'Periodic Trends: Atomic Radius' Chemistry Worksheet (Page 2 of 2) in pdf

Draw a diagram of an atom. Label these parts: nucleus, edge of electron cloud, and atomic radius.

Unfortunately, the definition of atomic radius is not precise. The electron cloud that surrounds

the nucleus is spherical, but it does not have an exact boundary. Nevertheless, there are several ways to

estimate the radius of an atom. These methods are based on a variety of techniques used to measure the

distance between the centers of atoms in various materials. For example, the distance between two

chlorine nuclei in a Cl

molecule is found to be 0.198 nanometers. Therefore, a value of 0.198 ÷ 2 =

0.099 nanometers is a good estimate of the radius of a chlorine atom. Similarly, the distance between

carbon atoms in diamond (a form of pure carbon) is 0.154 nanometers. The radius of a carbon atom is

0.154 ÷ 2 = 0.077 nanometers.

What is a diamond composed of? __________________________________________________________

You might think that atoms with more electrons would have larger atomic radii. However, this

idea is only half true at best. Study Figure 5-16 (see front of this paper), and you will notice two distinct

trends:

1. Atoms get larger going down a group. In Group 1A, the atomic radii progress from 0.152 nm

for a lithium atom to 0.262 nm for a cesium atom.

2. Atoms get smaller moving from left to right across each period. For example, across the

second period, the atomic radii decrease from 0.152 nm for a lithium atom to 0.064 nm for a

fluorine atom.

Across a row, atomic radius __________________. Down a column, atomic radius _________________.

Why do these trends exist? The first trend is the easier to explain. As you move down a group,

the principal quantum number of the outermost electrons increases. For example, as you move down

Group 1A, the outermost electron resides in orbitals from 1s

to 2s

to 3s

and up to 7s

. Electrons with a

larger principal quantum number are found in orbitals that extend farther away from the nucleus, which

makes the atomic radius larger.

Why does cesium have a larger atomic radius than lithium? _____________________________________

________________________________________________________________________ _____________

The second trend requires a bit more thought to understand. Recall that in any period, the outer

electrons of each element are in orbitals with the same principal quantum number. If principal quantum

number were the only factor that influenced the size of an orbital, then all elements in a period would

have atoms of the same size. So, another factor must be at work.

Recall that as you move from left to right across a period, the atoms’ nuclei gain more protons.

As a rule, atoms that have more positive charge in their nuclei exert a stronger pull on the electrons in a

given principal quantum level. In other words, as you look at the elements from left to right across a

period, the atoms’ outer electrons are increasingly attracted to the nucleus. A stronger attractive force

shrinks the electrons’ orbitals and makes the atoms smaller.

Why does one atom have more positive charge in the nucleus than another? ________________________

___________________________________________________________________________ __________

What causes atoms to get smaller as you move from left to right across a row? ______________________

________________________________________________________________________________ _____

'Periodic Trends: Atomic Radius' Chemistry Worksheet Page 2