Basic Chemistry
Matter is any substance which occupies space and has mass. All matter is made up of a
limited number of elements, substances which cannot be broken down by chemical means. The
known elements are listed in the periodic table. Try looking at the periodic table at this site:
http://chemistry.about.com/library/blperiodictable.htm You will want to keep the periodic table
page open while you are working through this exercise.
Every element has a name (carbon, hydrogen, oxygen) and a symbol, which is a one- or twoletter shorthand way of identifying the element (C, H, O). Each element is also identified by its
unique atomic number. Use the periodic table to determine the atomic numbers for elements
listed below.
Carbon ______
Hydrogen ______
Oxygen ______ Nitrogen ______
Sodium ______
Potassium ______
Calcium ______ Chlorine ______
Elements are composed of small particles called atoms. So, while elements are the building
blocks of all matter, atoms are the building blocks of an element. Atoms, in turn, are composed
of 3 different types of subatomic particles. The atomic nucleus is located at the center of an
atom. It contains two types of subatomic particles. Protons, which are small positively charged
particles, and neutrons, which are small neutral (not charged) particles, are found in the nucleus.
The third type of subatomic particle is the electron, an incredibly tiny negatively charged
particle that is found in electron shells or orbitals surrounding the nucleus.
Overall, the nucleus of an atom is positively charged because it contains only neutral and
positively charged particles. The electron shells are negatively charged because they contain
only negatively charged particles. The atom as a whole has no net electric charge because the
positive charges of the protons are balanced by an equal number of negative charges of the
electrons. Look at the atom pictured below. Count the number of protons in the nucleus. _____
now count the number of electrons in the orbitals surrounding the nucleus. _____ Do you see
that the number of positive charges is exactly equaled by the number of negative charges?
Therefore, the atom is neutral; it has no net charge.
Number of Protons
The number of protons in an atom is very important because that is what distinguishes the
atom of one element from an atom of another element. All oxygen atoms have 8 protons, so any
atom with 8 protons is an oxygen atom. Therefore, 8 is the atomic number of oxygen. All
hydrogen atoms have 1 proton, so the atomic number of hydrogen is 1. Look back at the atom
pictured on page 1. How many protons do you see? _____ What element is this? ___________
Number of Neutrons
The number of neutrons is often equal to the number of protons. For example, carbon
atoms have 6 protons, and they most frequently have 6 neutrons as well. But some carbon
atoms have a different number of neutrons; for example, some carbon atoms have 8 neutrons
and 6 protons. This alternative form of carbon is known as an isotope.
Look at the atoms pictured above. First count the number of protons in each of the atoms.
All of them have a single proton; therefore, all of them are hydrogen atoms. The atom on the
left has 0 neutrons, the middle atom has 1 neutron, and the atom on the right has 2 neutrons.
They are isotopes of hydrogen.
Neutrons and Protons
Atoms have very little mass. Virtually all of the mass that they do have comes from the
neutrons and protons in the nucleus. One atomic mass unit is equal to the mass of a proton or
neutron. (They are virtually the same size.) The atomic mass of an atom is determined by
adding the number of protons to the number of neutrons. Because different atoms of the same
element have different numbers of neutrons, i.e., they are isotopes, the atomic mass is an
average. Look again at the periodic table. Click on “C” in the periodic table. You should see
that the atomic number for carbon is 6. The atomic mass, however, is 12.011, not 12. That is
because most carbon atoms have 6 protons and 6 neutrons, but there are isotopes of carbon
that contain 8 neutrons. Now click on “H”. You should see that the atomic number of hydrogen
is 1. You might expect the atomic mass to be 2 (one proton, one neutron, but the atomic mass is
listed as 1.0079. This is relatively low, so there must be a lot of hydrogen atoms with no
neutrons that are bringing down the average.
Number of Electrons
The number of electrons in an atom is equal to the number of protons. Carbon has 6 protons;
it also has 6 electrons. The electrons occupy electron shells around the nucleus. The lowest
energy shells are closest to the nucleus, the highest energy shells are farthest from the
nucleus. The first electron shell is capable of holding 2 electrons. This is fine for hydrogen,
which has one electron, and helium, which has 2. Lithium has 3 electrons; 2 of them would be
found in the first shell, the third would be in the second shell. The second shell can hold up to
8 electrons. Sodium has 11 electrons; 2 of them would be found in the first shell, 8 of them
would be found in the second shell, and 1 would be found in the third shell. See the
arrangement of electrons in the diagram below.
The number of electrons in the outermost shell determines the chemical properties of an
atom. An atom “likes” to have its outermost shell full of electrons. Hydrogen and helium would
each like to have 2 electrons in the first shell or orbital. Lithium and sodium would be willing to
give up the lone electron in the outer shell. Chlorine with 17 electrons would be willing to accept
an electron into its outermost shell or valance shell so that it will be full. Draw in the electrons
that would be found in a chlorine atom on the diagram below. Do you see an open space in the
outermost shell where an additional electron would fit?
An element like helium or argon whose outermost shell is already filled with electrons does
not try to seek electrons or give up electrons. Therefore, they are not likely to enter into
chemical reactions and are said to be chemically inert.
