I cant remember whether the atomic radius increases or decreases going across the Periodic Table. And if it increases or decreases going down the periodic table. Help Pls.

 

Also, can someone explain electron affinity to me.

 

 

Thanks in advance.

Atomic radius decreases as you go down a group or across (towards the right of) a period.

 

Electron affinity is the energy change or energy given off when an electron is added to an atom (the atom will be in a gaseous state). Electron affinity is negative if energy is released.

 

[edit] or like it says here:

http://www.chemguide.co.uk/atoms/properties/eas.html

 

Defining first electron affinity

 

The first electron affinity is the energy released when 1 mole of gaseous atoms each acquire an electron to form 1 mole of gaseous 1- ions.

 

This is more easily seen in symbol terms.

 

[math] X_{(g)} + e^- \to X^{-}_{(g)}[/math]

 

It is the energy released (per mole of X) when this change happens.

That's why Hydrogen is placed at the top left=hand corner of the periodic table.

Umm, the reason I asked about atomic radius is because i think i missed it on my test today and I really want to know if I got it right or not.

 

The question was:

Atomic size generally_______

 

a. increases as you move left to right across the period

b. decreases as you move down a group

c. remains constant within a period

d. decreases as you move left to right across a period

 

 

I put b. So which one would be correct???

That's why Hydrogen is placed at the top left=hand corner of the periodic table.

 

 

So it has the greatest atomic radius of all the elements, right?

Atomic radius decreases as you go down a group or across (towards the right of) a period.

 

 

No no no no. That is definitely not correct. The sizes of the atoms INCREASES as you move down a group. Cesium is bigger than rubidium which is bigger than potassium which is bigger than sodium which is bigger than lithium. The atomic radius increases because as you move down the group you are adding more electron shells to the atoms which results in a bigger electron cloud. In addition, the nucleus is getting larger too, so the distance between the center of the nucleus and the edge of the outermost electron cloud increases as you move down.

 

When you move left to right, however, the addition of the extra protons actually outdoes the addition of the extra electrons. No new energy levels are added as you move left to right, only subshells. (I.E. you get a 3p and 3d subshell when you move towards the right, but not a 4s). So the combination of the extra protons which add an additional pull on the outer electrons, and by not adding any new electron shells, the radius of the atoms decrease. So fluorine is smaller than oxygen which is smaller than nitrogen which is smaller than carbon etc. etc.

 

Left to right, atomic radius decreases.

Top to bottom, atomic radius increases.

 

(Also, these trends exclude the noble gases and their full shells).

Just to make sure, is atomic radius the same thing as the atomic size??