### Correct Option: A. Atomic size decreases due to increased nuclear charge pulling electrons closer to the nucleus. #### Detailed Explanation: 1. **Understanding Atomic Size**: - Atomic size, or atomic radius, refers to the distance from the nucleus of an atom to the outermost shell of electrons. It is a measure of how large an atom is. 2. **Trends Across a Period**: - The periodic table is organized into rows (periods) and columns (groups). As you move from left to right across a period, you are adding protons to the nucleus and electrons to the same principal energy level (shell). 3. **Increased Nuclear Charge**: - As you move from left to right, the number of protons in the nucleus increases. This increase in positive charge (nuclear charge) attracts the negatively charged electrons more strongly. - For example, in the second period, lithium (Li) has 3 protons and 3 electrons, while fluorine (F) has 9 protons and 9 electrons. The greater positive charge in fluorine pulls its electrons closer to the nucleus compared to lithium. 4. **Shielding Effect**: - Although electrons are added as you move across a period, they are added to the same energy level. The shielding effect (where inner electrons repel outer electrons) does not significantly increase because there are no new inner shells being added. Thus, the effective nuclear charge felt by the outermost electrons increases. - This means that the outer electrons experience a stronger attraction to the nucleus, leading to a decrease in atomic size. 5. **Conclusion**: - Therefore, the correct statement is that atomic size decreases as you move from left to right across a period due to the increased nuclear charge pulling electrons closer to the nucleus. #### Why Other Options Are Incorrect: - **Option B: Atomic size increases because of the addition of electron shells.** - This statement is incorrect because, while it is true that atomic size increases with the addition of electron shells, this phenomenon occurs as you move down a group (not across a period). Across a period, no new shells are added; instead, electrons are added to the same shell. - **Option C: Atomic size remains constant as the number of protons and electrons increases equally.** - This option is misleading. While it is true that protons and electrons increase in number, the atomic size does not remain constant. The increasing nuclear charge leads to a stronger attraction between the nucleus and the electrons, resulting in a decrease in atomic size. - **Option D: Atomic size fluctuates unpredictably across a period.** - This statement is false. The trend in atomic size is predictable and consistent; it decreases steadily across a period due to the reasons explained above. ### Summary for Revision: - Atomic size decreases from left to right across a period due to increased nuclear charge. - Increased nuclear charge pulls electrons closer to the nucleus, reducing atomic radius. - The shielding effect does not significantly change across a period since no new electron shells are added. - Understanding these trends is crucial for predicting the behavior of elements in the periodic table.