Carbon, hydrogen, oxygen, sodium, and nitrogen (common elements in the human body) are all
seeking to get or lose electrons. Therefore, they are likely to enter into chemical reactions and
are said to be chemically reactive.
Ionic Bonds
An ionic bond is formed when one atom gives up electrons to another. A classic example is
sodium and chlorine. Sodium is willing to give up an electron. If it does this, it will have 2
electrons in its inner shell and 8 electrons in its next shell. The shells are full, and sodium is
“happy”. A chlorine atom is willing to accept an electron. If it does, it will have 2 electrons in
its innermost shell, 8 electrons in its second shell, and 8 electrons in its outermost shell. The
shells are full, and chloride is “happy”. So, sodium and chloride form an ionic bond, and form a
new chemical compound – sodium chloride. Sodium has lost an electron, so it has an excess
positive charge. It is an ion, specifically, a cation. Chloride has gained an electron, so it has an
excess negative charge. It is an ion, specifically, an anion.
Adapted from: http://iss.cet.edu/electricity/pages/images/B/b11_5.jpg
Questions:
1.
Look at the chlorine atom pictured above. It has 17 protons and 18 neutrons in the
nucleus. It has 17 electrons. What do we have if we take away a proton? _________
If we add a proton? ___________
2.
What do we have if we take away a neutron? ___________If we add a neutron?
___________
3.
What do we have if we take away an electron? ___________ If we add an electron?
___________
Electrolytes
Salts are formed by ionic bonds, and when they are placed in water, they dissociate or
separate into their component ions. If a teaspoon of sodium chloride is placed in water, the
sodium chloride will separate into sodium ions (Na+) and chloride ions (Cl-). These charged
particles are capable of conducting small electrical currents like we would find in a battery.
Substances that are capable of conducting electricity in solution are called electrolytes.
Sodium (Na+), chloride (Cl-), potassium (K+) and calcium (Ca2+) are examples of important
electrolytes in the human body.
Covalent Bonds
Not all chemical compounds are formed by ionic bonds. Sometimes, instead of giving up or
accepting electrons, atoms decide to share them. Atoms can share a single pair of electron, two
pairs of electrons, or even three pairs of electrons. Again, the goal is for the atoms to fill their
outermost electron shell. Look at the hydrogen atom pictured below (left). It has 1 electron in
its outer (and only) shell, but it would like to have 2. The oxygen atom in the center has six
electrons in its outer shell, but it would like to have eight. If two hydrogen atoms share their
single electron with one oxygen atom, two single covalent bonds would form. Each hydrogen has
two electrons in its outer shell, and the oxygen atom now has eight electrons in its outer shell.
A new compound, water, is formed.
Look at the carbon atom pictured below. It has four electrons in its outer shell, but it would
like to have eight. Carbon would be willing to share electrons with other atoms in order to have
a full shell. It can share with other carbons, oxygen, hydrogen or other atoms. A single carbon
atom is able to form multiple covalent bonds, a property that makes it a key player in the
macromolecules that form living organisms.
Polar vs. Nonpolar Covalent Bonds
Not all covalent bonds are the same. If two atoms of oxygen form a covalent bond, they will
share the electrons equally; each atom exerts the same amount of “pull” on the electrons.
Therefore, O2 would be a nonpolar molecule. When an oxygen atom binds with two hydrogen
atoms to form a water molecule, the larger oxygen atom doesn’t share quite fairly. It exerts a
stronger pull on the electrons than the smaller hydrogen atoms do. The shared electrons will
stay closer to the oxygen end of the molecule, making the oxygen end a little bit negative and
the hydrogen a little bit positive. This is a polar covalent bond; the atoms are not shared
equally between the participating atoms.
+
-
Hydrogen Bonds
A third type of bond is the hydrogen bond. Look again at the polar water molecule. The
negative end of the water molecule will be attracted to the positive end of another water
molecule. Hydrogen bonds don’t create new compounds. They are an “attraction” between
molecules. Two water molecules are attracted to each other, but they do not form a new
compound H4O2. In a long macromolecule, hydrogen bonds can form between different parts of
the molecule, causing the molecule to fold, or pleat, or form a helix (spiral).
Questions:
1.
If two nitrogen atoms form a covalent bond, would you expect the bond between them
to be a polar or nonpolar covalent bond? _______________
2.
In a sodium chloride solution, which end of the water molecule would be attracted to
the sodium ion? ______________ The chloride ion? ______________
Reactions
Chemical reactions involve the breaking and making of chemical bonds. Typically, the
reactants are written on the left side of the equation and the products are written on the
right. The equations need to be balanced, that is, there should be the same numbers of each
type of atom on each side of the equation. For example:
2H2 + O2
2H2O
Reactants
Product
Theoretically, reactions are reversible. H2O could be broken down into its component parts.
2H2O
2H2 + O2
Reactant
Products
So you can think of reactions as going both forwards and backwards. If the reaction is
making product just as quickly, or at the same rate, that the product is breaking down into the
initial reactants, then equilibrium has been reached. That would be indicated like this:
2H2 + O2
2H2O
Energy can be required to break chemical bonds. A chemical reaction that requires the input
of energy is said to be endogonic. Other reactions result in the release of energy that has been
stored in chemical bonds. These reactions are said to be exogonic.