Do Now:
Sodium Chloride Pre-Assessment
Instruction:
Defination of atomic radius:
 ½ distance between nuclei of two atoms of the same element when atoms are joined

Marshmallow analogy for measurement
What changes as you go across a period?

Number of protons and electrons changes.
What stays the same?

Valance shell (highest energy level) stays the same.
What does this mean?
The electrons are all added to the same energy level. (Draw energy level 2 and keep adding more electrons to it).
At the same time, you are increasing the number of protons. This increases the pull by the nucleus on the negative electrons.
So, if the electrons are not getting placed farther away and the positive charge is increasing, what will happen to the size of the atom?
It is smaller because it has a bigger positive charge pulling on the valence electrons.
To further explain this, chemists look at the Effective Nuclear Charge:
Z eff
= Z – S
Z eff
= Effective Nuclear Charge
Z = Number of Protons
S = Shielding Constant, which can be approximated as the number of electrons in lower energy levels.

What the heck is shielding??
Electrons in lower energy levels shield electrons in higher energy levels. Think of it like insulation. Shielding blocks, or insulates, the valence electrons from some of the positive attraction to the protons in the nucleus.
For Lithium, Z = 3 and S = 2, so Z eff
= 1
For Boron Z = 5 and S = 2 so Z eff
= 3
So Boron has a stronger pull on it’s valence electrons than lithium does (3 vs 1). Boron pulls it’s valence electrons more strongly and thus has a smaller atomic radius.
So, as you go across a period, the valence shell and the amount of shielding stay the same.
However, the number of protons increases. This increases the effective nuclear charge, which pulls the valence electrons closer to the nucleus and makes the atomic radius do what? Get smaller.
Examples:
Which has a larger atomic radius?
1) carbon or beryllium
2) chlorine or argon
3) calcium or bromine
4) strontium, tin, or antimony
What changes as you go down a group?

Valence shell (highest energy level) changes

Number of protons changes
What stays the same as you go down a group?

Number of valence electrons stays the same
How do we reconcile these competing effects? More protons should pull the electrons closer but the higher energy levels should be further away!

Let’s look at the Z eff
for lithium and sodium.
Lithium: Z = 3, S = 2. Z eff
= 1
Sodium Z = 11, S = 2+8. Z eff
= 1.
If they both have the same Z eff
, which one has a bigger atomic radius? Well, sodium’s valence electron is located in which valence shell? The 3 rd . Lithium’s valence electron is located in the
2 nd
valence shell. The 3 rd
shell is further away, so if they both have the same Z eff
, then the one in the higher energy shell will be further away and thus have the bigger atomic radius.
So, as you go down a group, the effective nuclear charge stays the same, but the shells get further away and the atomic radius does what? It increases.
Practice Problems.
Which has the smaller atomic radius:
1) strontium or magnesium
2) silicon or carbon
3) iodine, fluorine, or chlorine
4) gallium, indium, or aluminum
Let’s see a review (6 min) – Note – This has to be done through finding the video via Google search. http://education-portal.com/academy/lesson/atomic-and-ionic-radii-trends-among-groups-andperiods-of-the-periodic-table.html#lesson
HW – Atomic Radius Worksheet