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which of the following will have a lower ionization energy than scandiu…

Question

which of the following will have a lower ionization energy than scandium (sc)?
nesium (mg)
helium (he)
calcium (ca)
titanium (ti)

Explanation:

Brief Explanations

Ionization energy generally increases across a period (left to right) and decreases down a group (top to bottom) in the periodic table. Scandium (Sc) is in period 4, group 3.

  • Magnesium (Mg): Period 3, group 2. It is to the left and above Sc. Ionization energy increases across a period and decreases down a group, but Mg is in a lower period and left, but its ionization energy is actually higher than Sc? Wait, no, let's re - evaluate. Wait, Sc has electron configuration [Ar] 3d¹ 4s². Mg has [Ne] 3s². Helium (He) is a noble gas with very high ionization energy. Calcium (Ca) is in period 4, group 2. Sc is period 4, group 3. In the same period, as we move from left to right, ionization energy generally increases (because effective nuclear charge increases). But Ca is in group 2, Sc in group 3. However, Ca has a larger atomic radius than Sc (since in the same period, moving right, atomic radius decreases). Also, the outermost electron of Ca is in 4s², and for Sc, the outermost electrons are in 4s² (and one in 3d). But the key is that Ca is in group 2, Sc in group 3. Wait, actually, the trend for ionization energy in period 4: from K (group 1) to Ca (group 2) to Sc (group 3) to Ti (group 4), etc. The ionization energy for Ca: the electron is removed from 4s², which is a filled subshell? No, 4s² is a filled subshell, but Sc has 4s² 3d¹. Wait, no, the first ionization energy: Ca has electron configuration [Ar] 4s². Sc has [Ar] 4s² 3d¹. When we remove an electron from Ca, we are removing from 4s² (a filled subshell, but in Ca, the 4s electrons are more shielded? Wait, no, the effective nuclear charge on Ca's 4s electrons: Ca has atomic number 20, Sc has 21. So Sc has a higher effective nuclear charge. But the electron in Ca is in 4s, and in Sc, the outermost electron (for first ionization) is also in 4s? Wait, no, the first ionization energy of Sc: we remove the 4s electron? Wait, actually, the electron configuration of Sc is [Ar] 3d¹ 4s². The 4s electrons are lower in energy than 3d? No, actually, 4s fills before 3d, but once filled, 3d is lower in energy. Wait, maybe I got it wrong. Let's recall the trend: in general, ionization energy increases across a period. But there are exceptions. For example, from group 2 to group 3, sometimes ionization energy decreases because the electron is going into a p - orbital (but Sc is group 3, d - orbital). Wait, Ca is group 2, period 4. Sc is group 3, period 4. The first ionization energy of Ca: let's check the actual values. The first ionization energy of Ca is about 589.8 kJ/mol, and Sc is about 633.1 kJ/mol. Wait, that can't be. Wait, no, maybe I mixed up. Wait, no, actually, the first ionization energy of Mg is about 737.7 kJ/mol, Ca is about 589.8 kJ/mol (decreases down the group, group 2: Mg, Ca, Sr, Ba; ionization energy decreases down). Sc is in group 3, period 4. So Ca is in the same period as Sc, but left of Sc. Wait, but in the same period, moving from left to right, ionization energy generally increases. But Ca is group 2, Sc group 3. However, the electron in Ca is in 4s², and in Sc, the outermost electron (for first ionization) is in 4s² (before 3d). Wait, maybe the key is that Ca has a larger atomic radius than Sc, and the effective nuclear charge on the valence electrons of Ca is less than that of Sc. Wait, Sc has atomic number 21, Ca has 20. So Sc has one more proton. But the valence electrons of Ca are in 4s, and Sc's valence electrons are in 4s and 3d. Wait, maybe the correct way: Helium has the highest ionization energy (noble gas, very stable). Mg is in period 3, group 2, ionization…

Answer:

Calcium (Ca)