Animated Solution for Chemistry - Periodicity in Properties: The first ionisation energy (in kJ/mol) of Na, Mg, Al and Si respectively, are :

Imagine you are trying to take a toy away from a child. If the child is holding it loosely, it's relatively easy to take. But if they have a perfect, tight grip on it, you're going to need a lot more energy. This is exactly what happens at the atomic level when we talk about Ionization Energy—the energy required to rip an electron away from an atom.

Let's look at four elements sitting right next to each other in the 3rd period of the periodic table: Sodium (Na), Magnesium (Mg), Aluminum (Al), and Silicon (Si).

The general rule of thumb in chemistry is quite straightforward. As we move from left to right across a period, the number of protons in the nucleus increases. This increases the effective nuclear charge ($Z_{\text{eff}}$), which pulls the outermost electrons closer and holds them tighter.

Because of this tighter grip, we naturally expect it to get harder and harder to remove an electron. The expected order of first ionization energy should be a smooth, continuous climb:

But chemistry loves its exceptions! If we plot the actual experimental values, we see a sudden, unexpected drop. Magnesium's ionization energy is quite high, but instead of continuing to climb, Aluminum's energy actually dips down before Silicon goes back up.

Why does this happen? To understand this anomaly, we have to look past the general trends and dive deep into the quantum mechanical structure of the atoms.

The Magnesium Fortress

Let's write out the electronic configuration for Magnesium ($Z=12$):

Notice that the outermost subshell, the $3s$ orbital, is completely full. In the quantum world, a fully filled subshell is a state of immense stability. It is perfectly balanced and symmetrical. The Magnesium atom is incredibly "happy" in this state and strongly resists any attempt to disrupt it. Therefore, it demands a massive amount of energy to give up one of those $3s$ electrons.

Now, let's look at Aluminum ($Z=13$):

Aluminum has that same stable $3s^2$ core, but it has one extra, lonely electron sitting in the higher-energy $3p$ orbital. This single $3p$ electron is slightly further from the nucleus and is shielded by the $3s$ electrons.

More importantly, if Aluminum loses this one electron, it gets to drop back down to that super-stable, fully-filled $3s^2$ configuration! Because losing the electron actually leads to a highly stable state, Aluminum is quite willing to let it go. Consequently, the energy required to remove it is significantly lower than expected.

Because of this orbital stability interplay, Magnesium actually has a higher first ionization energy than Aluminum. The true, actual order is:

Now, let's match this with the numerical values given in the options: $496,577,737,786$.

Following our corrected order: - Sodium (Na) is the lowest: $496\text{ kJ/mol}$ - Magnesium (Mg) shoots up: $737\text{ kJ/mol}$ - Aluminum (Al) drops down: $577\text{ kJ/mol}$ - Silicon (Si) is the highest: $786\text{ kJ/mol}$

This perfectly matches the sequence $496,737,577,786$, which corresponds to option (d).

Pro Tip: Always be on high alert when crossing from the s-block to the p-block (Group 2 to Group 13), or when dealing with half-filled p-orbitals (Group 15 to Group 16). These are the classic "trap" zones where general periodic trends break down!