Atoms, Molecules and Chemical Reactions:

Explained By Lego Analogy


This is a chemistry analogy that will help explain how atoms are the basic building for everything and how they combine to form molecules.

This analogy deals with your favorite childhood toy, Legos. Remember how much fun they were? They came in many colors and sizes. When you put them together the possibilities were endless.

Concepts Explained

atoms, elements, atomic symbols, molecules, molecular formula, subscript, chemical bond, chemical reaction, reactant, product, chemical equation, stoichiometric coefficient, balanced equation
Atoms are like Legos. Similar to the way that Legos are the basic building block for the Lego house in the picture below, atoms are the basic building block for everything. Everything around you, from the chair you're sitting in, to the air you breathe and even your body, is made up of tiny building blocks called atoms.


Take a look at the Lego house pictured above. Let's say you wanted to divide it into as many pieces as possible. Eventually, you would be left with a pile of individual legos. However, you would not be able to take apart the individual legos. That's because legos are indivisible.

Similar to Legos, atoms are also
indivisible (the word atom actually comes from, atomos, the greek word for indivisible). Like a Lego, an atom is the most basic of building blocks.

When your mom bought you a new box of Legos, the box contained many different types of Legos. They came in many different colors. Some were red, some were yellow.


Similarly, there are many different types of atoms. Each type of atom is called an
element. Just like how different legos come in different sizes and colors, elements have also have properties that make them unique.

For instance, the element carbon is a black solid. The element oxygen is an invisible gas.


Similar to the way we abbreviate NJ for New Jersey and CA for California, there are abbreviations for the elements called atomic symbols.

The atomic symbol for the element oxygen is O. For carbon it is C. For hydrogen it is H.

Going back to our lego analogy, let's abbreviate the red lego R and the yellow lego Y.

Red lego = R
Yellow lego = Y

When you were a kid you put the Legos together to make larger Lego structures. Take for example, the Lego structure in the picture below. It is made of 1 red Lego and 2 yellow Legos.


Similarly, atoms can attach together to form something called a

The attachment between atoms is called a chemical bond. Water is an example of a molecule. A water molecule is made up of 2 hydrogen atoms and 1 oxygen atom attached together.

watermolecule2 copy

A molecular formula is a way of expressing what's in a molecule.

A molecular formula tells you two things:
  1. What types of atoms are in a molecule
  2. How many of each type of atom are in a molecule
In a molecular formula the atomic symbols of each type of atom in the molecule are written, followed by the number of that type of atom.

Lego Example:

If we were to write a molecular formula for the lego structure above it would be: Y

In the molecular formula, the Y
means that the structure has yellow Legos. The 2 after the Y is called the subscript and tells you that there are 2 yellow legos. The R means there is a red lego in the structure. The fact that there is no number after the R means that there is only 1 red lego.

Chemistry Example:

In the case of water, which is made of 2 hydrogen atoms and 1 oxygen atom, the molecular formula would be H
When chemical substances (elements or molecules) break or form chemical bonds with each other, we call it a chemical reaction.

A chemical reaction is similar to a red Lego and 2 yellow Legos forming "Lego bonds" with each other to form a Lego structure in the picture below:


When we combine 2 hydrogen atoms with 1 oxygen atom a chemical reaction occurs. The hydrogen atoms and oxygen atoms form bonds to each other and a water molecule is formed.


In a chemical reaction, the chemical substances that undergo change are known as the
reactants. The chemical substances that are formed are known as the products.

In the reaction above, the oxygen and hydrogen atoms are the reactants and the molecule of water is the product.

Chemical Equation

A chemical equation is a simple way of writing down a chemical reaction. If we were to write the chemical equation for the reaction of a red lego with 2 yellow legos it would like this:

R + 2Y --> Y2R

Here are 3 things to know about chemical equations:
  1. In a chemical equation, the reactants are written on the left side of an arrow and the products are written on the right side of the arrow:
  2. The number written in front of the reactants and products is called a a stoichiometric coefficient (this is not to be confused with the subscript). The stoichiometric coefficient tells you how many of that reactant or product are involved in the reaction. The 2 in front of the Y means that 2 yellow legos react with 1 red lego.
  3. When there is no number in front of a reactant or product, the stoichiometric coeffecient is 1. There is no number in front of the R which means that one red lego is involved in the reaction. There is no number in front of the Y2R, which means that one Y2R Lego structure is formed.
Now let's look at the chemical equation for the reaction of oxygen and hydrogen to make a molecule of H

O + 2H --> H

In this chemical equation we see that the reactants are hydrogen and oxygen and the product is a molecule of H
2O. The stoichiometric coeffecients tell us that 1 oxygen atom is combined with 2 hydrogen atoms to form 1 molecule of H2O.
Take a look at the “Lego reaction” below and see if you can point out anything that seems wrong:

In this “Lego reaction” you combine 1 red lego with 1 yellow lego and end up with a “Lego molecule” with 1 red Lego and
2 yellow Lego. You may ask yourself, where did that second yellow lego come from?

If you have a certain number of red and yellow Legos in your hand before a “Lego reaction” you should have the same number of red and yellow Legos in your hand after the “Lego reaction”, they will just be connected together in different ways. You can’t make an extra yellow Lego appear out of thin air. On that same note, you couldn’t make a certain type of Lego dissapear into thin air either.

The same principle holds true for chemical reactions involving atoms. In a chemical reaction, the total number of each type of atom present on the reactant side of the equation must be the same as in the product side of the equation. A chemical equation that shows the same number of each type of atom in the reactants and products is known as a
balanced equation. All chemical reactions must be balanced.

Let’s examine the chemical reaction between oxygen and hydrogen to make H
2O again.

O + 2H --> H2O

This reaction is balanced. On the reactants side we have 1 oxygen atom reacting to form a product that contains 1 oxygen atom. We also have two hydrogen atoms reacting to form a product with 2 hydrogen atoms.

At some point in your chemistry class, you be given a problem in which you have to balance an unbalanced chemical reaction by plugging in the right stoichiometric coeffecients. To learn more about how to solve these types of problems click below: