Download K has two shells (n=4 vs n=2) more than Li, so it

In-Class Activity: Predicting Trends in the Periodic Table
Sparks, CH301
We can utilize the effective nuclear charge, Zeff, to predict trends in atomic radii, ionic radii, and
ionization energy.
Atomic radii: The “size” of the atom. Keep in mind that most of an atom is actually empty space and
there is no defined edge of an atom. We measure the distance BETWEEN two atoms and from that infer
a radius.

Which would you expect to have the larger atomic radius: Li or K?
K has two shells (n=4 vs n=2) more than Li, so it would be much larger.

Which would you expect to have the larger atomic radius: C or O? (Consider all you know about
the electronic structure of atoms: principle quantum number, effective nuclear charge, etc.)
C will have the slightly larger radius because it has one less proton and thus, a lower
effective nuclear charge.
Ionic Radii
 Cations (positive ions) are always _______ their respective neutral atoms. Why?
A. smaller than
B. the same size as
C. larger than
When you remove an electron, its corresponding proton remains, so all of the electrons still on the
atom now feel an additional pull from that proton which shrinks the radius.

Anions (negative ions) are always _______ their respective neutral atoms. Why?
A. smaller than
B. the same size as
C. larger than
In the opposite case, you’re adding a negatively charged electron to an area that already has electrons.
They’re going to repulse each other and spread out to make room.
An isoelectronic series consists of ions that have the same number of electrons.

Which would have a larger radius: Na+ or Mg2+? Why?
Na+ has the larger radius because it has a fewer number of protons to pull in the same
number of electrons, so each electrons feels less effective nuclear charge.

The three ions, N3-, O2-, and F- have these three ionic radii (in no particular order): 1.71 A, 1.19 A,
and 1.26 A. Which of these three values would be the radius for N3-?
N3- will have a radius of 1.71 A. It has the largest negative charge and has the least
number of protons.

Arrange these elements in order of increasing ionic radii: S, K, Cl
K < Cl < S
First you have to think about what charged ion each element will make, K+ Cl- and S2-. Between these
three, there are equal numbers of electrons but different numbers of protons. Potassium has the most
protons and can pull in its electrons closest. This same trend applies to chlorine and then sulfur (in that
order)
Ionization Energy: Energy required to remove an electron from an atom. We expect that the first
electron to be removed will be an outer, valence electron.
First ionization energy: The minimum amount of energy required to remove the most loosely bound
electron from an isolated gaseous atom to form an ion with a +1 charge.
– Atom(g) + energy  ion+(g) + e– Mg(g) + 738kJ/mol  Mg+ (g) + eSecond ionization energy: energy needed to remove an electron from a singly ionized ion (an X+ ion)
– ion+ + energy  ion2+ + e– Mg+ + energy  Mg2+ + e–

Note that this is the energy to remove just the second electron, not the total energy to remove
both electrons. The total energy to remove both electrons would be IE1 + IE2.
Which is always higher: IE1 or IE2. Why?
IE2 will always be higher. For the second ionization energy, you’re trying to remove an electron from
something that is already positively charged (more protons than electrons)
 As you move across a period, you would expect IE1 to (increase/decrease). Why?
more answers after Oct 2nd class
 As you move down a group, you would expect IE1 to (increase/decrease). Why?
The trend as we move across a period for ionization energies isn’t perfect. The special stability of
partially and fully-filled subshells effects this trend.

Based on the general trends, you would expect the first element in each pair below to have a
lower IE1 than the second element. Which pair is likely an exception to the trend? (For which
pair would you expect the second element listed to have the lower IE1?)
A. Na and Mg
B. Mg and Al
C. Al and Si

Draw a graph of atomic number versus ionization energy for the 2nd period elements.