Unit 1: Back to the Basics

Section 3: Molecules, Ions, and the Periodic Table

* Introduction
* Molecules
* Ways of Showing the Composition of Compounds
* Ions
* The Periodic Table

Introduction

In the last section, Section 2, atoms were defined as the smallest division
of all substances. For example, water can be broken down into hydrogen atoms
and oxygen atoms. This unit deals with the questions, What holds atoms
together? How are substances created using atoms? How do people know what
atoms and how many are in certain compounds? And how are atoms related to
each other?

Molecules

Molecules are compounds that contain more than one atom of the same or
different type of atom. Examples of molecules are CO, CO2, SiO2, N2, O2, and
F2. (Notice that the main criteria is that there are more that one atom
present, but it doesn't matter if the atoms are the same or different.) The
atoms in molecules are held together by chemical bonds, or more
specifically, covalent bonds. Chemical bonds are the forces that hold atoms
together. Covalent bonds are a specific type of chemical bond and this type
of chemical bond occurs when elements "share" electrons. Sharing electrons
means that one atom uses the electron for a while and then the other atom
uses the electron for a while and it keeps switching back and forth.

Ways of Showing the Composition of Compounds

All compounds are composed of different elements and different numbers of
them. To identify which elements and how many of them are present in
different compounds, a set of rules were set up to identify compounds. The
rules are very simple. The first rule is that chemical symbol is written to
tell what element is used in a compound and the second rule is a subscript
that follows the symbol to tell how many of these elements are present. The
name of the substance using this method of naming is called the molecular
formula. An example of the molecular formula are H2O (2 atoms of hydrogen
and 1 atom of oxygen), CO2 (1 atom of carbon and 2 atoms of oxygen), and Br2
(2 atoms of bromine).

TRY IT FOR YOURSELF: Practice Problems

There are still other ways of showing what atoms and how many are in
compounds. Another way might be to draw the structural formula. The
structural formula is a picture that shows what atoms are bonded to what
atoms and what the relative positions of the atoms are in the compound. In
this type of formula lines are drawn between the atoms to show how atoms are
bonded together.

Ions

Ions are atoms or groups of atoms with a net positive or negative charge.
For example, H+, OH-, H3O+, Na+, Mg+2, B+3, F- are all examples of ions.
Ions are created by elements giving away or gaining an electron or two. An
example might be that Na (sodium) might give away and electron which would
make it Na+ [Remember that Na starts out neutral and then gives away a
negatively charged electron so it becomes positive.]. Then Cl (Chlorine)
might come along and pick up that extra electron and then it would become
Cl- [Remember that Cl starts out neutral and then gains a negatively charged
electron, so it becomes negative.] An ion with a positive charge on it is
called a cation. An ion with a negative charge on it is called an anion.
(People remember that the cation is positive because the "t" looks like a +
sign, and the anion is negative because there is a "n" in it.

Above, in the molecules section, chemical bonds were described as the forces
that hold atoms together and covalent bonds were described as a specific
type of chemical bond in which electrons are being "shared." There is
another type of bonding and that is called ionic bonds. Ionic bonds involve
ions of different charges, and these ions would pull each other together
because of the opposite charges. They interact with each other much like the
positive side of a magnet would react with the negative side of a magnet. An
example of ionic bonding would be when Na+ and Cl- interact with each other
to produce NaCl. (Notice that there is not charge on the final product. This
is because the negative charge balances out the positive charge to leave a
charge of zero.)

The Periodic Table

The periodic table is a table that shows a list of all the elements, their
weights, and the number of protons of each element in an organized manner.
The periodic table is used to show trends, similarities, and differences of
elements. The periodic table is exactly what its name is, a table, so it has
columns and rows. The rows are called periods. The columns are called
families or groups. Each group (column) of elements has similar chemical
properties. Later in this Supplemental Text Book, more in depth information will
be presented regarding the periodic table, but for now as you learn about
different elements through the examples, make a mental note of what position
that element is in on the periodic table.