Many Electron Atoms And Periodic Trends

Question 1

What were Bohr’s postulates?

Answer 1

Electrons orbit the nucleus in distinct circular paths at certain distances

Each orbit/shell corresponds to a certain energy level n

Electrons can only gain and lose energy by jumping between shells (∆E=hv)

Question 2

What is quantum number n?

Answer 2

The principal quantum number - shell number

n = radial nodes + angular nodes + 1

Question 3

What is quantum number l?

Answer 3

Angular momentum quantum number - the shape of the orbital / number of angular nodes

L = 0 s orbital
L = 1 p orbital
L = 2 d orbital

Question 4

What is quantum number ml?

Answer 4

Magnetic quantum number - gives direction of the orbital

ml runs from -l to +l in steps of 1

Question 5

How many orbitals are there in any given shell?

Answer 5

n^2

Question 6

How do you find the number of nodes an orbital has?

Answer 6

n - 1

Question 7

What’s the Pauli exclusion principal?

Answer 7

It dictates that two electrons with the same spin cannot occupy the same orbital

Question 8

What is the Aufbau principle?

Answer 8

Orbitals are filled from the lowest energy state orbital first

Question 9

What is Hund’s rule?

Answer 9

In degenerate orbitals, orbitals will fill up with single electrons of parallel spin before they are paired up - spin-pairing costs energy

Question 10

What do angular nodes have to do effective nuclear charge?

Answer 10

For a given shell, orbital with higher quantum number l have more angular nodes - thus penetrating the nucleus to a lesser extent and hence have higher energies.

E.g. 2p orbital has one angular node at the nucleus, experiencing a lesser effective nuclear charge than a 2s orbital and therefore is of higher energy

Question 11

Define relative atomic mass Ar

Answer 11

Ar is defined as the ratio of the average mass of atoms of a chemical element to the atomic mass constant, which is a twelfth of the mass of an atom of carbon 12

Question 12

What is Avogadro’s constant?

Answer 12

NA = 6.022 x 10^23 mol^-1

Question 13

What are the general principles of atomic radius when going down a group or across a period?

Answer 13

Atomic radii increase down a group due to increasing number of shells

They decrease across a period as effective nuclear charge increases

Question 14

Why does atomic size matter?

Answer 14

Because it determines the coordination sphere of an atom

Si can expand its coordination sphere whereas C cannot

Question 15

Define ionisation energy

Answer 15

Ionisation energy is the minimum energy needed to remove an electron from a gaseous atom

Question 16

Why is the first ionisation energy always endothermic?

Answer 16

Because it requires energy to remove an electron from a neutral atom

Question 17

What’re the trends of ionisation energies across periods and down groups?

Answer 17

Ionisation energies decrease down groups to to increased shielding

However they increase across periods because of increase in nuclear charge - lack shielding

Question 18

Why is Boron’s 1st IE less than Beryllium’s?

Answer 18

Because the electron in Boron’s 2p orbital is less tightly bound than Beryllium’s 2s electron

Question 19

Why is Nitrogen’s 1st IE higher than Oxygen’s?

Answer 19

Because the nitrogen atom has a relatively stable half-filled p-shell, more stable than the spin paired electron in oxygen’s 2p orbital

Question 20

Why are 2nd IE higher - and why is Li and Na so extreme?

Answer 20

2nd ionisation energies are higher because electrons are now being removed from a +vely charged ion

Li and Na are extremely high because electrons are now being stripped from core electrons

Question 21

What is electron affinity?

Answer 21

EA is a measure for an atom to gain an electron

Question 22

What is electron affinity defined as?

Answer 22

It’s defined as the negative of electron gain

EA = -EG

Question 23

What happens to electron affinity going left to right cars a period?

Answer 23

EA increases because the energy of the vacant orbital decreases

Question 24

1st electron gain energies can be exo or endothermic, what are 2nd EG energies?

Answer 24

ALWAYS endothermic

Question 25

Define electron negativity

Answer 25

Electronegativity is defined as the power of an atom in a molecule to attract electron density to itself