Chemistry 9701/12 — October/November 2023

Background Concept

Atoms are represented by the notation $^{A}_{Z}\text{X}^{\text{charge}}$, where:

• $Z$ (subscript) is the atomic number, equal to the number of protons. It identifies the element.
• $A$ (superscript) is the mass number, equal to the total number of protons and neutrons.
• The charge, written top right, is the number of protons minus the number of electrons.

In a neutral atom, the number of electrons equals the number of protons. A negative ion has more electrons than protons; a positive ion has fewer.

Understanding the Question

The question gives the composition of a particle: 8 protons, 9 neutrons and 10 electrons. We must choose the correct notation from four oxygen species. The options differ in mass number (16 or 17) and charge ($-$ or $2-$). To answer, we need to work out the atomic number, mass number and charge from the particle counts.

Approach

1. Use the proton number to identify the element: 8 protons means $Z = 8$, which is oxygen.
2. Add protons and neutrons to get the mass number: $8 + 9 = 17$.
3. Compare electrons with protons to find the charge: 10 electrons vs 8 protons gives a $2-$ charge.
4. Match these three pieces of information to the options.

Step-by-Step Reasoning

• Protons: 8 protons means the atomic number is 8, so the element is oxygen, symbol O.
• Neutrons: 9 neutrons. Mass number $A =$ protons + neutrons $= 8 + 9 = 17$. This eliminates options A and B, which both have mass number 16.
• Electrons: A neutral oxygen atom has 8 electrons. This particle has 10 electrons, which is 2 more than the number of protons. Therefore the charge is $2-$.
• The correct notation is therefore $^{17}_8\text{O}^{2-}$, which is option D.

Why the other options are wrong:

• A $^{16}_8\text{O}^-$ has 8 protons, 8 neutrons and 9 electrons. It has the wrong mass number and the wrong charge.
• B $^{16}_8\text{O}^{2-}$ has 8 protons, 8 neutrons and 10 electrons. It has the wrong mass number.
• C $^{17}_8\text{O}^-$ has 8 protons, 9 neutrons and 9 electrons. It has the correct mass number but the wrong charge.

Key Takeaways

• In nuclear notation, the superscript is the mass number and the subscript is the atomic number.
• Mass number = protons + neutrons.
• Charge = protons $-$ electrons. A negative ion has more electrons than protons.

Common Mistakes

• Confusing mass number with atomic number: 16 is not the mass number when there are 9 neutrons.
• Thinking that 10 electrons means a $1-$ charge. The charge must be found by comparing the electron count with the proton count, not by counting electrons alone.
• Mixing up the superscript and subscript positions in the notation.

Things to Be Careful About

• Always read the superscript as the mass number and the subscript as the atomic number.
• For an ion, the charge sign matters: 10 electrons and 8 protons gives $2-$, not $2+$.
• Check that the mass number equals protons plus neutrons, not just the number written in the option.