HELP - TEST TOMMOROW ...Why does the atomic radii decrease as you go down the periodic table?

Question

When you go down the periodic table, you get more protons AND more energy levels. My impression is that the Z-effective would cancel out with the shielding when going down a group.. but I know that isn't the case.

Why is the #energy levels more important than the Z-effective when going down the periodic table?

err typo, I meant increase.. as in bigger ;)

would it be fair to say the following:

The atomic radius increases as you go down a group on the periodic table. While both the nuclear charge and the # of energy levels increases down a group, (n) is more important than (Z) in determining the atomic radius, so the end result is the atomic radius increasing.

Feedback would be appreciated. Thanks to all for your help so far.

Answer 1

The atomic radii increase as you go down a group in the periodic table. They do not decrease.

Energy levels are more important because electrons are arranged according to Pauli's exclusion principle which ensures that no two electrons have the same quantum number.

Answer 2

Vocabulary
atomic nucleus
atomic radius
effective nuclear charge
electron
period
valence electron
valence shell

Suppose you wanted to determine the size of a swarm of bees. Counting the bees won't do: you want the size or volume of the swarm, not the number of bees or the mass. One approach is to draw an imaginary line around the swarm. A few bees will cross the line, now and then, but the size of the boundary should give you some idea of the size of the swarm.
Like swarms of bees, atoms don't really have well-defined surfaces. They're soft, diffuse objects. Electrons can be found at any distance from the nucleus. After a certain distance from the nucleus the swarm of electrons gradually thins, but never entirely disappears. However, an imaginary boundary surface can be drawn around the atom that contains (say) 99% of the electron density, and we can then refer to the size of the boundary surface as the size of the atom.

What effect does an additional electron have on atomic size? Dropping extra bees into a swarm usually makes the swarm larger: the bees each have a "personal space" that the other bees won't violate. Similarly, dropping extra electrons into an atom should cause it to swell slightly, because the electrons repel each other.

What effect does an additional proton have on atomic size? The extra proton in the atom's nucleus pulls all the inner electrons closer. The outer electrons are screened by the inner electrons, but in general they too are pulled in closer, because the inner electrons can't hide all of the additional +1 charge from those in the outer shell. So an additional proton will make the atom smaller. This is rather like dropping a queen bee into the center of a swarm. The bees are attracted to the queen, and they close in to attend her. The size of the swarm goes down, even though you've increased the mass of the swarm.

Why do atoms tend to get smaller going left to right across a period? Elements on the left side of a period have fewer electrons and protons than elements on the right side. More protons make the atom smaller; more electrons make it slightly larger. The effect of the protons wins out when the electron was added to an incomplete valence shell.
Bees are attracted to the queen
in the center of the swarm. Two queens attract the bees more
than one, making the swarm smaller.



Additional bees can't penetrate
the swarm of attendants
easily. They extend the
boundaries of the swarm.
Why do atoms tend to get larger going down a column on the periodic table? Adding electrons and protons to an atom with a completely filled valence shell is another story. That will usually make the atom larger. Imagine the swarm again. This time, most of the bees are shoulder-to-shoulder around the queens. An additional bee won't be able to penetrate the dense inner layer of bees attending the queen. This bee also may not be as strongly attracted to the queen, because the queen is effectively screened by her attendants. The additional bee will have to fly around the outer edges of the swarm. The swarm will be larger.

For example, both sodium and potassium have a single valence electron. The potassium atom is much larger than the sodium atom, because potassium has a larger swarm of core electrons screening the valence electron from the nucleus. The valence electron roams the outer edges of the core. All of the smaller orbitals are occupied, and the valence electron must occupy a larger orbital (with higher n). The size of the atom is effectively determined by the size of its valence shell.

Answer 3

As we move down the group, there is new addition of shell in theexising in the so shielding effec increases and the nuclear charge is reduced considerably..so as we move down theperiod the atomic radii increases andso does the size of the element..