Biology Summer Assignment Background Information
The Atom:
The concept of atoms was once again introduced to the scientific world by John Dalton.
Dalton put forth the concept of all matter being composed of small particles, atoms,
which varied in weight and size. An element was a substance that contained only one
particular type of atom. The atoms of one element are different from the atoms of any
other element. The atoms of elements combine in small whole number ratios to form the
many chemical compounds found on earth and in the rest of the universe. All atoms are
composed of a given set of subatomic particles: protons, neutrons, and electrons. These
particles have definite arrangements for any given element. The important thing to
remember is that the protons, electrons, and neutrons of one element are exactly the
same as the protons, electrons, and neutrons of any other element. It is their number
and arrangement that make the elements different. The basics of atomic structure are as
follows:

Protons are positively charged particles, weighing 1 atomic mass unit and
located in the nucleus.

Neutrons are neutrally charged particles, weighing approximately 1 atomic
mass unit and located in the nucleus.

Electrons are negatively charged particles weighing zero atomic mass units and
located in the various orbitals of the energy levels outside the atomic nucleus. We
do not count the mass of the electrons in calculating the mass of the atom.
So an atom is composed of protons, neutrons, and electrons, but what makes an atom of
one element different from the atoms of every other element?
The atomic number of an element is equal to the number of protons in the nuclei of
its atoms.
The atomic mass or mass number of an element is equal to the number of protons
and neutrons in the nuclei of its atoms.
1
An ion is an atom in which the number of protons differs from the number of electrons.
If an atom has more electrons than protons, the ion is known as an anion; if it has
fewer electrons than protons, it is known as a cation.
An isotope is an atom in which the number of protons and neutrons differ.
The number of electrons in an atom may be changed by adding or subtracting electrons
to form positive or negative ions of the element. The number of neutrons in the nucleus
of an atom may also be changed to create various isotopes of an element (some more
stable than others). But add or subtract even one proton from an atom of any element
and you no longer have the original element in any form. Now you have a different
element! The number of protons in an atom determines what element it is.
Chemical Bonding:
Though the periodic table has only 118 or so elements, there are obviously more
substances in nature than 118 pure elements. This is because atoms can react with one
another to form new substances called compounds. Compounds are formed when two or
more atoms chemically bond together, the resulting compound is unique both
chemically and physically from its parent atoms.
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 charged ions are attracted to each other by electrostatic
forces, which are the basis of the ionic bond. Ionic bonds form between metals and
nonmetals.
For example, during the reaction of sodium with chlorine:
sodium (on the left) loses its
one valence electron to
chlorine (on the right),
resulting in
a positively charged sodium
ion (left) and a negatively
charged chlorine ion
(right).
2
The second major 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 an effort to fill their valence shells. A good example of a covalent bond
is that which occurs between two hydrogen atoms. Atoms of hydrogen (H) have one
valence electron in their first electron shell. Since the capacity of this shell is two
electrons, each hydrogen atom will "want" to pick up a second electron. In an effort to
pick up a second electron, hydrogen atoms will react with nearby hydrogen (H) atoms to
form the compound H2. Because the hydrogen compound is a combination of equally
matched atoms, the atoms will share each other's single electron, forming one covalent
bond. In this way, both atoms share the stability of a full valence shell.
There are, in fact, two subtypes of covalent bonds. The H2 molecule is a good example
of the first type of covalent bond, the nonpolar covalent 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 a nonpolar covalent bond is formed.
Whenever two atoms of the same element bond together, a nonpolar bond is formed.
A polar covalent bond is formed when electrons are unequally shared between two
atoms. 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 a polar covalent bond is the hydrogen-oxygen bond in the water molecule.
Water molecules contain two hydrogen atoms 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 hydrogen,
completing the H valence shells.
The primary difference between the H-O bond in water and the H-H bond is the degree
of electron sharing. 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, it
preoccupies their time, and this leads to unequal sharing and the formation of a polar
covalent bond.
3
Strong intermolecular forces called hydrogen bonds are formed between water
molecules. It is a force between a partially positive hydrogen atom, and a partially
negative atom (either nitrogen or oxygen). Hydrogen bonding is responsible for many of
the unusual characteristics of water, namely its relatively high boiling point for a
molecule of its size, the wide range of temperature that this small molecule exists in
liquid form, its lower density in the solid form compared with its liquid form, and its
propensity to form dome-like droplets on surfaces.
Hydrogen bonds are formed between electronegative atoms of one molecule and
hydrogens that are bound to electronegative atoms of another molecule. For water, the
hydrogen bonds form between the oxygen of one water molecule and a hydrogen atom
of another water molecule.
Unique properties of water include cohesion and adhesion. Cohesion is when water
molecules are attracted to other water molecules because there is an attractive force
between the positive and negative parts of the water molecules. Adhesion is when
water molecules are attracted to other materials such as a glass beaker.
4
The pH Scale:
Acids are ionic compounds (a compound with a positive or negative charge) that break
apart in water to form a hydrogen ion (H+). The strength of an acid is based on the
concentration of H+ ions in the solution. The more H+ the stronger the acid.
Bases are ionic compounds that break apart to form a negatively charged hydroxide ion
(OH-) in water. The strength of a base is determined by the concentration of Hydroxide
ions (OH-). The greater the concentration of OH- ions the stronger the base.
Solutions containing bases are often called alkaline.
When acids and bases are added to each other they react to neutralize each other if an
equal number of hydrogen and hydroxide ions are present. This is called a
neutralization reaction. When this reaction occurs -salt and water are formed.
HCl + NaOH
NaCl + H2O
(Acid) (Base)---(Salt) (Water)
The strength of an acid or base in a solution is measured on a scale called a pH scale.
The pH scale is a measure of the hydrogen ion concentration. It spans from 0 to 14 with
the middle point (pH 7) being neutral (neither acidic or basic). Any pH number greater
than 7 is considered a base and any pH number less than 7 is considered an acid. 0
is the strongest acid and 14 is the strongest base.
5
6