Chemistry I: Periodicity Notes
What have you learned about the periodic table up to now?
Let’s learn some more:
Atomic Size (aka atomic radius)
Group Trend:
As you move down the periodic table, what happens to the atomic size?
Why?
Periodic Trend: As you move right across the periodic table, what happens to the atomic size?
Why?
Summary:
Sample Problem: Arrange the following atoms in order from smallest to largest:
Ge, Sr, S, Si, Rb, F, Ca, P, Ga
Review of Atomic Radius
We said that atomic radius decreases as you move across a period to the right on the periodic table. This is
because moving to the right increases the number of protons in the nucleus. This increased strength of the
nucleus pulls the electrons in tighter, making the atom smaller.
We also said that atomic radius increases as you move down a group on the periodic table. This is because
moving down increases the number of energy levels. The higher the energy level, the further away from
the nucleus the energy level is.
I’m confused about something:
Since the highest energy level of Rb is 5, and the highest energy level of K is 4, Rb should be
bigger than K, right?
But Rb has 37 protons, and K has 19 protons. If Rb’s nucleus is stronger, shouldn’t that pull the
electrons in tighter, thus making it smaller than K?
Explain:
Ion Radius
Try to do this on your own:
What do the following atoms/ions have in common: Ne, F-, O2-, Mg2+, Na+
Arrange the above atoms/ions in order from smallest to largest. (Hint: consider the nucleus)
In the above example, the number of electrons for each atom was the same. Let’s consider examples where
the numbers of electrons are different:
Which is bigger: F or F-? Why?
Which is bigger: Na or Na+? Why?
Atom/Ion Size Sample Problems
Arrange each of the following sets of atoms/ions in order from smallest to largest radius:
1) Eu, U, Am, Tb
2) Rn, Ra2+, At-, Fr+
3) Cl, Ar, Li+, Se, S, Se2-, He
4) Fe, Fe2+, Fe3+
Periodic Trends Continued…
Ionization Energy
What is ionization energy?
What is the periodic trend, and why is it so?
Examples
Na
496 kJ/mol
Mg
738 kJ/mol
K
419 kJ/mol
What does “1st Ionization Energy” refer to as opposed to “2 nd Ionization Energy?”
Sample Problems
1. The ionization energies of all noble gases are extremely high. Explain.
2. Arrange the following atoms in order of increasing ionization energy: In, Ar, Rb, Ga, Kr, Cs
3. Examine the following ionization energies. Explain.
Si
786.2 kJ/mol
P
1012 kJ/mol
S
999.6 kJ/mol
Cl
1255 kJ/mol
Ar
1520 kJ/mol
Electronegativity
What is electronegativity?
What is the periodic trend, and why is it so?
We do not include noble gases in the electronegativity scale. Why not?
Reactivity
What is the periodic trend, and why is it so?
Electron Affinity
What is electron affinity, and what is the periodic trend?
Sample Problem: Arrange the following atoms in order of increasing electron affinity:
Si, Cl, K, Al