Trends in atomic radius of period 3 elements
02
atomic radius (mm)
0.12
0.08
na
mg
al
st
element
a. are these elements in a period or group?
b. what is the trend for atomic radius as shown in the graph above?
c. use atomic structure and attractive forces to explain this trend.

Here's what I find  

Explanation:

A. Period or Group?

Na Mg Al Si

These elements are in Period 3.

B. Trend in atomic radius

The graph below shows that the atomic radius decreases as the atomic number increases, that is, as you go from left to right across the Period.

C. Explanation

As you go from left to right across a Period, you are adding electrons to the n = 3 shell. The electrons repel each other, so the atoms tend to increase in size.

However, you are also adding protons to the nucleus, so the attraction between the protons and the electrons increases. Each added electron can shield only about 0.3 of a proton's charge, so the nuclear attraction wins out and the atomic radius decreases.