1. The number of dots = number of valence electrons
QUESTION:
How many dots are shown in the Lewis symbol for a nitride ion? A. 5, B. 7, C. 8, D. 14
The number of dots = number of valence electrons
Nitrogen atom has 7 electrons. 1s2 2s2 2p3 3-
SCRIPT: SLIDES 3 and 4
How many dots are shown in the Lewis symbol for a nitride ion…. 5, 7, 8, or 14?
PAUSE 3 seconds CLICK The number of dots shown in the Lewis symbol for an atom is equal to the number of valence electrons. Valence electrons are the electrons in the outermost shell of the atom.
CLICK A nitrogen atom has 7 electrons….
CLICK and its electron configuration is one ess two, two ess two, two pee three.
There are 2 electrons in the first shell (HIGHLIGHT 2 in 1s2)…
and five electrons in the second shell. (HIGHLIGHT 2 and 3 in 2s2 and 2p3) The second shell is the valence or outermost shell. Therefore, …
CLICK a nitrogen atom has five valence electrons.
CLICK This is the Lewis symbol for a nitrogen atom. The symbol for the element is surrounded by five dots.
But… the question is asking for the the Lewis symbol for the nitride ion….CLICK.
Which has a charge of negative three. A negative three charge means a gain of three electrons.
Seven plus three is ten. So a nitride ion has ten electrons and it is isoelectronic with Neon.
It has the same electron configuration as a neon atom.
So instead of five dots…. We need to show three more dots. CLICK CLICK CLICK
Whenever we draw the Lewis symbol for an ion, we must enclosed it in square brackets CLICK CLICK
And indicate the charge as a superscript CLICK
The correct answer is 8.
PAUSE 2 seconds
We can easily figure out the answer to this question by referring to the periodic table.
CLICK
The elements are arranged so that the group number is equal to the number of valence electrons.
CLICK We can see that nitrogen belongs to group “five A.”
Therefore, a nitrogen atom has five valence electrons. The same is true for phosphorus and other members of this group.
When nonmetals form negative ions, their tendency is to gain enough electrons to be come isoelectronic with a noble gas. CLICK Nitrogen tends to gain three electrons to become isoelectronic with a noble gas… in this case, neon.
A similar behavior is observed for phosphorus and arsenic. CLICK CLICK
Since noble gases have 8 valence electrons…. Except for Helium…. This propensity of atoms to acquire a noble-gas-like
Configuration is know as the octet rule.
Nitride ion is N3-. It has 10 electrons and is isoelectronic
with Ne: 1s2 2s2 2p6

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