To begin our discussion of the trend in atomic radii lets consider the electron configuration for the elements in the third period, sodium through argon.

Element

Nuclear
Charge

Electron
Configuration

Na

11

1s22s22p63s1

Mg

12

1s22s22p63s2

Al

13

1s22s22p63s23p1

Si

14

1s22s22p63s23p2

P

15

1s22s22p63s23p3

S

16

1s22s22p63s23p4

Cl

17

1s22s22p63s23p5

Ar

18

1s22s22p63s23p6

 

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To develop the next portion of the table we need to discuss two new terms; valence electrons and inner core electrons.

 

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So lets determine the number of valence electrons and inner core electrons for each of the elements in our table.

Element

Nuclear
Charge

Electron
Configuration

Inner Core
Electrons

Valence
Electrons

Na

11

1s22s22p63s1

10

1

Mg

12

1s22s22p63s2

10

2

Al

13

1s22s22p63s23p1

10

3

Si

14

1s22s22p63s23p2

10

4

P

15

1s22s22p63s23p3

10

5

S

16

1s22s22p63s23p4

10

6

Cl

17

1s22s22p63s23p5

10

7

Ar

18

1s22s22p63s23p6

10

8

 

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How many valence electrons in each of the following?

a) N Answer

b) Ca Answer

c) Br Answer

Inner core electrons shield valence electrons from the nucleus. If we are on a valence electron looking back at the nucleus, the inner core electrons shield a portion of the nuclear charge from us. So the valence electron does not feel the attraction of all of the protons in the nucleus, but the attraction of an effective nuclear charge which is less than the total charge on the nucleus. The effective nuclear charge is determined by subtracting from the number of protons in the nucleus (Z), the number of inner core (I.C.) electrons that shield the valence electron from the nucleus.

Zeff = Z - I.C.

 

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What is the effective nuclear charge experienced by the valence electrons in each of the following?

a) N Answer

b) Ca Answer

c) Br Answer

 

So lets consider the valence electrons in magnesium and consider the shielding effect of its inner core electrons and whether electrons in the same shell shield the electron we are considering.

 

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So here is a question to see whether the concept of effective nuclear charge is clear.

 

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Let's review what we've discussed and extend the idea of shielding and effective nuclear charge. You have to be able to determine the effective nuclear charge on any electron in an atom in the A groups.

 

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