Chemistry Reference
In-Depth Information
6.
The atomic mass unit (amu) is defined as exactly 1/12 of an atom of ___.
a)
b)
c)
d)
88
38
2+
4
12
6
14
6
137
56
Sr
C
C
Ba
Lesson 3-3: Quantum Numbers
Compared to the old planetary model of the atom, the quantum-mechanic
model seems much less certain. Instead of thinking of electrons as occupy-
ing fixed orbits with predictable paths, we now think in terms of probability.
Although you shouldn't think of the electrons as being locked into fixed
orbits, the way planets are, we are able to predict the areas where elec-
trons are most likely to be found. Imagine that you needed to find your
chemistry teacher outside of class, perhaps to give him or her a lab report.
You might start by going to the floor of the building where the science
classes are found. You might then narrow your search to the specific hall-
way in which the classroom is located. You might then check in the specific
classroom. Your teacher might not be in that classroom, but this may be
the place where he or she probably is.
Our model of the electron cloud is broken up into areas, as in the ex-
ample of finding the chemistry teacher. Each electron is located in a spe-
cific energy level (in our example, the floors of a building). Each energy
level is broken up into sublevels (the different hallways on the floor of a
building). Each sublevel contains a certain number of orbitals (just as a
hallway can contain some number of rooms).
Just as levels of a building are given numbers, energy levels are given
numbers as well. As a floor, or level of a building, can contain several hall-
ways, each energy level is broken up into sublevels, which are designated
by specific letters. Each sublevel can contain a certain number of orbitals,
which are analogous to the rooms in the hallway. Each orbital can contain
up to two electrons.
The rooms in many buildings contain numbers on their doors, so that
they can be found easily. So to, numbers are given to electrons, so that we
can picture where they are likely to be found. The numbers that are given
to electrons are called quantum numbers, and each electron is given four.
n
The principal, or first, quantum number is used to indicate the energy
level that the electron is found in. The value for n will always be a whole
number, and the higher the number, the further away from the nucleus
the electron described by n tends to be. For example, an electron with a
value of 3 for n is in the third energy level, so it is likely to be located
further away from the nucleus than an electron with a value of 1 for n.
