Periodic Trends

What’s it all mean?

Atomic Radius

Atomic radius is simply how small or

large the atom is. (measured in picometers or Angstroms)

Radius is measured from the nucleus to the edge of the electron cloud. (we often use covalent radius when an atom is interacting with another atom because then its hard to tell where on electron cloud ends and the other’s begins)

Small atomic radius(hydrogen possibly)

Larger atomic radius(Polonium possibly)

At this point it’s important to remember that electrons reside in the electron cloud, but are also organized into “shells”. Think of electron shells as layers of an onion, or the various layers of a jawbreaker candy ball.

Electron Shells

Notice how each layer of electron shell gets further and further away from the positively charged center of the atom.

That means the atoms on the outer shells feel less of a magnetic pull toward the nucleus, both because of distance and the “shielding” effect of the inner layers of electrons.

Back to Atomic Radius…

So, as you move DOWN the periodic table (moving down a column or “group”), you are adding additional electron shells.

Makes sense, right?

More Atomic Radius…

However, when you move left to right across the periodic table, you aren’t adding any additional shells. You’re just cramming more protons into the nucleus and more electrons into the current shell. It gets a little CROWDED!

If all the protons (and electrons) act like little magnets, imagine what happens to the total magnetic charge of the nucleus as you add more protons. (“effective nuclear charge”) and don’t add any electron shells…

Beryllium nucleus with 4 protons Oxygen nucleus with 8 protons

The effective nuclear charge of the oxygen atom is greater and therefore has a greater magnetic pull on the electrons in the electron cloud, bringing them in a little closer. Therefore, as you move left to right on the periodic table (across a period), atomic radius actually decreases!

Beryllium nucleus with 4 protons Oxygen nucleus with 8 protons

Atomic Radius Data

See? Atomic radius increases as you go down a group but decreases as you go across a period!

Ionization Energy

First, remember that ions are atoms that have either lost or gained electrons.

Here, a sodium atom has lost an electron (giving it a + charge) and the chlorine atom has gained an electron (giving it a – charge).

Ionization energy is the energy required to rip an electron away from an atom.

It is directly related to the size of the atom so make sure you understand atomic radius before you move on to ionization energy.

Remember these slides?

You better, because ionization energy is the result of these two concepts put together!

If atomic radius increases as you move down a group, then the electrons in the outer shells feel less and less of a pull from the effective nuclear charge—the magnetic pull of the nucleus.

Therefore, the larger the atomic radius, the easier it is to pull an electron away.

Therefore as you move down a group on the periodic table, the ionization energy decreases.

And, if atomic radius decreases as you move left to right across a period due to the increased magnetic pull of the nucleus, that must make it harder to pull an electron away!

Therefore, the ionization energy as you move left to right increases.