Overview: This section provides
a review of the structure of the atom and its constituents.
In chemistry the atom is the fundamental building block of chemical structures.
Unique combinations of atoms lead to the diverse array of compounds that form
the universe. Democritus, around 400 B.C., first proposed the idea of an unchangeable
atom. However, the Greeks had no experiments to test this idea. Around 1800
John Dalton performed a series of experiments to measure the ratios of the masses
of elements in different compounds. From the results of these experiments, he
hypothesized that matter consists of atoms and in 1808 he described the Atomic
Theory of Matter in his book A New System of Chemical Philosophy, Part I.
Though his points are not entirely true, it is very important to understand
them and appreciate the foresight of Dalton. Remember there was no way that
Dalton could see the atom at the time of his publishing. Today, however, modern
instruments provide direct evidence that atoms exist.
The atom continues to be fundamental in the field of chemistry as well as other sciences; however, today we
understand the atom in more depth than in Dalton's time.
This helium atom contains 2 protons
and 2 neutrons
within the nucleus. The 2 electrons
(in a neutral helium atom) are outside the nucleus
in the electron cloud.
The protons have a
positive charge, the neutrons are neutral and the electrons
are negatively charged. The radius of the atom in this cartoon is not
drawn to scale-- the radius of the atom is much greater than the radius
of the nucleus. In fact ratom > 104rnuc.
Based on this fact what conclusions can you draw about the atom? Answer
For a short biographical note on John Dalton, click here.
p+, proton mass: 1.67262 x 10-27 kg
no, neutron mass: 1.67493 x 10-27 kg
e-, electron mass: 9.1094 x 10-31 kg
Now what can you conclude about the atom? Answer
Each element is named by a symbol. When writing symbols for the elements, we
usually include the mass number and occasionally the atomic number. If we continue
with the example above, helium is represented by:
The mass number (A = # of protons + # of neutrons) is four. This is the total
number of protons plus the number of neutrons (i.e., the total number of nucleons).
The atomic number (Z) is 2 and is the number of protons. The atomic number often
is not included because the element name (or element symbol) also tells the
number of protons. If the number of protons changes, then it becomes a different
element. For example, helium will always have 2 protons; if you add a proton
then it becomes lithium (Li). By the way, this is a nuclear process and is not
easy to accomplish!
In a neutral atom, the number of electrons must equal the number of protons.
However if the atom has a non-zero charge, the number of electrons does not
equal Z, and the atom is referred to as an ion.
, # of e-
= Z, a neutral atom
, # of e-
< Z, a He cation
of e- > Z, a He anion
Note that the number of protons and neutrons are not changing. It is only the
number of electrons that change.
Ion: a charged atom produced by adding or removing an electron or electrons
to or from a neutral atom.
Cation: a positively charged ion.
Anion: a negatively charged ion.
In the type of calculations you will encounter in Chemistry 111/112, you will
probably make extensive use of the Periodic Table of the Elements. It is basically
a catalog of the properties of the elements. Each entry in the periodic table
looks something like the example entry for phosphorus shown below.
You can see that if you only have the atomic number for an element, you can use the periodic table to
determine the name of the element. Alternatively, if you are given the name of an element, you can easily
tell how many protons are in the nucleus of an atom of that element just by inspecting the periodic table.
How about determining the number of neutrons in the nucleus from the periodic
table if you are given the name of the element? You cannot do it. We will talk
about this when we discuss isotopes in the next section on Atomic
1. How many protons, neutrons and electrons are there in each of the following
2. Complete the following chart:
Now you should feel more comfortable with the structure and parts of the atom.
Also you should be able to write out atomic symbols and use the periodic table
to get some basic information about the elements.