1. SO3: one Sulfur atom: 16 protons
QUESTION:
How many electrons are in a sulfur trioxide (SO3) molecule and a sulfite ion (SO32-)? A. 40, 40, B. 40, 42, C. 38, 40, D. 78, 80
The subscripts tell us the number of atoms of each kind present in each molecule or ion.
SO3: one Sulfur atom: protons
three oxygen atoms: 3x8, or 24 protons
Total #protons: = 40
Total #electrons: 40
SO32-: Total #protons: 40
Total #electrons: = A -2 charge means 2 more electrons
SCRIPT:
How many electrons are in a sulfur trioxide (SO3) molecule and a sulfite ion (SO32-)? A. 40, 40, B. 40, 42, C. 38, 40, D. 78, 80
PAUSE Since our molecule and polyatomic ion are made up of sulfur and oxygen, let’s look these up in the periodic table.
CLICK
Oxygen has an atomic number of 8… An oxygen atom has 8 protons.
Sulfur has an atomic number of 16… A sulfur atom has 16 protons.
CLICK The subscripts in the formula of a molecule or ion tells us how many atoms of each kind is present in one molecule or ion.
CLICK So let’s count the protons in the sulfur trioxide molecule.
The subscript of sulfur is one CALLOUT
Whenever a subscript is not written in the formula, it’s implied to be one.
And the subscript of oxygen is three.
HIGHLIGHT So one sulfur trioxide molecule consists of one sulfur atom ---- that’s 16 protons
HIGHLIGHT 16
And three oxygen atoms. Since each oxygen atom has 8 protons, we get a total of 24 protons from the oxygen atoms.
Therefore, the total number of protons in a sulfur trioxide molecule is 40… equals 40.
Since a molecule is neutral, the number of protons must be equal to the number of electrons.
The number of electrons is, therefore, 40.
Therefore, choices C and D are wrong.
CROSS OUT 38 and 78 in choices B and D.
Let’s take a look at sulfite ion. It has the same atoms --- one sulfur and three oxygen atoms .
Therefore, the total number of protons is the same …. 40.
HIGHLIGHT 40
But the sulfite ion has –2 charge. A –2 charge means two more electrons than protons.
So, 40+2 equals 42.
Therefore choice A is wrong. The correct answer is choice B.
PAUSE CLICK END RECORDING

2. 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