Periodic Table Class #2

OB: Trends of the periodic table. (atomic mass + atomic size)Trend: A general pattern or tendency; consistent and repetitive

On the periodic table there are many periodically repeating patterns.

The table itself is arranged in order of increasing atomic number (which is the number of electrons = the number of protons.

There are group trends (that follow a pattern going down groups).

There are period trends (that follow a pattern going across periods).

We’ll start to examine these patterns now.

The first and simplest trend is atomic mass.

To determine the group trend for atomic mass we’ll look at atomic mass of four elements in two different groups. Let’s do group 2 and group 16:

Group 2

Atom mass

Be

Mg

Ca

Sr

Group 16

Atom mass

O

S

Se

Te

It should be clear that going down these and all the groups of the periodic table, that the atomic mass gets larger (because the atoms get larger.

Group 2

Atom mass

Be 9 amu

Mg 24 amu

Ca 40 amu

Sr 88 amu

Group 16

Atom mass

O 16 amu

S 33 amu

Se 79 amu

Te 128 amu

To state this properly: The group trend for atomic mass is increasing.

This time let’s do the period trend for atomic mass, we’ll examine period 3 and 4.

Remember, whenever possible, use 4 atoms in a row to measure a trend.

atoms Na Mg Al Si

masses

atoms K Ca Sc Ti

masses

This time let’s do the period trend for atomic mass, we’ll examine period 3 and 4.

Remember, whenever possible, use 4 atoms in a row to measure a trend.

atoms Na Mg Al Si

masses 23 amu 24 amu 27 amu 28 amu

atoms K Ca Sc Ti

masses 39 amu 40 amu 45 amu 48 amu

How can we state this trend???

The period trend for atomic mass in increasing.

When ever you state a trend, it’s always as a complete sentence.

Start with: “The period trend…” or “The group trend…”

Then, state the trend itself…

Finish with increasing or decreasing.

It’s easy, the rules are clear. Memorize the way to do this, but not the actual trends. The trends are right on the table to figure out every time you need them.

Always use 4 atoms in a row to measure a trend.

Trend #2 Atom Size (atomic radius in picometers)

How big is a picometer?A meter is a basic unit of length in the metric system.

A picometer is 1 x 10-12 of one meter, or:

One picometer = 1 pm = 0.000000000001 m

That’s one trillionth of one meter. With a “T” - a Trillionth!

Really, really tiny, but NOT ZERO.

Using four atoms in a row from groups 1 and 17, determine the group trend for atomic size, and state it in one sentence in proper form.

Group 17

Atom radius

F

Cl

Br

I

Group 1

Atom radius

Li

Na

K

Rb

Group trend for atomic

radius:

Using four atoms in a row from groups 1 and 17, determine the group trend for atomic size, and state it in one sentence in proper form.

Group 17

Atom radius

F 60. pm

Cl 100. pm

Br 117. pm

I 136 pm

Group 1

Atom radius

Li 130. pm

Na 160. pm

K 200. pm

Rb 215 pm

The group trend for atomic radius is

increasing.

Because each time you go lower in the group, the atoms are adding orbitals.

Each period number increase is an increase of orbital number.

Why is the group trend for atomic radius size increasing?

(write this down!)

This time we’ll look over the period trend for atomic radius using the first 4 atoms of period 4 and of period 5.

Period 4 Atoms K Ca Sc Ti

Radiusin pm

Period 5 Atoms Rb Sr Y Zr

Radiusin pm

Period 4 Atoms K Ca Sc Ti

Radiusin pm 200. 174 159 148

Period 5 Atoms

Rb Sr Y Zr

Radiusin pm 215 190. 176 164

The period trend for atomic radius is decreasing!

How could the atomic radius decrease as you go across the period?

What do these atoms have changing that could affect that?

Let’s look at period 4 to start:

Atom: Rb Sr Y Zr

# of orbitals 4 4 4 4

# protons 37 38 39 40

Radius in pm 215 190. 176 164

Going across a period, each atom has the SAME NUMBER OF ORBITALS for the electrons, but each successive atom has ONE MORE PROTON, pulling harder and harder on those negative electrons in this 4th orbital.

The nucleus becomes increasingly more positive, the inward pull on the electrons becomes greater and greater, making the atomic radius shrink.

The Trend for Atom Size, in picture form.

What happens with the atomic sizes of

Co, Ni, Cu and Zn?

Look at those atomic radii now.

Atom: Co Ni Cu Zn

Atomic radius

in pm

Atom: Co Ni Cu Zn

Atomic radius

in pm118 117 122 120.

If the period trend for atomic radius is decreasing, how can copper be bigger than cobalt nickel? How can zinc be bigger than cobalt and nickel?

Trends are patterns that are regular and predictable, but not the law.

These are exceptions to the trend. You already know a few another “exceptions to a trend”, The AlPo dog food exception to the metalloid “rule”.