Chemical Bonding
A molecule is a combination of two or more atoms. These atoms may be
of the same elements, as in the oxygen molecule (02), or of different elements
as in the hydrogen chloride molecule (Hcl). A more complicated molecule is
that of glucose, C6H12O6. Atoms are held together by bonds that may be
classified into two main types: ionic and covalent.
Most atoms are considered stable (nonreactive) when their highest (outer)
energy level is filled to eight. The noble gases – neon, argon, krypton, xenon,
and radon—all have eight electrons in their highest energy level, they are
stable. One exception to this rule of eight (the octet rule) is helium, which is
stable even though it has only two electrons in its outer energy level since
that energy level (the first) can hold only two electrons.
Atoms that do not have eight outer electrons may lose, gain, or share their
valence electrons with other atoms in order to reach a more stable structure
with lower chemical potential energy. This process of rearrangement of the
valence electrons is responsible for chemical reactions between atoms.
The electron-dot structure of an atom (also called a Lewis structure) is an
abbreviated representation for the structure of that atom.
In this system, the nucleus and all of the energy levels except the outer one
are represented by the symbol for that element. Each valence electron is
indicated by a dot. For example, the element sodium (symbol Na, atomic
number 11) has its nucleus surrounded by 11 electrons, 2 in the first energy
level, 8 in the secondenergy level, and 1 in the third (outer) energy level. The
electron-dot structure for the sodium atom is Na., with the dot representing
the one valence electron and the symbol Na representing the remainder of
the atom. Carbon, atomic number 6, has the electron configuration 2e) 4e).
The electron- dot representation for carbon is
.
.
C .
.
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There are many types of chemical bonds exist to allow the creation of
matter?
A chemical bond formed when two atoms are held together by a force of
attraction, sowhen two chemicals bond, it creates a new substance named
compound.
The element sodium is a silver-colored metal that reacts so violently with
water that flames are produced when sodium gets wet. The element chlorine
is a greenish-colored gas that is so poisonous that it was used as a weapon
in the First World War and when chemically bonded together, these two
dangerous substances form the compound sodium chloride, a compound so
safe that we eat it every day - common table salt.
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Covalent bonding:
This type of atomic bonding occurs when atoms share electrons. As opposed
to ionic bonding in which a complete transfer of electrons occurs, covalent
bonding occurs when two (or more)elements share electrons. Covalent
bonding occurs because the atoms in the compound have a similar tendency
for electrons (generally to gain electrons). This most commonly occurs when
two nonmetals bond together. Because both of the nonmetals will want to
gain electrons, the elements involved will share electrons in anstrength to fill
their valence shells.
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Polar and nonpolar covalent bonding
There are, in fact, two subtypes of covalent bonds. The H2 molecule is a good
example of the nonpolarcovalent bond. Because both atoms in the H2
molecule have an equal attraction (or affinity) for electrons, the bonding
electrons are equally shared by the two atoms and anonpolar covalent bond
is formed. Whenever two atoms of the same element bond together, a
nonpolar bond is formed.
Polar covalent bonding occurs because one atom has a stronger affinity for
electrons than the other (yet not enough to pull the electrons away completely
and form an ion). In a polar covalent bond, the bonding electrons will spend a
greater amount of time around the atom that has the stronger affinity for
electrons. A good example of apolar covalent bond is the hydrogen-oxygen
bond in the water moleculeH2O.
Water molecules contain two hydrogen atoms (pictured in red)
bonded to one oxygen atom (blue). Oxygen, with six valence electrons, needs
two additional electrons to complete its valence shell. Each hydrogen
contains one electron. Thus oxygen shares the electrons from two hydrogen
atoms to complete its own valence shell, and in return shares two of its own
electrons with each hydrogencompleting the H valence shells.
The primary difference between the H-O bond in water and the H-H bond is
the degree of electronsharing. The large oxygen atom has a stronger affinity
for electrons than the small hydrogen atoms. Because oxygen has a stronger
pull on the bonding electrons, and this leads to unequal sharing and the
formation of a polar covalent bond.
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Ionic bonding: In
this type of bonding, electrons in the atom are
transported from one atom to the next. When atoms react-meaning that the
electrons are either being negatively charged or losing the charge-they form
ions. Metals and nonmetals reacting are a sign of ionic bonding. When
produced to extreme heat, ionic compounds can form crystals. It is also
possible for them to conduct electricity. Table salt can again be used as an
example. When the sodium loses a valence electron, the chlorine gains one.
The sodium decreases in size while the chlorine grows. When listing ionic
compounds, the metal appears before the nonmetal. e.g. sodium chloride.
In ionic bonding, electrons are completely transferred from one atom to
another. In the process of either losing or gaining negatively charged
electrons, the reacting atoms form ions. The oppositely cNotice that when
sodium loses its one valence electron it gets smaller in size, while chlorine
grows larger when it gains an additional valence electron. This is typical of
the relative sizes of ions to atoms. Positive ions tend to be smaller than their
parent atoms while negative ions tend to be larger than their parent. After the
reaction takes place, the charged Na+ and cl- ions are held together by
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electrostaticforces, thus forming an ionic bond. Ionic compounds share many
features in common:
•
Ionic bonds form between metals and nonmetals.
•
In naming simple ionic compounds, the metal is always first, the
nonmetal second (e.g., sodium chloride).
•
Ionic compounds dissolve easily in water and other polar solvents.
•
In solution, ionic compounds easily conduct electricity.
Ionic compounds tend to form crystalline solids with high melting
temperatures, which results from the intermolecular forces (forces between
molecules) in ionic solids. Ionic solids form crystals with high melting points
because of the strong forcesbetween neighboring ions and the charged ions
are attracted to each other by electrostatic forces, which are the basis of the
ionic bond.
Hydrogen bonding is the third type of chemical bonding. When two
molecules are shared in a hydrogen atom, hydrogen bonding occurs. Polarity
is also found in hydrogen. This means that the compounds are not balanced.
Hydrogen bonds with oxygen to make water molecules. Hydrogen and carbon
make methane. Hydrogen and nitrogen form ammonia. Hydrogen and
chlorine come together to make hydrogen chloride.
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