New Jersey NJSLA Science Grade 11 Practice Questions - New Jersey Student Learning Assessments - Science Grade 11 Exam

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A student compares the first ionization energies of sodium (Na) and magnesium (Mg), both in period 3. Magnesium has a higher first ionization energy than sodium. Which statement best explains this trend across the period?

As protons are added across a period, the nuclear charge increases while valence electrons stay in the same principal energy level, so they are held more tightly. Valence electrons move into a higher principal energy level as you move left to right, placing them farther from the nucleus. The number of occupied electron shells decreases from sodium to magnesium, so electrons are closer to the nucleus. Electronegativity decreases from sodium to magnesium, weakening the attraction between the nucleus and valence electrons.

Correct Answer:

As protons are added across a period, the nuclear charge increases while valence electrons stay in the same principal energy level, so they are held more tightly.

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