Are radii of ions always smaller than the radii of the corresponding atoms of the same element?

Answer 1

Electrons occupy most of the space of an atom -- more than 99.9% of it! Niels Bohr suggested that electrons exist is shells around the nucleus. In his simplest model for a one-electron atom, the size of the shells increased proportionally to the square of the distance. In other words, if the first shell was a distance 1 from the nucleus, the second shell of electrons would be four times farther in distance, the third shell 9 times, the fourth shell 16 times, and so on.

Now to answer the question.

Most metals, such as Scandium through Zinc and the periods below them on the Chart, have two electrons in an outer shell, and fill up electrons into an inner shell. A metal atom becomes a cation (positively charged) by losing electrons. If the metal atom loses two electrons to become +2 charged (and most transition metals have this as a possibility) then the atom lost a complete outer shell! Therefore, metal ions are smaller than their respective neutral atoms.

Even if a metal atom doesn't lose a complete shell, the ion will still be smaller because there are fewer electrons pushing away from each other compared to the neutral form. Negatively charged anions must therefore be larger than their neutrals for the very same reason.

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Answer 2

Yes they do. It's often a very tiny difference (and being chemists, we mean really really tiny) but still, yes.

Why is this? Because an element has perfectly balanced positive and negative charges (actually, that's not strictly true but it's true enough for college-level). If we take away a negative charge (one electron) we make the whole thing positive, because the nucleus is still at the same strength it's always been.

And yes, this means each e- will be attracted a little bit more to the nucleus, pulling them in slightly and decreasing atomic radius.

Another good way to look at this is through ionisation energies. 1st IE (pulling off the first electron) usually pretty simply. 2nd IE (pulling off a 2nd electron) is much harder...and 3rd IE can be a VERY difficult task indeed, depending on the element.

Each time we pull one off, we decrease the overall negative charge, so each electron is attracted a little more by the nucleus - making the atom small and making the next electron a little harder to pluck off - **Even if that electron is on the same energy level/shell of the last atom**.

Hope that helps you picture the situation.

Answer 3

The differences depend on whether the atom in question gained an electron (anion) or lost an electron (cation). Electrons are negatively charged particles and thus repel themselves, the repulsion is balanced by the pull on all the electrons due to the positively charged nucleus. When an atom gains an electron there are more electrons available to repel and as a result the atomic radii increases as the electrons expand their orbital to accomodate the extra negative charge. When an atom becomes a cation one less electron is available to contribute to the repulsion so the atomic radii generally decreases.

This is just a general trend, there are usually always examples that contradict this trend due to complexities not exhitibited by all atoms.

Answer 4

Some metal form positive ions by losing electrons in an outer energy level which basically makes that energy level disappear. This makes the ion smaller than the atom. If the atom doesn't lose an entire energy level, but only a few electrons from that level the remaining electrons are attracted more strongly by the nucleus making the ion somewhat smaller.

Nonmetals do the opposite. They gain electrons to form negative ions. The extra electrons in the outer energy level repel other electrons already in the outer level. This repulsion makes the energy level larger thus the ion becomes larger than the atom that formed it.

Answer 5

True.