```How many electrons are in each energy level?
The total number of electrons in an energy level is the sum of the electrons in each sub-shell of that energy
level. Just add the numbers in superscript together to find the number of electrons in an energy level. The
number of electrons in each energy level in an atom of gold is shown below:
How many electrons are in an atom's outer energy level?
This is just a combination of the previous two examples. Use the electron configuration to find that atom's
highest energy level and then add up the numbers in superscript to find the number of electrons that are in it.
There is one electron in the outer energy level of an atom of gold, as shown below:
Electron
Configuration
The electron configuration of an atom is a form of
notation which shows how the electrons are
distributed among the various atomic orbital and
energy levels. The format consists of a series of
numbers, letters and superscripts as shown below:
1s2
Here we see the electron configuration for the
element helium. This electron configuration provides
us with the following information:


The large number "1" refers to the
principle quantum number "n" which
stands for the energy level. It tells us
that the electrons of helium occupy the
first energy level of the atom.
The letter "s" stands for the angular

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momentum quantum number "l". It
tells us that the two electrons of the
helium electron occupy an "s" or
spherical orbital.
The exponent "2" refers to the total
number of electrons in that orbital or
sub-shell. In this case, we know that
there are two electrons in the spherical
orbital at the first energy level.
Before we continue, we must review some
information that you will need to fill in electron
configurations correctly.
I. Principle Quantum Number (n) and Sublevels
The number of sublevels that an energy level can
contain is equal to the principle quantum number of
that level. So, for example, the second energy level
would have two sublevels, and the third energy level
would have three sublevels. The first sublevel is
called an s sublevel. The second sublevel is called a p
sublevel. The third sublevel is called a d sublevel and
the fourth sublevel is called an f sublevel. Although
energy levels that are higher than 4 would contain
additional sublevels, these sublevels have not been
named because no known atom in its ground state
would have electrons that occupy them.
II. Sublevels and Orbitals
An orbital is a space that can be occupied by up to two
electrons. Each type of sublevel holds a different
number or orbitals, and therefore, a different number
of electrons. s sublevels have one orbital, which can
hold up to two electrons. p sublevels have three
orbitals, each of which can hold 2 electrons, for a total
of 6. d sublevels have 5 orbitals, for a possible total of
10 electrons. f sublevels, with 7 orbitals, can hold up
to 14 electrons. The information about the sublevels is
summarized in the table below:
Table 3-6a - Orbital and Electron
Capacity for the Four Named Sublevels
Sublevel
# of orbitals
Maximum
number of
electrons
s
1
2
p
3
6
d
5
10
f
7
14
III. Total Number of Orbital and Electrons per
Energy Level
An easy way to calculate the number of orbitals found
in an energy level is to use the formula n2. For
example, the third energy level (n=3) has a total of 32,
or nine orbitals. This makes sense because we know
that the third energy level would have 3 sublevels; an s
sublevel with one orbital, a p sublevel with 3 orbitals
and a d sublevel with 5 orbitals. 1 + 3 + 5 = 9, so the
formula n2 works!
IV. Total Number of Electrons per Energy Level
An easy way to calculate the total number of electrons
that can be held by a given energy level is to use the
formula 2n2. For example, the fourth energy level
(n=4) can hold 2(4)2 = 32 electrons. This makes sense
because the fourth energy level would have four
sublevels, one of each of the named types. The s
sublevel hold 2 electrons, the p sublevel holds 6
electrons , the d sublevel holds 10 electrons and the f
sublevel holds 14 electrons. 2 + 6 + 10 + 14 = 32, so
the formula 2n2 works! We can summarize this
information in the table below:
Table 3-6b Orbitals and Electron
Capacity of the First Four Principle
Energy Levels
Number Maximum
Number
Principle
of
number
Type of
of
energy
orbitals
of
sublevel orbitals
level (n)
per
electrons
per type
level(n2)
(2n2)
1
s
1
1
2
2
3
4
s
1
p
3
s
1
p
3
d
5
s
1
p
3
d
5
f
7
4
8
9
18
16
32
V. Order of Filling Sublevels with Electrons
The next thing that you need to recall is the fact that
the energy sublevels are filled in a specific order that
is shown by the arrow diagram seen below:
Remember to start at the beginning of each arrow, and
then follow it all of the way to the end, filling in the
sublevels that it passes through. In other words, the
order for filling in the sublevels becomes; 1s, 2s, 2p,
3s, 3p, 4s, 3d, 4p, 5s, 4d, 5p, 6s, 4f, 5d, 6p, 7s, 5f,
6d,7p.
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