Question Video: Explaining Why an Oxygen Ion Has a Larger Radius Than a Sodium Ion Chemistry

Which of the following statements best explains why an oxygen ion (O²⁻) has a larger ionic radius than a sodium ion (Na⁺)? [A] An oxygen ion gains electrons and forms a negatively charged ion. [B] There are more subatomic particles in the nucleus of an oxygen ion than in that of a sodium ion. [C] Metal ions are always smaller than nonmetal ions. [D] An oxygen ion has fewer protons in its nucleus than a sodium ion. [E] A sodium ion only has a single charge, but an oxygen ion has a double negative charge.

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Video Transcript

Which of the following statements best explains why an oxygen ion O2− has a larger ionic radius than a sodium ion Na+? (A) An oxygen ion gains electrons and forms a negatively charged ion. (B) There are more subatomic particles in the nucleus of an oxygen ion than in that of a sodium ion. (C) Metal ions are always smaller than nonmetal ions. (D) An oxygen ion has fewer protons in its nucleus than a sodium ion. Or (E) a sodium ion only has a single charge, but an oxygen ion has a double negative charge.

To answer this question, we need to evaluate why an oxygen ion has a larger ionic radius than a sodium ion. Before we consider the ions, let’s first consider a sodium atom and an oxygen atom. An atom of sodium has 11 protons and 11 electrons. The 11 protons are in the nucleus, two electrons are in the first electron shell, eight electrons are in the second electron shell, and one electron is in the third electron shell. An atom of oxygen has eight protons and eight electrons. The eight protons are found in the nucleus, two electrons are in the first electron shell, and six electrons are in the second electron shell.

When an atom of sodium forms a positive ion, the outermost electron is lost. So a sodium ion has 11 protons in its nucleus, two electrons in the first electron shell, eight electrons in the second electron shell, and no electrons in the third electron shell. Because there are no electrons in the third electron shell, we can remove it from this diagram. When an oxygen atom forms an oxygen ion with a two minus charge, the atom gains two electrons. So the oxygen ion has eight protons in its nucleus, two electrons in the first electron shell, and eight electrons in the second electron shell.

We can see that a sodium ion and an oxygen ion have the same number of electron shells and the same number of electrons. Furthermore, the outermost electrons in a sodium ion and an oxygen ion experience a similar degree of repulsion from the innermost electrons. The outermost electrons in each ion also experience a force of attraction to the positively charged nucleus. But because the positive charge of a sodium-ion nucleus is greater than the positive charge of an oxygen-ion nucleus, the outer electrons in a sodium ion will experience a stronger force of attraction to the nucleus than the outer electrons in an oxygen ion.

Due to the stronger force of attraction, the outer electrons in a sodium ion will be drawn closer to the nucleus. Therefore, even though a sodium ion and an oxygen ion have the same number of electron shells and the same number of electrons, a sodium ion is smaller because it has a greater number of protons. Or in other words, an oxygen ion is larger because it has fewer protons. With this in mind, let’s evaluate the answer choices to identify the statement that best explains why an oxygen ion has a larger ionic radius than a sodium ion.

Answer choice (A) says that an oxygen ion gains electrons and forms a negatively charged ion. While a species did gain electrons to form a negatively charged ion, this does not explain why an oxygen ion has a larger atomic radius. Therefore, we can eliminate answer choice (A).

Answer choice (B) says that there are more subatomic particles in the nucleus of an oxygen ion than in that of a sodium ion. The most common isotope of sodium is sodium-23, an atom of which has 11 protons and 12 neutrons in its nucleus. The most common isotope of oxygen is oxygen-16, an atom of which has eight protons and eight neutrons in its nucleus. When these atoms become ions, the number of subatomic particles in the nucleus does not change. Thus, answer choice (B) is an incorrect statement as a sodium ion has more subatomic particles in the nucleus than an oxygen ion.

Let’s look at answer choice (C). While it’s true that sodium ions, a metal ion, are smaller than oxygen ions, a nonmetal ion, the statement in answer choice (C) is far too general to be applied to all ions. For example, the ionic radius of a rubidium ion, a metal ion, is 166 picometers, while the ionic radius of an oxygen ion, a nonmetal ion, is 126 picometers. So statement (C) is incorrect and cannot be the answer to this question.

Answer choice (D) states that an oxygen ion has fewer protons in its nucleus than a sodium ion. We know this statement to be true. We also know that because oxygen ions have fewer protons, the outer electrons will experience a lesser force of attraction to the nucleus than the outer electrons in a sodium ion. Thus, oxygen ions are larger. We have identified the correct statement, but let’s take a look at answer choice (E). Answer choice (E) suggests that the amount of charge affects the ionic radius, but the amount of charge does not result in a pattern in terms of ionic radius.

So the statement that best explains why an oxygen ion has a larger ionic radius than a sodium ion is answer choice (D), an oxygen ion has fewer protons in its nucleus than a sodium ion.

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