5.8.1. Atomic and Ionic Radius

Let's start by discussing what we mean by atomic radius. The atomic radius is essentially the size of an atom, which we measure as the distance from the nucleus to the outermost shell of electrons. Can anyone tell me how atomic radius might change as we move across a period in the periodic table?

I think it decreases as you move from left to right across a period, right?

Exactly! As we go from left to right, the effective nuclear charge increases because more protons are added to the nucleus without adding extra energy levels. This increased charge pulls the electrons closer, thereby decreasing the atomic radius. Can anyone think of a reason why the atomic radius increases when moving down a group?

It’s because we add more energy levels, right? So the electrons are further from the nucleus?

Correct! Each new electron shell is farther away, and even though the positive charge increases, the shielding effect from inner electrons prevents the outer electrons from feeling the full attraction of the nucleus. So, remember: Across a period, radius decreases; down a group, it increases. A good mnemonic is 'AC does not go up!' where A represents atomic radius, C for across, and D for down.

Now that we understand atomic radius, let's discuss ionic radius. Can someone explain what an ionic radius is and how it differs from atomic radius?

The ionic radius is how big an ion is, right? It's different because cations and anions are not the same size.

That's spot on! Cations, which are positively charged ions, are smaller than their neutral atoms. This reduction in size occurs because when an atom loses electrons to form a cation, the electron-electron repulsion decreases, allowing the remaining electrons to be pulled in closer to the nucleus. Can anyone give an example of a cation and its corresponding neutral atom?

For example, Na loses an electron to become Na⁺. The atomic radius of Na is bigger than that of Na⁺.

Exactly! On the other hand, anions gain electrons and thus increase in size due to higher electron-electron repulsion. Let's summarize: cations are smaller due to loss of electrons, and anions are larger due to gain of electrons. Remember this difference as it's crucial for understanding bonding and reactivity!

Now, let's shift to the concept of isoelectronic series. What do you all understand by this term?

I think it's when different ions have the same number of electrons.

Correct! An isoelectronic series consists of ions that have the same electron configuration but different numbers of protons. Can someone explain how this would affect their size?

The one with more protons will be smaller because its positive charge pulls the electrons closer.

Right again! For instance, in the isoelectronic series of O²⁻, F⁻, Na⁺, Mg²⁺, and Al³⁺, even though they all have ten electrons, Al³⁺, with its thirteen protons, is the smallest because it has the highest nuclear charge. So always remember: in isoelectronic species, the higher the nuclear charge, the smaller the ionic radius. This understanding will help when predicting how these ions behave in compounds!