This presentation reviews chemical bonds. There are two main types of chemical bonds:
Ionic and Covalent. Chemical bonds are formed from the sharing of
electrons through one of two methods. Ionic bonds are formed through electron transfer,
whereas covalent bonds are formed through the
sharing of electrons. Ionic bonds are formed between metals and
nonmetals. Recall that metals are located to the
left of the metalloids on the periodic table. The zigzag line shown on the periodic table
divides the metals from the nonmetals. The nonmetals are located to the right of the
zigzag line. The valence electrons of the metals and
nonmetals are responsible for the bonding
behavior of the elements. The number of valence electrons for an element
can be determined by looking at what group
number the element is in. Recall that the periodic table is divided into
eighteen groups. The A group elements follow a trend when it
comes to the number of valence electrons they
have. If an element is located in group IA, that means
they have one valence electron. If it is in IIA,
there are two valence electrons. That means an element in VIIA will have seven
valence electrons. The B group elements do not
follow a trend, so we will not discuss those here. Lewis Structures are drawn to represent the
number of valence electrons around an atom. Here is an example of a Lewis structure for an
element from each of the A group elements. Lithium has 1 valence electron, beryllium has 2,
boron has 3 valence electrons, and carbon has
4. Notice that the filling order shows an electron
drawn singularly on each side of the symbol
before they are paired up. Looking at nitrogen, which has 5 valence
electrons, oxygen that has 6, fluorine with 7,
and neon with 8 shows this. Every atom desires to have a total of 8 valence
electrons. They achieve this by either giving away their
valence electrons or by gaining valence
electrons from other atoms. If an atom has less than 4 valence electrons it
tends to give the electrons away, whereas if an atom has more than 4 valence
electrons it tends to gain them. Looking at the elements here, lithium will give up
1 electron, beryllium will give up 2, and boron
will give up 3 electrons. Carbon behaves a little differently and does not
usually give up or take electrons. It tends to share electrons and form covalent
bonds, which we will talk about later. Nitrogen, oxygen, and fluorine will gain electrons
to fill up their octets. To show how an ionic bond is formed, the Lewis
structures are used and electrons are moved
from the metal to the nonmetal. Here you see the formation of ionic bonds
between magnesium and fluorine. Magnesium has 2 valence electrons and wants
to give them away. Fluorine has 7 valence electrons and desires to
gain 1 electron. Magnesium will give 1 of its valence electrons to
the fluorine atom. Fluorine is now complete with 8 electrons, but
magnesium still has 1 electron left. This means that another fluorine atom is needed
to give away that last electron. Once magnesium has given away its electrons,
it becomes an ion with a two plus charge. Each fluorine atom has gained 1 electron and
they become ions with a one minus charge. If you notice, you have 1 ion with a two plus
charge and 2 ions with a one minus charge. If you add all of these charges up, they add up
to be neutral. All compounds must be neutral. The other type of bond that can be formed is a
covalent bond. Covalent bonds are formed
through the sharing of electrons. Hydrogen only has 1 valence electron. It and
helium both only want to have 2 valence
electrons. Hydrogen will achieve this by sharing 1 electron
each. Once they share the electrons, they have
access to 2 electrons each. When 2 electrons are being shared, that is
called a single bond. Fluorine has 7 valence electrons and desires to
have 1 more electron. It can obtain this electron
by sharing 1 electron as well. This results in each fluorine atom having access
to 6 of its own electrons and 2 shared electrons. Atoms can also share 4 or 6 electrons. When
this happens, the bonds are called double or
triple bonds respectively. Oxygen only has 6 electrons. It needs to obtain
2 more electrons. It does this by sharing 4
electrons with another oxygen atom. This gives each oxygen a total of 4 shared
electrons and 4 non-shared electrons. The shared electrons are referred to as a
bonding pair and the non-shared electrons are
referred to as a lone pair. Triple bonds are formed when an atom needs to
share 6 electrons to obtain an octet. Nitrogen has a total of 5 valence electrons and
needs to obtain 3 more to have an octet. 2 nitrogen atoms will share 3 electrons each to
obtain an octet. This results in 3 bonding pairs
of electrons and 1 lone pair of electrons. Covalent bonds can also be formed between 2 or
more different elements. Here you see 1 atom of
carbon and 4 atoms of hydrogen. Carbon has 4 valence electrons and would like
to obtain 4 more. In this compound, carbon
shares its 4 electrons with 4 hydrogen atoms. Notice that 4 separate single bonds are formed.
1 electron from each hydrogen is shared with 1
electron from carbon. Each hydrogen now has access to 2 electrons
and carbon has access to 8.