Periodic Trends - Ionic Radius
Topics Covered: ionic radius, ion, cation, anion, electron-electron repulsion
Today, we are back with another series of periodic trends! In this post, we will discuss the trends in ionic radius and explore how the radius of a monatomic ion changes across the periods and groups.
Before we discuss the trend, let’s talk about ions. An ion is an atom or molecule that has a net electric charge. The elements in the periodic table start with neutral charge, but once they lose or gain electrons, an atom becomes an ion because they will now have different numbers of protons and electrons. If an atom loses electrons, it becomes positively charged and becomes a cation (remember, electrons are negative and they are losing “negative” charge!). If an atom gains electrons, it becomes negatively charged and becomes an anion.
Another concept to cover is electron-electron repulsion. Electron-electron repulsion simply refers to the negative-negative interactions of electrons in which they repel one another. Remember, “like charges repel,” so when there are a lot of electrons, they will try to repel one another, creating a repulsion force.
Ok then, how is this all relevant to the ionic radius? First of all, when you go down a group on a periodic table, the ionic radius increases because ions, just like atoms, are gaining more electron shells.
Across the period, ionic radius depends on whether the ion is a cation or anion. When they are in the same period, anions are larger than cations (and other neutral atoms) because when anions gain electrons, the electron-electron repulsion increases, so electrons repel each other and thus increase the ionic radius. Cations are smaller than anions because when they lose electrons, they usually lose a whole energy shell (as the electrons they lose tend to be the valence electrons).
When moving from left to right across the period, the ionic radius decreases for both cations and anions. For cations, moving left to right means the atoms are losing more electrons. Also more protons are added, so the nucleus draws in the electrons more tightly! (remember effective nuclear charge?!) In the case of anions, they gain less electrons across the periodic table, so the electron-electron repulsion decreases from left to right of the periodic table!
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