Atomic Orbitals, Electronic Configurations and Electron Dot Symbols
Introduction
We are studying atomic orbitals and electron configurations because both of these topics describe the
locations of electrons in atoms. If we understand the basic structure of where electrons are in atoms, we can
better understand what happens when a chemical transformation occurs.
Orbitals are volumes of space in which an electron is most likely to be found. We say “most likely” because
it is not currently possible to tell exactly where an electron is, and so we talk instead about the probability of
where an electron is in an atom. This is similar to not being able to say exactly where a wave is at the beach.
It is kind of spread out all along the coast so you cannot point to just one spot and say that all of the wave is
there. We don’t have to worry about this probability; the main thing is that when we discuss orbitals, they
are similar to the coastline area, the wave is there, but it keeps moving and it occupies lots of locations at
once so we cannot say exactly where the wave is. Similarly, because electrons are extremely small and move
extremely fast, it is not possible to say exactly where the electron is.
Sublevels, sometimes called subshells, are a way of grouping similar orbitals. Subshells are a group of
orbitals that have similar shapes. All orbitals in a subshell have similar features in their shapes, even if the
shapes themselves are not identical. There are four types of subshells and they are labeled s, p, d and f. The
“s” type of orbital is spherical in shape. The “p” type of orbital looks like an 8. You should
remember these two shapes. The d and f orbitals have more complex shapes and you do not
have to remember what they are.
There is only one orbital in the s subshell.
There are three orbitals in the p subshell.
There are five orbitals in the d subshell, and
There are seven orbitals in the f subshell.
Energy Levels, sometimes called shells are another way of grouping orbitals. This is a broader category
than subshells. Shells contain one or more subshells. These groups of orbitals have similar energy. The
shells are labeled 1, 2, 3, 4, 5, etc... The lowest energy orbital is in Shell 1. The orbitals in shell 2 are higher
in energy than those in shell 1, but lower in energy than those in shell 3. So as the shell number gets higher,
the electrons in the orbitals, have more energy.
All atoms have electrons. An atom of hydrogen has only one electron, whereas an atom of calcium has 20
electrons. No matter what the atom is, the orbital structure is the same. In class we learned how to use the
periodic table to remember the orbital structure, and then write it using the shorthand notation of electron
configurations.
Some things to remember:
• Each orbital can contain 0, 1, or 2 electrons (and no more!).
• Electrons always fill up the lowest energy orbital first. The lowest energy orbital is in shell 1 and
subshell s. The subshells contained within a shell are ordered in this way:
• Lowest energy s < p < d < f highest energy
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Orbital Notation
Orbital notation is a drawing of the electron configuration. It is very useful in determining electron
pairing and thus predicting chemical reactions.
The orbital notation for sulfur would be represented as follows:
The electrons are numbered as to the filling order. Notice electrons 5, 6, and 7 went into their own
orbitals before electrons 8, 9, and 10 forced a pairing to fill the 2p sublevel. This is an application of
Hund’s rule which minimizes electron-electron repulsions. The same filling order is repeated in the 3p
sublevel.
Directions: Draw the orbital notation of each of the following atoms.
Example:
F
1s
2s
2p
2p
2p
1. Chlorine:
2. Nitrogen:
3. Aluminum:
4. Oxygen:
5. Sodium:
6. Potassium:
7. Sulfur:
8. Calcium
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An Electron Configuration is a shorthand notation that is used to describe the locations of all of the
electrons in an atom.
The electron configuration for a helium atom (He) is: 1s2
• This means that there are 2 electrons in an s orbital. The s orbital is located in the first energy level
(energy shell 1).
The electron configuration for a fluorine atom (F) is:. 1s2 2s2 2p5
• An atom of fluorine has a total of 9 electrons, and so all of the superscripts in an electron
configuration should add up to 9 (2+2+5 = 9).
• The 1s2 again means that the first 2 electrons in a fluorine atom go into the lowest energy shell (1)
and into the s subshell, which contains the lowest energy orbital.
• The 2s2 means that the next 2 electrons go into the s orbital in the second energy level.
• The 2p5 means that the next 5 electrons go into the p orbitals in the second energy level or second
shell. Remember that 5 electrons cannot fit into one orbital. The p subshell actually contains 3
orbitals, which all together, could hold up to 6 electrons.
In the space below, indicate the order in which the orbitals of an atom are filled by electrons.
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Electron Configuration Elements (atoms)
Write the electron configuration for the following Atoms:
Element
Atomic
number
# of e-
Electron Configuration
He
Be
N
O
F
Na
Ca
Al
Ne
Se
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Noble Gas Configuration
Valence electrons are the electrons in the outermost, or highest, energy level. For a chemist, the valence
electrons are quite possibly the most important electrons an atom has. "Why the valence electrons?" you
might ask. Well, since the valence electrons are the electrons in the highest energy level, they are the most
exposed of all the electrons ... and, consequently, they are the electrons that get most involved in chemical
reactions. To simplify the electron configuration and focus only on the “important” electrons in an atom –
the valence electrons, chemists have come up with a method of identifying the arrangement of only the
valence electrons; the Noble Gas Configuration. To write the Noble Gas Configuration, follow the steps
below.
1.
2.
3.
4.
5.
Find the element on the periodic table.
Find the noble gas that comes before the element.
Determine which period (row) the element is in.
Write the noble gas identified in step 2 in brackets.
List the energy sublevels (main energy level and orbital block) beginning with the period identified in
step 3.
6. When you get to the block that the element is in, count how far into the block the element is. This
will be the number of electrons in the last sublevel.
The electron configuration for a fluorine atom (F) is: 1s2 2s2 2p5.
The noble gas configuration of a fluorine atom is: [He]2s2 2p5.
The electron configuration for a silicon atom (Si) is: 1s2 2s2 2p63s23p2.
The noble gas configuration of a silicon atom is: [Ne] 3s23p2.
The electron configuration for a palladium atom (Pd) is: 1s2 2s2 2p63s23p64s23d104p65s24d8.
The noble gas configuration of a palladium atom is: [Kr]5s24d8.
Draw the noble gas configurations of the following atoms:
1. phosphorus
2. calcium
3. bromine
4. boron
5. molybdenum
6. erbium
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Electron Dot Symbols
In addition to the noble gas configuration, chemists use a notation called electron dot symbols, also known as
Lewis symbols, to show how many valence electrons a particular element has and how those electrons can
participate in bonding with other atoms. An electron dot symbol consists of the element's symbol surrounded
by dots that represent the valence electrons. Typically the dots are drawn as if there is a square surrounding
the element symbol. Up to two dots are drawn on each side of the imaginary square. (An element will never
have more than eight valence electrons.)
Hydrogen has 1 electron (and therefore 1 valence electron) and an electron dot symbol of:

