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Periodicity Review Answer Key

1) Explain the phenomenon of shielding. In your explanation, indicate which electrons are

effected and why.

Shielding occurs when an atoms outermost electrons feel less of the positive pull of the nucleus

due to inner electrons getting in the way.

Which elements are more prone to be affected by electron shielding?

Electron shielding increases when there are more inner electrons to block the nuclear pull of

valence electrons. Since elements with higher atomic mass have more energy levels, shielding willbe greater when moving down any group in the periodic table.

2) What is electronegativity and how is it influenced by shielding?

Increased shielding causes valence electrons not to be held to an atom as tightly, thereby increasing

the chance that theyll be given off. Electronegative elements tend to attract electrons; therefore,

shielding decreases electronegativity.

3)Explain how the atomic radius of an element can be used to gather information aboutelectronegativity; both within rows and within columns.

Moving left to right within a row, the decrease in atomic radius is an indication of increased nuclear

charge pulling on the same number of energy levels. This greater pull is associated with greater

electronegativity -- the tendency to attract and hold electrons more closely.

Moving down a group, the addition of energy levels causes an increase both in radius and in the

resulting amount of shielding of outer electrons. Such elements are less likely to hang on to theirvalence electrons and are therefore considered to be less electronegative.

4)Moving side to side within a period, explain how electronegativity of elements changes and

why.

Elements in the same period have the same number of energy levels and therefore the same amount

of shielding. The only variable affecting electronegativity within a period is therefore the increase

in nuclear pull. This causes electronegativity to increase while moving left to right within a period,since elements to the right have more protons pulling on the same # of energy levels.

5)Explain what impact electronegativity has upon cation formation. Do the same for anion

formation.

Electronegative elements have a greater ability to hang onto valence electrons and to attract even

more electrons, thus attaining a negative charge. Elements lacking electronegativity cannot hold hevalence shell very well, resulting in the loss of valence electrons and a resulting positive charge.

6) Which two groups (columns) of elements are least likely to ionize? Explain your

answer.

The noble gases in column 18 have the greatest nuclear pull in their respective rows; and are

therefore the least likely to lose electrons and become cations. Since the addition of electrons

would require the creation of an entirely new energy level that will be heavily shielded, they wontbecome anions either.

The elements in column 14 are less likely to ionize because of the medium nuclear pull. Its greatenough to keep the 4 valence electrons that are already there, but not enough to readily attract

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additional electrons. Elements to the left of this column will become cations because of the lesser

nuclear pull; elements to the right will become anions because of the greater nuclear pull.

7) Chlorine has less of a nuclear pull than Argon; yet it is MORE likely to pull in an extra

electron. They both have the same number of energy levels! Explain in detail what

prevents Argon from doing this very easily.

Chlorine does not have to add an additional energy level to accommodate an extra electron. Argon,

with its full outer shell, would have to add an extra electron to a new energy level that would be tooheavily shielded. Argons extra proton compared to Chlorine is not enough to make up for the

increased shielding of a new energy level.

8) Draw energy level diagrams for Lithium and for Bromine. Which one experiences a

greater amount of shielding of outer electrons?

Bromine has more energy levels and therefore more shielding.

9) Explain why, despite your answer to #9, Lithium is still more likely than Bromine to

lose an electron. Given the energy level diagrams, why is it so easy for a person to be

mislead into believing otherwise?Generally, greater shielding results in less electronegativity and a greater tendency to lose electrons.However, nuclear pull is also a factor. Lithium has the least nuclear pull within its period, making

it easier to lose an electron. Bromine has the greatest nuclear pull within its period, making it easier

to GAIN an electron.

11) How does a metals electronegativity influence its reactivity?

The less electronegative a metal atom, the less likely it is to hang on to its outermost electrons. A

metal that loses its electrons more readily is more reactive.

12) Electron shielding and nuclear pull are the two competing factors that influence

electronegativity. Which of these is the primary reason Calcium reacted more readily than

Magnesium? Explain in detail, using energy level diagrams.

Calcium experiences greater electron shielding than Magnesium because it is lower in the AlkalineEarth metals group and has more energy levels. Calcium will therefore be less electronegative, lose

its electrons more readily, and have a higher reactivity. This is why Calcium fizzed when placed in

water, whereas Magnesium required the addition of an acid in order to react in water.

13) Which of the two factors from the previous question is the primary reason Sodium is

more reactive than Magnesium and Aluminum is less reactive than both? Explain in detail,

using energy level diagrams.

Sodium, Magnesium, & Aluminum are in the same energy level and will therefore experience

the same shielding. Sodium however, has less nuclear pull; which will make it less

electronegative and more likely to lose its outer electron. Aluminum has the greatest nuclearpull (and highest electronegativity) of the three, making its electrons less likely to be pulled

from the atom in a chemical reaction.