QUESTION: Which of the following describes a cation? A. an aluminum ion (Al3+), B. an oxygen atom that has gained 2 electrons C. an ion with 17 protons and 18 electrons D. an ion with more neutrons than protons A cation (pronounced “cat”, “eye”, “on”) is a positively charged ion. Obviously, Al3+ is a cation. A positive charge means loss of electrons. Gain of electrons produces a negatively charged ion (“anion”, pronounced “an”, “ion”). Choice B is O2- Charge = (#protons) – (#electrons). Choice C has a charge of –1; it’s an anion. In fact, it’s chloride (Cl-). Cations have more protons. Anions have more electrons SCRIPT: Which of the following describes a cation? A. an aluminum ion (Al3+), B. an oxygen atom that has gained 2 electrons C. an ion with 17 protons and 18 electrons D. an ion with more neutrons than protons PAUSE CLICK A cation is a positively charged ion. So obviously, the correct answer is A. The charge of the aluminum ion is +3. HIGHLIGHT 3+ Let’s see what’s wrong with the other choices…. CLICK A positive charge is obtained when an atom loses electrons. A gain of electrons, as in choice B, HIGHLIGHT “gained 2 electrons” in choice B Produces a negatively charged ion, or an anion. How about choice C? If a cation is formed by the lsoss of electrons, then a cation must have more protons than electrons. In choice C, we have more electrons. CLICK In fact, we calculate the charge of an ion from the number of protons and electrons. The charge of an ion is equal to the number of protons… HIGHLIGHT Minus The number of electrons… So, for choice C, the charge is the number of protons, 17… Minus the number of electrons, 18… Or –1. In fact, the ion described in choice C is the chloride ion. Finally let’s look at choice D… It doesn’t tell us if we have more protons than electrons, or more electrons than protons. Neutrons have no charge. So choice D does not give us enough information to determine if the ion is positive or negative. If you’re wondering how we figured out that choice C describes a chloride ion, here’s how it’s done. We refer to the periodic table and find that the atomic number of chlorine is 17. Any atom with 17 protons is a chlorine atom and any ion that has 17 protons is derived from a chlorine atom. Since an atom is neutral, it has the same number of protons and electrons. So, a chlorine atom has 17 protons and 17 electrons. An ion with 17 protons and 18 electrons is an ion derived from a chlorine atom, with a -1 charge. We call this ion the chloride ion. CLICK PAUSE END RECORDING

Chlorine atom (Cl) has 17 protons, 17 electrons Chloride ion (Cl-) has 17 protons, 18 electrons.

Video ID: 1-15-1 © 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