Question

periodic trends practice
i. atomic radius
trend across periods
as you move from left to right across a period, atomic radii will ______.
why: the number of protons in the nucleus increases, resulting in a stronger nuclear charge.
therefore, electrons are pulled in toward the nucleus. notice that the number of energy levels remains the same.
trends down groups
as you move down a group, atomic radii will ______.
why: this is because there are more energy levels and
electrons in these levels are not as attracted to the nucleus. notice that the number of protons in the nucleus is increasing (resulting in an increase in nuclear charge) is offset by increased distance.
which of the following factors contributes to the relatively greater atomic size of the higher atomic number elements within a particular group of the periodic table?
a. more energy levels
b. larger nuclei
c. greater number of protons
d. smaller number of valence electrons
(images: beryllium, boron, magnesium atomic models with electron shells and nuclear charge labels)

Explanation:

First, let's analyze the first blank (Trend across periods):

When moving from left to right across a period, the number of protons in the nucleus increases, which leads to a stronger nuclear charge. This stronger nuclear charge pulls the electrons in toward the nucleus more effectively. Since the number of energy levels (electron shells) remains the same across a period, the atomic radius will decrease (because the electrons are pulled closer to the nucleus).

Now, the second blank (Trend down groups):

As we move down a group, the number of energy levels (electron shells) increases. Even though the number of protons in the nucleus increases (which would tend to pull electrons in), the increase in the number of energy levels means that the outermost electrons are further from the nucleus. The increased distance offsets the increased nuclear charge, so the atomic radius will increase.

For the multiple - choice question (Which factor contributes to larger atomic size of higher atomic number elements in a group):

• Option A: More energy levels. As we move down a group, the number of energy levels (electron shells) increases. Each new energy level is further from the nucleus, so the outermost electrons are further away, leading to a larger atomic radius. This is the correct factor.
• Option B: Larger nuclei. A larger nucleus (more protons) would tend to pull electrons in, which would decrease the atomic radius, not increase it. So this is incorrect.
• Option C: Greater number of protons. Similar to option B, more protons mean a stronger nuclear pull, which would make the atomic radius smaller, not larger. So this is incorrect.
• Option D: Smaller number of valence electrons. The number of valence electrons in a group remains the same (elements in the same group have the same number of valence electrons). So this is not a factor for the increase in atomic size down a group.

Answer:

s:

1. Trend across periods: decrease
2. Trend down groups: increase
3. Multiple - choice answer: A. more energy levels