Question

Which of the following explains the reason why atomic radii decrease across a period?
A. As you move across the period, the number of protons increases, which increases the nuclear charge. The number of electrons remains the same creating a larger Coulombic attraction and decreasing the radii.
B. As you move across the period, the number of protons and electrons increases. The increased Coulombic attraction decreases the overall size of the atom.
C. As you move across the period, the number of protons remains constant while the number of electrons increases. The decreased Coulombic attraction decreases the overall size of the atom.
D. The atomic radii do not decrease across a period. It actually increases, duh!

Answer 1

Hint : In this question, we need to comment on the reason behind the decrease in the atomic radii of the atom across a period. For this, we will go about the factors in which the radius of an atom depends on. Considering all the factors, we can find out the reason behind the decrease in the atomic radii across a period.

Complete Step By Step Answer:
Across a period, both the number of electrons and the protons increases. As the number or electrons increases, so due to repulsion the size of the atom should increase. However, the number of protons also increases. Due to which the effective nuclear charge experienced by the outermost electrons also increases so they tend to come closer to the nucleus. Thus, the electrons are attracted to the nucleus more as we go right in a period. So, the increase in nuclear charge is the dominant factor.
Hence option (A) is the correct answer.

Note :
The periodic table consists of periods and groups, the periods are the horizontal rows and groups are the vertical columns. It is interesting to note here that the atomic size increases while going down the group. This is because the electrons are added to a new shell. Thus, the factor of increase in nuclear charge is not dominant in a group. In both the cases, both the number of electrons and protons increase equally but in the case of periods, the increase in the nuclear charge is dominant.