mass of an atom of C-12

Question 10

give 2 reasons why its necessary to ionise isotopes of element before analysed in TOF MS

Answer 10

• ions will interact w and be accelerated by electric field
• ions create a current when hitting detector plate

Question 11

describe process of electrospray ionisation, give eqn to represent ionisation of P in this process

Answer 11

• P dissolved by being added to solvent
• solvent injected thru needle at high voltage
• so it gains a H+
• P + H+ -> PH+