2.2 Molecules and Compounds

Nonpolar covalent bond

A bond in which the sharing of electrons between atoms is fairly equal.

At the completion of the reaction,

each atom has eight electrons in the outer shell, but each also carries a charge as shown.

Now both atoms have

eight electrons in their outer shells.

Figure 2.7 Formation of sodium chloride (table salt). (a) During the formation of sodium chloride, an

electron is transferred from the sodium to the chlorine atom.

Sometimes, atoms share more than one pair of

electrons to complete their octets.

In this case, we

say that the atom that has a greater attraction for a shared pair of electrons has a greater electronegativity

For example, one hydrogen atom will

share with another hydrogen atom.

Its valence shell has seven electrons,

so if it acquires only one more electron it has a stable outer shell.

In water,

the oxygen atom is more electronegative than the hydrogen atoms; as a result, water molecules are polar.

Moreover, because of its non symmetrical shape,

the polar bonds cannot cancel each other, and water is a polar molecule.

The result is crystals of

salt as in the table salt.

Covalent Bonding

A covalent bond results when two atoms share electrons in such a way that each atom has an octet of electrons in the outer shell (or two electrons, in the case of hydrogen)

Formula

A group of symbols and number used to express the composition of a compound.

(a)

A molecule of hydrogen (H2) contains two hydrogen atoms sharing a pair of electrons.

Polar covalent bond

Bond in which the sharing of electrons between atoms is unequal.

For example, the formula for glucose is:

C6H12O6 indicating 6 carbons, 12 hydrogens and 6 oxygens.

Ions

Charged particle that carries a positive or negative charge.

Covalent bond

Chemical bond in which atoms share one or more pairs of electrons.

Ionic bond

Chemical bond in which ions are attracted to one another by opposite charges.

List the different types of bonds that occur between elements.

Ionic Hydrogen Covalent Nonpolar Polar

Compare and contrast an ionic bond with a covalent bond.

Ionic bonding is when element accept or donate electrons in order to have a stable outer shell for both elements. Covalent bonds results when two atoms share electrons in such a way each atom has an octet of electrons in the outer shell. Ionic bond-Chemical bond and which ions are attracted one another by opposite charges. Covalent bond-Chemical bond in which atoms share one or more pairs of electrons

Explain why methane is non polar but water is polar.

Methane shares its electrons equally. While carbon is larger and has more protons than a hydrogen atom, the symmetrical nature of a methane molecule cancels out any polarities. Water is polar because it does not share electrons equally. The oxygen atom is more electronegative than the hydrogen atoms; as a result, water molecules are polar. Moreover, because of its non symmetrical shape, the polar bonds cannot cancel each other, and wait is a polar molecule. The more electronegative endow the molecule is designated slightly negative and the hydrogens are designate slightly positive.

Explain the difference between a polar and a non polar covalent bond.

Polar bonds don't share electrons but Nonpolar do share electrons.

Compound

Substance having atoms of two or more different elements in a fixed ratio.

After therefore, it has a net charge of -1 (symbolized by Cl-).

These charged particles are called ions.

Molecule

Union of two or more atoms of the same element; also, the smallest part of a compound that retains the properties of a compound.

Describe the process by which ions are formed.

When ionic bonds come together and have one or more protons than electrons or vice versa they form ions.

Figure 2.8 Covalently bonded molecules. In a covalent bond, atoms share electrons, allowing each atom to have

a completed outer shell.

The polarity of molecules

affects how they interact with other molecules.

Spontaneous reactions, which occur freely,

always release energy.

Ionic compounds are held together by

an attraction between negatively and positively charged ions, called ionic bond.

Chlorine (Cl), on the other hand, tends to be

an electron acceptor.

When a sodium atom and a chlorine atom come together,

an electron is transferred from the sodium atom to the chlorine atom.

While carbon is larger

and has more protons than a hydrogen atom, the symmetrical nature of a methane molecule cancels out any polarities; thus, methane is a non polar molecule.

