Electron Dot Models (Lewis Structures)
Def - A simple model of the atom that can be used to illustrate chemical bonding
Electron dot models consist of the Chemical symbol of the element surrounded by the valence
electrons.
Example: Sodium
The Bohr-Rutherford model for sodium is:
This atom has only one valence electron so the electron dot model would be
Rules:
1) No more than 2 electron dots per side
2) Do not pair electron dots until there is one dot per side
3) Single electrons are placed on adjacent sides of the model
Samples:
Metals lose electrons to become + ions. The number of electrons lost is the valence number.
Since these are the valence electrons the number of dots will be equal to the valence number.
# of dots = valence number
Ex 1/ Sodium has a +1 valence, it therefore has 1 valence electron and its electron dot model
would be:
Ex 2/ Scandium has a +3 valence, it therefore has 3 valence electrons and its electron dot model
would be:
Nonmetals form – ions by gaining electrons to fill their outer most energy level. The valence
number tells the number of electrons they are short by. To find the number of dots subtract the
valence number from 8.
# of dots = 8 – |valence number|
Ex 1/ Fluorine has a valence of -1. The number of dots = 8 – 1 = 7 and the model would be:
Ex 2/ Phosphorus has a -3 valence, the number of dots would be 8 – 3 = 5 and the model would
be:
Chemical Bonds
A chemical bond - force that joins 2 or more atoms together.
Bonds result from atoms gaining, losing or sharing electrons in order to fill their outer most
energy level. Therefore, the electrons involved in bonding are the valence (outer most)
electrons.
Metals always lose electrons to nonmetals.
Nonmetals can either:
1) gain electrons from metals
or
2) share electrons with other nonmetals
The different ways that nonmetals fill their outer most energy level results in two types of
bonds:
1) Ionic Bond – A bond due to a transfer of electrons from a metal to a nonmetal.
2) Covalent Bond – A bond due to the sharing of electrons between nonmetals
Ionic Bonds - as the name implies, must involve ions (charged atoms)
Metals become ions by losing electrons to form + ions
Nonmetals become ions by gaining electrons to form - ions
The opposite charges produced are attracted to each other forming the ionic bond.
If a metal is brought near a nonmetal the valence electrons on the metal will move to the
nonmetal.
Example 1:
Sodium has 1 valence electron while chlorine has 7 valence electrons.
Sodium will lose its valence electron to become a + 1 ion, while the Chlorine will become -1.
The chemical formula for this compound is NaCl.
Example 2: Magnesium and Sulfur
Magnesium has 2 valence electrons while Sulfur has 6 valence electrons
Magnesium will lose both of its electrons to the Sulfur.
This makes the Magnesium a +2 ion and Sulfur a -2 ion.
The ions attract each other and stay together as MgS, Magnesium sulfide.
Example 3: Sodium and Sulfur
Sodium has one valence electron and Sulfur has 6 valence electrons.
The sodium can provide one electron to the Sulfur.
The Sulfur must get to 8 valence electrons. This requires that there be a second sodium atom to
provide the required electron.
The Sodium atoms become +1 ions and the Sulfur atom becomes a -2 ion.
This results in the formation of Na2S, Sodium sulfide.
Example 4: Magnesium and Chlorine
Magnesium has 2 valence electrons while chlorine has 7 valence electrons.
The Chlorine can only accept one electron from the Magnesium, but the Magnesium must lose
both of its electrons.
Therefore, a second Chlorine is required to pick up Magnesium’s second electron.
The Magnesium becomes a +2 ion. The Chlorine atoms become -1 ions.
The ions attract and form MgCl2, Magnesium chloride.
Example 5: Titanium and Oxygen
Here is a Bohr-Rutherford model of Titanium.
If Titanium loses 2 electrons the outer electron energy level will be left with 10 electrons. It,
therefore, must lose 4 electrons.
The Titanium becomes +4 and the Oxygen becomes -2.
The ions attract and stick together to form TiO2, Titanium oxide.
Each compound is given a chemical formula
Chemical formula - group of symbols telling the composition of a chemical compound.
Chemical Formulas tell:
1.
2.
Elements present
# of atoms of each element in the molecule or the ratio between the atoms.
Important!!! The metal always goes first.
Subscript: Is a number written after and below a chemical symbol, indicating the number of
atoms.
(If there is only 1 atom, a 1 is not written, it is understood.)
Covalent Bonds
Covalent Bond: a chemical bond due to a sharing of electrons between two nonmetals or a
nonmetal and hydrogen.
Examples:
Chlorine + Fluorine
Chlorine has 7 valence electrons as does Fluorine.
They are both very good at picking up electrons, but neither is willing to give up any electrons.
So when they bump into each other they grab hold of each other’s unpaired electron and try to
take it away, but neither is able to let go of their electrons and so they end up sharing the
electrons.
The formula for this compound is ClF
Hydrogen and Oxygen
Hydrogen has one valence electron, if it loses this electron it is left as a nucleus with no
electrons.
Oxygen has 6 valence electrons and a very high electronegativity.
The Oxygen grabs hold of the electrons on 2 Hydrogen atoms to fill its need for 2 more
electrons. The Hydrogen in turn grabs one of Oxygen’s unpaired electrons and these electrons
are shared to form the covalent bond.
The formula for this compound is H2O.
Oxygen has 2 unbonded electrons. These are both available to be shared in the formation of a
chemical bond. i.e.
There are 2 pairs of shared electrons (2 covalent bonds) this is called a double
bond.
Nitrogen has 3 pairs of unbonded electrons that are available to be shared to form covalent
bonds.
There are 3 pairs of shared electrons (3 covalent bonds) this is called a triple bond.