Physical and Chemical Properties

Learn About Atoms and Molecules

All physical things in the universe or matter are made up of tiny building blocks called atoms. They make up the air we breathe, the food we eat, the planet that we live on and even our bodies.If we could see atoms, we would discover they are made up of smaller particles.

Atoms are made up of three primary particles: protons, neutrons, and electrons.Protons and neutrons make up the nucleus or center of the atom. Protons are positively charged while neutrons carry no charge. Electrons are found outside of the nucleus of the atom. Electrons circle or orbit around the nucleus at very fast speeds and carry a negative charge.

Each atom is unique because of the number of protons, neutrons, and electrons that it has. These unique atoms are called elements. An element is the smallest unit of a chemical substance that can not be broken down by chemical means and still retain its unique properties. Each elemental atom has the same number of protons to electrons.For example, the most simple atom or element known is hydrogen. The hydrogen atom is made up of one proton, one neutron, and one electron. The element hydrogen has the same number of protons and electrons.The number of protons or electrons for each element is called the atomic number. Hydrogen's atomic number is 1, since it has only one proton (and also one electron).

Hydrogen is only one example of an element but there are many others. In fact there are over 100 known elements in the universe. To keep track of all the elements, scientists have built a chart called the “Periodic Table of Elements.”

The Periodic Table arranges each element according to its atomic number. The elements are also grouped according to whether the elements are metals (such as iron, gold, and silver) or non-metals (such as hydrogen, helium and oxygen).

Since atoms and elements are the building blocks that make up matter, elements can combine to form different substances.Two or more elements that combine to form new substances are called compounds and the smallest compound that a substance can be reduced down and still retain its physical properties is called a molecule.  For example, salt is a compound of sodium (Na) and chloride (Cl). A molecule of salt is composed of 1 sodium atom and 1 chloride atom.   This compound is written NaCl to represent one atom of each element.

A water molecule is a compound of 2 hydrogen (H) atoms and 1 oxygen (O) atom. The notation H2O represents a water molecule.The little number two, or subscript, represents the number of hydrogen atoms needed to make the water molecule.The O has no number following it, since only one oxygen atom is needed to make the water molecule.

Understanding the molecular structure of compounds is very important because if a molecule is broken down into smaller parts or if another atom is added to the molecule, the molecule will change and become another substance.  This change is known as a chemical change or a change in the chemical property.

For example, if another oxygen atom is added to a water molecule (H2O) it will become H2O2, hydrogen peroxide, which is very different than water.

Remember our discussion of salt (NaCl)? If you were to separate sodium (Na) from ordinary table salt the remaining substance would be pure chlorine (Cl), which is very poisonous. If you were to separate chlorine (Cl) from the molecule then the remaining substance would be pure sodium (Na), which can explode if it comes into contact with water. However, when sodium and chlorine are joined together as the sodium chloride molecule, NaCl, they become very stable. In fact, sodium chloride has been a part of people's diets for centuries.

Create a Compound

Welcome to the Create a Compound game!

Here is how you play.First, you will be given a molecular structure for a particular substance. On the left side of the screen is the “Periodic Table of Elements”.When prompted, you will drag the appropriate number of elements from the “Periodic Table” that you need to build the molecule over to the beaker. When you are finished building the molecule, press the test button to determine if you correctly built the molecule.

Water (H2O)
Iron Oxide (Fe2O3)
Polypropylene (C3H6)
Salt (NaCl)
Propane (C3H8)
Ethanol (C2H6O)
Nitrous Oxide (N2O)
Chloric Acid (HClO3)
Phosphoric Acid (H3O4P)
Phosphorous Pentachloride (Cl5P)

What’s Missing?

Hi! I’m the Chemical Plant manager, Maria.  If you have gone over the lesson about atoms and molecules you experienced how combining atoms into molecules will create a different substance. Now, let’s look at the opposite effect.If we take atoms and elements away from molecules and other chemical compounds, what will be the effect?

Chemical Properties

If we take away atoms from a molecule, the compound is no longer in its original state; meaning that it has changed its chemical property. Chemical properties are usually changed due to chemical reactions. An example of a chemical property of iron is its reaction with oxygen in the air. When iron is exposed to oxygen in the air, it begins to change its chemical property from iron to iron oxide or rust.

A shovel, for example, will undergo a chemical reaction when it is exposed to the oxygen in the air. If it is left out in the weather, this reaction happens much faster and eventually, the shovel would fall apart.

To illustrate chemical properties, below is a box of items (a salt shaker full of salt, a container filled with water, and a plastic jug) made up of chemical compounds that you may encounter every day.Click on an item to see what will happen if its chemical properties are changed.


Table Salt is made up of sodium (Na) and chlorine (Cl); if the sodium is removed, then table salt becomes chlorine gas.Of course this doesn’t happen when it is left out in the weather like the shovel. Instead, a process known as electrolysis is used to separate the two elements. The word "lysis" means to dissolve or break apart, so the word "electrolysis" literally means to break something apart (in this case salt) using electricity.


Water is composed of 2 hydrogen atoms and 1 oxygen atom. If the oxygen is removed it becomes hydrogen gas.Electrolysis is also used to separate this compound.


A plastic jug is typically made from a polymer called polypropylene, which is composed of 3 carbon atoms and 6 hydrogen atoms.If the carbon atoms are removed, it will become hydrogen gas.

Physical Properties

Physical properties are the parts of a compound or chemical that can be measured and directly observed. Physical properties include:

  • Color
  • Smell
  • State of matter
  • Taste

Not all physical properties are constant. Physical changes can occur when the state of matter of the chemical compound is changed but the chemical or chemical compound still retains the same number of atoms and molecules.

Usually physical changes will occur when a chemical or compound’s state of matter is changed.For example, when water is exposed to temperatures 32 degrees F or lower it becomes ice or its solid state. If water is exposed to temperatures greater than 32 degrees F and lower than 212 degrees F it will be in its liquid state.If water is exposed to temperatures greater than 212 degrees F then it will be converted to steam or its gas state.

To illustrate the process of physical changes, below is a box of items (a salt shaker full of salt, a container filled with water, and a plastic jug) made up of chemical compounds that you may encounter every day.


One example of a physical change is by dissolving salt when it is added to water.The salt seems to disappear when it dissolves, yet if the water evaporated, the salt would still be there


Water will freeze into a solid state (ice) at 32 degrees F or below, causing it to change its physical property.This is known as water’s freezing point.


A physical change for polypropylene or plastic occurs when it is introduced to temperatures at or above 330 degrees F causing the plastic to melt. This is known as plastic’s melting point.