Because they form a covalent bond

and share the electron pair, each atom has a completed outer shell.

Their two electron shells overlap,

and the electrons are shared between them.

This electron transfer, however,

causes imbalance in each atom.

(b) In a sodium chloride crystal, ionic bonding between Na+ and Cl-

causes the atoms to assume a three-dimensional lattice in which each sodium ion is surrounded by six chloride ions, and each chloride ion is surrounded by six sodium ions.

A more common way to symbolize that atoms

are sharing electrons is to draw a line between the two atoms, as in the structural formula H--H.

In practice, these two terms

are used interchangeably, but in biology we usually speak of molecules.

Electrons possess energy,

as do the bonds between atoms.

When a chemical reaction occurs,

as when glucose is broken down, electrons shift in their relationship to one another, and energy is released.

Single covalent bonds

between atoms are quite strong, but double and triple bonds are even stronger.

Nonpolar and Polar Covalent Bonds: When the sharing of electrons

between two atoms is equal, the covalent bond is said to be a non polar covalent bond.

Sodium (Na+) and chloride (Cl-) are not the only

biologically important ions. Some, such as potassium (K+), are formed by the transfer of a single electron to another atom; others, such as calcium (Ca2+) and magnesium (Mg2+), are formed by the transfer of two electrons.

To show that oxygen gas (O2)

contains a double bond, the molecule can be written as O--O.

(c) A molecule of methane (CH4)

contains one carbon atom bonded to four hydrogen atoms.

In a hydrogen atom, the outer shell is complete when it

contains two electrons. If hydrogen is in the presence of a strong electron acceptor, it gives up its electron to become a hydrogen ion (H+).

(b) A molecule of oxygen (O2)

contains two oxygen atoms sharing pairs of electrons.

However, in some cases one atom is able to attract electrons to a greater

degree than the other atom.

This results in a

double covalent bond.

As you may know, organisms routinely break

down glucose, the sugar show above to obtain energy.

Organisms are directly dependent on chemical-bond

energy to maintain their organization.

A molecule

exists when two or more of the same type of atoms bond together; it is the smallest part of a compound that retains its chemical properties.

For example, the amine

group (-NH2) is polar, and this causes amino acids and nucleic acids to exhibit polarity, as we will see in the next chapter.

Water (H2O) is a molecule that contains atoms of

hydrogen and oxygen.

But if this is not possible,

hydrogen can share with another atom and thereby have a completed outer shell.

Just as a handshake requires two hands,

one from each person, a covalent bond between two atoms requires two electrons, one from each atom.

Water is not the only polar molecule

in living organisms.

Salts are solid substances that usually separate and exist as

individual ions in water, as discussed in section 2.3.

The shape of a molecule may also

influence whether it is polar or non polar.

When sodium reacts with chlorine, an

ionic compound called sodium chloride (NaCl) results.

A compound

is a molecule containing atoms of at least two different elements.

When electrons are

not shared equally, the covalent bond is a polar covalent bond.

Sodium chloride is an example

of a salt.

The more electronegative end

of the molecule is designated slightly positive, and the hydrogens are designated slightly positive.

The single covalent bond can be

represented in any of the three ways show

A formula

tells you the number of each kind of atom in a molecule.

Ionic Bonding: Sodium (Na), with only one electron in its valence shell,

tends to be an electron donor.

Electronegativity

the ability of an atom to attract electrons toward itself in a chemical bond.

In a molecular formula,

the line is omitted and the molecule is simply written as H2.

After giving up an electron,

the sodium atom has one more proton than it has electrons; therefore, it has a net charge of +1 (symbolized by Na+)

it is commonly called table salt that is commonly used

to season food.

It is also possible for atoms to form

triple covalent bonds, as in nitrogen gas (N2), which can be written as N quadruple bond N.

A double covalent bond occurs

when two atoms share two pairs of electrons.

Once it gives up this electron, the second shell,

with its stable configuration of eight electrons, becomes its outer shell.