H
The dot can be drawn on any one of the four “sides” of the imaginary square.
Boron has 5 electrons, which includes 3 valence electrons and thus has a dot symbol of:

B
Each of the dots is placed on its own side of the symbol.
Phosphorus has 15 electrons, which includes 5 valence electrons and a dot symbol of:

One electron is placed on each side of the square before a second electron is
placed on any of the sides of the square.

The last dot can be drawn as the second electron on any one of the four sides of
the square.
P
Electrons, Valence and Lewis Dot Symbols
1.
How many total electrons are present in:
helium (He) ______
carbon (C) ______
neon (Ne) ______ sodium (Na) ______
2. How many valence electrons are present in:
helium (He) ______
carbon (C) ______
neon (Ne) ______ sodium (Na) ______
3. Draw Lewis dot symbols for the following elements
helium (He)
carbon (C)
neon (Ne)
sodium (Na)
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Electron Configuration Practice - Homework
In the space below, write the orbital notation (arrows) of the following elements:
1)
aluminum
2)
magnesium
3)
nitrogen
4)
phosphorous
5)
calcium
In the space below, write the electron configurations of the following elements:
6)
sodium
7)
potassium
8)
chlorine
9)
silicon
10)
oxygen
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Write the noble gas configuration of the following elements.
11)
fluorine
12)
strontium
13)
indium
14)
bismuth
15)
iodine
Determine what elements are denoted by the following electron configurations:
16)
1s22s22p63s23p64s23d104p65s24d3
17)
1s22s22p63s23p64s2
18)
1s22s22p63s23p64s23d104p65s24d105p66s24f145d106p67s25f-11
19)
1s22s22p4
20)
1s22s22p63s23p64s23d104p65s24d105p3
Draw electron dot symbols for the following elements.
21)
beryllium
22)
silicon
23)
aluminum
24)
fluorine
25)
potassium
26)
sulfur
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