The Lewis symbol for an atom depicts its valence electrons as dots
around the symbol for the element.
LEARNING OBJECTIVES [ edit ]
Learn to write Lewis symbols for atoms
Memorize the conventions for drawing Lewis structures
KEY POINTS [ edit ]
The columns, or groups, in the periodic table are used to determine the number
of valence electrons for each element.
The noble/inert gases are chemically stable and have a full valence level of electrons.
Other elements react in order to achieve the same stability as the noble gases.
Lewis symbols represent the valence electrons as dotssurrounding the elemental symbol for
the atom.
TERMS [ edit ]
Lewis symbol
Formalism in which the valence electrons of an atom are represented as dots.
Noble Gases
Inert, or unreactive, elements in the last group in the periodic table which are typically found in
the gaseous form.
group
A column in the periodic table that consists of elements with similar chemical reactivity, because
they have the same number of valence electrons.
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Determining the Number of Valence Electrons
In order to write the Lewis symbol for an atom, you must first determine the number
of valence electrons for that element. The arrangement of the periodic table can help you
figure out this information. Since we have established that the number of valence
electrons determines the chemical reactivity of an element, the table orders the elements by
number of valence electrons.
Each column (or group) of the periodic table contains elements that have the same number
of valence electrons. Furthermore, the number of columns (or groups) from the left edge of
the table tells us the exact number of valence electrons for that element. Recall that any
valence level can have up to eight electrons, except for the first principalenergy level, which
can only have two.
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Lv
Periodic table of the elements
Group numbers shown by Roman numerals (above the table) tell us how many valence electrons there
are for each element.
Some periodic tables list the group numbers in Arabic numbers instead of Roman numerals.
In that case, the transition metal groups are included in the counting and the groups
indicated at the top of the periodic table have numbers 1, 2, 13, 14, 15, 16, 17, 18. The
corresponding roman numerals used are I, II, III, IV, V, VI, VII, VIII.
Survey of the Groups in the Periodic Table
Take the first column or group of the periodic table (labeled 'I'): hydrogen (H), lithium (Li),
sodium (Na), potassium (K), etc. Each of these elements has one valence electron. The
second column or group (labeled 'II') means that beryllium (Be), magnesium (Mg), calcium
(Ca), etc., all have two valence electrons.
The middle part of the periodic table that contains the transition metals is skipped in this
process for reasons having to do with the electronic configuration of these elements.
Proceeding to the column labeled 'III', we find that those elements (B, Al, Ga, In, ...) have
three valence electrons in their outermost or valence level.
We can continue this inspection of the groups until we reach the eighth and final column, in
which the most stable elements are listed. These are all gaseous under normal conditions
of temperature and pressure, and are called 'noble gases.' Neon (Ne), argon (Ar), krypton
(Kr), etc., each contain eight electrons in their valence level. Therefore, these elements have a
full valence level that has the maximumnumber of electrons possible. Helium (He), at the
very top of this column is an exception because it has two valence electrons; its valence level
is the first principal energy level which can only have two electrons, so it has the maximum
number of electrons in its valence level as well.
The Lewis symbol for helium
Helium is one of the noble gases and contains a full valence shell. Unlike the other noble gases in Group
8, Helium only contains two valence electrons. In the Lewis symbol, the electrons are depicted as two
lone pair dots.
The noble gases represent elements of such stability that they are not chemically reactive, so
they can be called inert. In other words, they don't need to bond with any other elements in
order to attain a lower energy configuration. We explain this phenomenon by attributing
their stability to having a 'full' valence level.
The significance in understanding the nature of the stability of noble gases is that it guides us
in predicting how other elements will react in order to achieve the same electronic
configuration as the noble gases by having a full valence level.
Writing Lewis Symbols for Atoms
Lewis symbols for the elements depict the number of valence electrons as dots. In
accordance with what we discussed above, here are the Lewis symbols for the first twenty
elements in the periodic table. The heavier elements will follow the same trends depending
on their group.
Once you can draw a Lewis symbol for an atom, you can use the knowledge of Lewis symbols
to create Lewis structures for molecules.
Valence Electrons and the Periodic Table
Electrons 'live' in energy levels or shells, which are varying distances from the nucleus, and can hold
varying numbers of electrons. The most important electrons in an atom are the valence electrons, which
are in the outermost energy level or shell. We'll look at how to determine the number of valence electrons
in an atom, based on which column it is in on the periodic table.