ENC and Shielding Effect

Question 1

Effective Nuclear Charge

Answer 1

The net force experienced by an electron in an atom due to the positively charged nucleus

Question 2

Shielding Electrons

Answer 2

The electrons in the energy levels between the nucleus and the valence electrons
“shielding” because they “shield” the valence electrons from the force of attraction exerted by the positively charged nucleus.
This is why valence electrons require less energy to remove than the inner electrons

Question 3

Shielding electrons to protons and nucleus

Answer 3

protons are the charge of the nucleus. consider shields as walls. they shield electrons from the nucleus, they feel less force from the nucleus. This is why it is easier to remove valence electrons since they feel less force from the nucleus. some attraction with the shields but they stay in a fixed position.

Question 4

Shielding Electrons in Fluorine

Answer 4

In fluorine, there are 9 protons in the nucleus and there are 2 shielding electrons in the first energy level between the nucleus and the outer shell

Question 5

Shielding Electrons to Sodium

Answer 5

In sodium, there are 11 protons and 10 shielding electrons.
2 in the first energy level and 8 in the second energy level
Thus the valence electron in sodium experiences less force from the nucleus than the valence electrons in fluorine

Question 6

Shielding electrons in periods and their trends

Answer 6

Now look at carbon, nitrogen, and oxygen to see that within a period there is NO change in the number of shielding electrons

Trend
The # of shielding electrons stays the same within a period (except for increasing gradually and erratically across transition metals)

Question 7

Shielding electrons example withing a group

Answer 7

Going down a group, the number of protons in the nucleus increases and so does the number of shielding electrons, due to there being more energy levels

Trend
Shielding electrons follow a pattern somewhat like the number of energy levels
They increase in steps as you go down a group

Question 8

Effective Nuclear Charge (Zeff)

Answer 8

The charge felt by the valence electrons after you have taken into account the number of shielding electrons that surround the nucleus.

ENC = nuclear charge - # of shielding electrons
ENC (Zeff) = NC - SE

what’s participating in reactions is the valence shell. protons charge the nucleus. Shielding elections stay the same, but as you go across, the ENC increases due to the protons. Can never get a negative calculation since protons=electrons and you are only accounting for shielding electrons.

Question 9

Effective Nuclear Charge Trends

Answer 9

Left-to-right in a period:
The number of protons increase, but the number of shielding electrons stays the same, thus the effective nuclear charge felt by the valence electrons increases

Top-to-bottom in a group:
As you go down a group, the increase in the nuclear charge is cancelled out by the increase in shielding electrons and the effective nuclear charge stays the same

Question 10

Atomic Radius Across the Period

Answer 10

Decreases
as ENC increases, and e- are added to the same shell
Greater attraction because positivley charged nucleus and e- cause a decrease in size
(ie. pulls electrons closer to nucleus)

Question 11

Atomic Radius down the group

Answer 11

Increases
The effective nuclear charge stays the same and electrons are added to new shells, therefore, causing an increas in size.

Question 12

Reactivity of metals

Answer 12

Reactivity increases down, since electrons are further from the nucleus (ie. with less force due to shielding). Decreases across a period since more electrons are added
*metals give electrons

Question 13

Reactivity of non-metals

Answer 13

Reactivity decreases down a group, due to electron shells being added.
(ie. electrons are less attracted). Increases across a period since they take electrons, the nucleus is more charged. This is because the proton number increases.