1. Valence configuration is the same.
QUESTION:
The decreasing trend in ionization energy going down a group is, is due to… A. Increasing repulsions among electrons in the valence shell B. Increasing effective nuclear charge C. Decreasing number of core electrons D. Increasing size of valence shell
Within a group,
Valence configuration is the same.
Increase in number of core electrons. But the effect is offset by increasing nuclear charge.
Effective nuclear charge is essentially the same.
Going down: distance of valence electrons from nucleus increases  weaker force
SCRIPT:
The decreasing trend in ionization energy going down a group is, is due to… A. Increasing repulsions among electrons in the valence shell B. Increasing effective nuclear charge C. Increasing number of core electrons D. Increasing size of valence shell
PAUSE
CLICK
All the atoms of elements belong to the same group have the same valence configuration. The number of electrons in the valence shell is the same.
Therefore, repulsions among electrons in the valence shell is not increasing. In fact, since the valence shells get larger, the electrons would be farther away from one another and that lessens the repulsions.
Therefore, choice A is wrong.
CLICK There is an increasing number of core electrons… which would tend to increase the push that a valence electron would feel from the core. So choice C is wrong.
The trend of increasing number of core electrons is offset by an increase in the number of protons in the nucleus. Therefore, the effective nuclear charge is essentially the same and choice B is wrong.
The correct answer is D. As you go down a group, the valence shell gets larger… That means the electrons are farther away from the nucleus ---- since the force that pulls them in is electrostatic, the strength of the force decreases.
Here’s a diagram to help you visualize the trend. In this diagram, we try to compare Beryllium and Magnesium. Both belong to group two A. Beryllium is in the second period, while magnesium is in the third period.
Note that both have two valence electrons
HIGHLIGHT 2s2 for Be and 3s2 for Mg
But Magnesium has more electrons in the core
HIGHLIGHT number of core electrons and core configuration for both Be and Mg
And Magnesium also has more protons.
HIGHLIGHT number of protons.
In this diagram, we represent the nucleus with the black circle in the middle.
The core electrons is in the small green shell around the nucleus.
The valence electrons are in the outer green shell.
Since the nucleus is positive, it will attract the electrons.
A valence electron will feel an inward pull from the protons.
CLICK CLICK But electrons repel each other. So a valence electron will also feel an outward push from the core electrons.
CLICK CLICK
And… an outward push from the other electrons in the valence shell
In other words, the inward pull on a valence electron by the nucleus is lessened due to the repulsions from the other electrons.
CLICK We could say that there is shielding effect. The electrons feel an effective nuclear charge, not the full nuclear charge
Because Beryllium and Magnesium belong to the same group, the increase in number of core electrons is offset by the increase in the number of protons in the nucleus.
So the effective nuclear charge that a valence electron on either atom would feel is essentially the same.
END RECORDING

2. Be: 4 protons 1s2 2s2
2 valence electrons 2 core electrons
Mg: 12 protons
1s2 2s2 2p6 3s2
2 valence electrons 10 core electrons

3. Video ID: © 2008, Project VALUE (Video Assessment Library for Undergraduate Education), Department of Physical Sciences Nicholls State University Author: Glenn V. Lo Narrator: Funded by Louisiana Board of Regents Contract No. LA-DL-SELECT-13-07/08