Trends in Physical Properties

Today we're going to discuss atomic radii. How do you think the size of an atom changes as you move from left to right across a period in the Periodic Table?

I think it gets smaller because there are more protons pulling the electrons closer.

That's correct! The effective nuclear charge increases, which pulls the electrons in more tightly. Can someone tell me what happens when you go down a group?

The atomic radius increases because there are more electron shells!

Exactly! More electron shells mean the outer electrons are farther from the nucleus. This principle is often summarized in a simple mnemonic: 'Increase down, decrease across.' Let's recap: atomic radius decreases across a period and increases down a group.

Now let's talk about ionic radii. What do you think happens to the size of an atom when it loses an electron?

I think it gets smaller because there's one less electron, so there's less repulsion.

Good job! So when an atom loses an electron and forms a cation, the ionic radius decreases. What about when an atom gains an electron?

The ionic radius would increase because the added electron causes more repulsion among electrons.

Well done! The general trend is that cations are smaller than their atoms, while anions are larger. Remember this: 'Lose for less, gain for gain.' This helps when thinking about ionic sizes compared to atomic sizes.

Next, let’s discuss ionization enthalpy. What do you think happens to ionization energy as you move across a period?

It should increase because the effective nuclear charge is greater.

Correct! More protons mean a stronger pull on the electrons, requiring more energy to remove them. And what about when we go down a group?

It decreases because the outer electrons are further from the nucleus, right?

Absolutely! This is a critical relationship. To remember it, think of the acronym 'IE-COPE': Increase Energy across, Decrease energy downward. Let’s summarize: Ionization energy increases across a period and decreases down a group.

Let's explore electron gain enthalpy. Who can explain what happens when an atom gains an electron?

The atom usually releases energy, so the electron gain enthalpy is negative.

Precisely right! What's interesting is that this trend is less predictable than others. It generally becomes more negative across a period but can vary down a group. How about we remember that with the mnemonic 'Gain some, lose some'? Let's wrap it up: Electron gain enthalpy can be negative for nonmetals as they tend to attract electrons.

Finally, let's tackle electronegativity. Who can tell me how electronegativity changes across a period?

It increases across a period because atoms become better at attracting electrons.

Great! What about down a group?

It decreases because the larger size means the nucleus has a weaker pull on the electrons.

Exactly! To remember this, think of 'Electro-up right, electro-down light.' That indicates where electronegativity is strongest and weakest. To summarize: Electronegativity increases across a period and decreases down a group.