1. QUESTION:
Which of the following describes an anion? A. a sodium ion (Na+), B. a calcium ion that has lost 2 electrons C. an ion with 8 protons and 10 electrons
An anion (pronounced “an”, “ion”) is a negatively charged ion. Na+ is not an anion; it’s positively-charged, a cation (pronounced “cat”, “ion”).
A negative charge means a gain of electrons. Therefore, an anion has more electrons than protons.
Charge = (#protons) – (#electrons). Choice C has a charge of –2; it’s an anion. In fact, it’s oxide (O2-).
SCRIPT:
Which of the following describes an anion? A. a sodium ion (Na+), B. a calcium ion that has lost 2 electrons C. an ion with 8 protons and 10 electrons PAUSE
CLICK An anion is a negatively charged ion. Obviously, choice A is wrong.
The charge of the sodium ion is +1.
HIGHLIGHT + in Na+
CLICK Remember that electrons have a –1 charge. So an atom acquires a negative charge
And becomes an anion by gaining electrons.
This means choice B is wrong.
CROSS OUT “lost” in choice B
Furthermore, since a negative charge is acquired by gaining electrons, there must more
Electrons than protons in an anion. Therefore, the correct answer is C. In choice C, we are have more electrons than protons. Ten is larger than eight.
CLICK
In fact, the ion described in choice C is the oxide ion.
HIGHLIGHT oxide (O2-)
The charge of an ion is equal to number of protons minus number of electrons.
In this case, we have 8 minus 10, or –2.
But how do we know that choice C describes an oxide ion? We refer to the periodic table.
An ion with 8 protons must be derived from an atom with 8 protons.
Any atom with 8 protons is an oxygen atom.
Since any atom is neutral, it must have the same number of protons and electrons.
So an oxygen atom has 8 protons and 8 electrons.
And an oxide ion, which has a –2 charge, has two more electrons.
CLICK PAUSE END RECORDING

2. Oxygen atom (O) has 8 protons, 8 electrons
Oxide ion (O2-) has 8 protons, 10 electrons.

3. Video ID: © 2008, Project VALUE (Video Assessment Library for Undergraduate Education), Department of Physical Sciences Nicholls State University Author: Glenn V. Lo Narrator: Funded by Louisiana Board of Regents Contract No. LA-DL-SELECT-13-07/08