Question

give reason___first ionization enthalpy of Ca is little heigher than that of Al.

Answer 1

The energy required to remove an electron f a neutral isolated gaseous atom and convert it into a positively charged gaseous ion is called first ionisation enthalpy.
➡️As first ionisation enthalpy increases on moving left to right in a given period and it decreases on moving down the group therefore first ionisation enthalpy of Ca is little higher than that of Al⬅️

Answer 2

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$<b>First Ionisation explanation$

Let's look at the electrons we are removing for first ionisation:
$<b> K: 1s² 2s² 2p⁶ 3s² 3p⁶ 4s¹$
$<b> Ca: 1s² 2s² 2p⁶ 3s² 3p⁶ 4s²$

Both electrons are removed from the same principal quantum shell, so we can use the concept of effective nuclear charge.

Across the Period, proton number increase so nuclear charge increase. Electrons are added to the same shell, so shielding effect is constant.

Effective nuclear charge increases from K to Ca, there is stronger attraction between nucleus and valence electron, more energy is required to remove electron, therefore first IE for Ca is higher.
$<b> Second Ionisation explanation$

Let's look at the electrons we are removing for second ionisation:
$<b> K: 1s² 2s² 2p⁶ 3s² 3p⁶$
$<b> Ca: 1s² 2s² 2p⁶ 3s² 3p⁶ 4s¹$

This time the electron of K is removed from an inner principal quantum shell n=3 while electron of Ca is removed from higher principal quantum shell n=4.

This means that the electron from inner shell is much closer to the nucleus, hence the attraction between nucleus and electron is much stronger and you need a lot more energy to remove electron from an inner shell.
$<b> Therefore 2nd IE of K is much higher.$

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