The Atom

Overview: This section provides a review of the structure of the atom and its constituents.


New terms:

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.

Dalton's Atomic Theory of Matter

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.

The General Structure of the Atom

The atom consists of protons and neutrons in a central nucleus (the constituents of the nucleus are called nucleons) with electrons distributed outside in an electron cloud or orbital. As you will learn later, determining the exact location of these electrons is very difficult to say the least.

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.

Masses of Atomic Components

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

Atomic Symbols

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.

Introduction to the Periodic Table

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 Mass.


1. How many protons, neutrons and electrons are there in each of the following elements?
a. Answer
c. Answer
d. Answer

2. Complete the following chart:

Element Symbol

# of protons # of neutrons # of electrons net charge mass number
  15 15 15    
  33 42   -3  
    77   0 131
    58 43 +2  


3. A single atom of the magnesium isotope 26Mg has a mass of 1.1783 x 10-20 g (nucleons plus electrons). The nuclear radius of 26Mg is 2.963 x 10-15 m, while the atomic radius is 160 x 10-12 m.

  1. What is the nuclear density of 26Mg in g/cm3? Assume the nucleus is roughly spherical and assume the atomic mass only arises from the nucleons. Answer
  2. What is the ratio of the atomic and nuclear volumes (Vatom/Vnuc)? Answer
  3. The Mg2+ ion is formed by removing two electrons from the magnesium atom. The ionic radius of the Mg2+ ion is 65 x 10-12 m. Why do you think the ionic radius is so much smaller? Answer

Advanced Applications: Two Washington University chemistry professors study the properties of nuclear matter.


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.

Some things to think about

Practice Quiz: The Atom

Note: You will need a pencil, scratch paper, calculator, periodic table and equation sheet to work the practice quiz. Quizzes are timed (approximately 4 minutes per question).

We suggest that you print out the periodic table and the constants/equations page before starting the quizzes.

Open Periodic Table in a separate browser window.

Open Equations/Constants Page in a separate browser window.

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© Washington University in St. Louis, 2005.