Video: Applying Knowledge of the Radii and Electron Removal Energies of the Li⁺ Ion and Li Atom

For statements I and II, state for each if they are true or false. I) The energy required to remove an electron from an Li⁺ ion is greater than the energy required to remove an electron from an Li atom. II) The radius of an Li⁺ ion is greater than the radius of a Li atom. If both are true, state if II is a correct explanation for I.

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Video Transcript

For statements I and II, state for each if they are true or false. I) The energy required to remove an electron from an Li⁺ ion is greater than the energy required to remove an electron from an Li atom. II) The radius of an Li⁺ ion is greater than the radius of the Li atom. If both are true, state if II is a correct explanation for I.

Statement I refers to the energies required to remove electrons from lithium plus ions and lithium atoms. The first of these processes is illustrated by this equation, where a lithium ion turns into a lithium two plus ion plus an electron. And the second process is illustrated by this equation, where a single lithium atom loses an electron to form a lithium ion. So the first question is, why is energy required to remove an electron from a lithium atom or ion in the first place?

Lithium is the third lightest element in the Universe, with atomic number three. It can be found all the way to the left of the periodic table in Group 1. A lithium atom consists of a dense, positively charged nucleus surrounded by electrons. All lithium nuclei contain three protons and naturally occurring lithium nuclei will contain either three or four neutrons. For this question, we’re only really concerned about the protons and the electrons. The protons in the nucleus all have a single positive charge. So the nucleus itself has an overall charge of three plus. Therefore, to be neutral, we need as many electrons as there are protons to provide an equal but opposite charge. So a lithium atom will have three electrons. It takes energy to remove electrons because the electrons are attracted to the nucleus. So we need to provide force to counteract that force of attraction.

Now, we can go back to the question, Why would it take more energy to remove an electron from a lithium plus ion than a lithium atom? For that, we’re going to need to understand the electron configuration for lithium. In a lithium atom, the first two electrons are in the lowest energy subshell, the 1s shell. This leaves the third electron in the 2s subshell. When removing an electron, we’ll always take the highest energy electron first, which is the one furthest from the nucleus. So the electron we take away from the lithium atom comes from the 2s subshell.

What about with a lithium ion? We already know that the electron configuration of Li⁺ is 1s². This means the electron we remove must be from the 1s subshell, leaving us with the electron configuration of 1s². The outer electron of the lithium atom is further from the nucleus than in the lithium ion. So it’s easier to remove. So it’s actually true. The energy required to remove the outer electron from a lithium plus ion is greater than the energy we’d need to remove the outer electron from a lithium atom. This is because we always start from the outer electron. And the outer electron in the lithium ion is closer to the nucleus.

The second statement is that the radius of the lithium plus ion is greater than the radius of the lithium atom. Well, let’s have a look at the lithium ion. The lithium ion has one less electron than a lithium atom. It lost that electron from the 2s subshell. And the 2s subshell is bigger than the 1s subshell. This means we’d expect the lithium ion to be smaller than the lithium atom, not bigger. So the radius of the lithium atom should be bigger than the radius of the lithium ion.

So statement II is definitely false. Since statement II is false, we don’t have to address the last part of the question. A false statement cannot be a correct explanation for a correct one.

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