The properties of the elements of the third period vary as one goes across the period from Na to Ar. Which of the following shows how the atomic radius, effective nuclear charge and first ionisation energy vary from Na to Ar?

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BlackTom AI · Accepted Answer

The question asks how three properties vary across the third period from Na to Ar: atomic radius, effective nuclear charge, and first ionisation energy.
First, consider the general trend across a period in the periodic table: as you move from left to right, the atomic radius generally decreases because the same principal energy level is being filled while the increasing number of protons pulls the electron cloud closer. The effective nuclear charge (the net positive charge experienced by valence electrons) increases across a period due to more protons in the nucleus without a corresponding increase in shielding for valence electrons. Consequently, the first ionisation energy typically increases across the period because electrons are held more tightly by the nucleus.
Option-by-option evaluation is not possible here in full because the answer options provided are missing in the prompt (the list is empty). Normally, one would compare each proposed set of trends against the well-established across-a-period pattern described above to determine which option aligns with: (1) decreasing atomic radius, (2) increasing effective nuclear charge, and (3) increasing first ionisation energy from Na to Ar.
Given the standard trends, the correct description would be: atomic radius decreases across Na→Ar; effective nuclear charge increases across Na→Ar; first ionisation energy increases across Na→Ar. If an option labeled, for example, III states: ‘Decreases’ for atomic radius, ‘Increases’ for effective nuclear charge, ‘Increases’ for ionisation energy, that would match the expected pattern for most properties except the radius (which should be decreasing, not increasing). If an option labeled III states exactly Decreases / Increases / Increases, that would be consistent with radius decreasing, ENC increasing, IE increasing, which matches the typical across-a-period trend.
In summary, with the missing answer choices, I can’t definitively identify why “c. III” is correct or incorrect. However, the standard across-a-period trend is: atomic radius decreases, effective nuclear charge increases, and first ionisation energy increases from Na to Ar. If option III conveys that exact combination (Decreases in radius; Increases in ENC; Increases in IE), it would be consistent with the expected pattern. The absence of the actual option text prevents a precise validation.

MUF0041 Chemistry Unit 1 - Semester 1, 2025 Area of Study 2 Test 1: (Atomic Structure/Periodic Table)

The properties of the elements of the third period vary as one goes across the period from Na to Ar. Which of the following shows how the atomic radius, effective nuclear charge and first ionisation energy vary from Na to Ar?

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