What is an Atom?

When starting your lesson on Atoms and Molecules make sure to start with the Atom and the changes that have been discovered. Atoms are the simplest building block of matter. Originally, it was thought that atoms were indivisible. However, we now know that an atom can be broken down further, although its chemical properties are not retained.  Therefore, an atom can be defined as the smallest particle of a given element that retains the elements chemical properties. For example, a gold coin is made up of a large number of gold atoms molded into the shape of a coin, with small amounts of other elements. Gold atoms cannot be broken down into anything smaller and still retain the properties of gold. A gold atom gets its properties from the tiny subatomic particles from which it is made.

Matter describes all physical substances.

Examples: table, a pencil, water

Non-examples: heat, sound, light

Matter has mass and takes up space.  It is made up of atoms.

Because atoms are quite small, scientists use large models to explain the structure.

There are theories for the structure of atoms.

How Big Are Atoms?

Atoms are very small, measuring 10^-10 meters in size. They are so small in fact, that when stacked, it would require millions of them to make a layer as thick as a sheet of paper.  The table below gives you an idea of the relative sizes of the radius of some everyday objects compared to the radius of a hydrogen atom:

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The atoms of different elements can vary in size and mass, due to the number of subatomic particles and the number of electron shells they possess.

Early Models of the Atom

Democritus

Key Points of this Theory

• Universe was made of empty space and tiny bits of stuff
• He called these tiny bits of stuff atomos.
• He claimed that atomos could not be divided.

Dalton

Key Points of this Theory

• All elements are composed of indivisible particles.
• He claimed atoms of the same element are the same.
• He imagined atoms as tiny, solid balls.

Thomson

Key Points of this Theory

• Blueberry muffin model
• He claimed atoms were made of a positively charges material with negative charges scatter through it.

Nagaoka

Key Points of this Theory

• He claimed that an atom had a large positively charge sphere in the center
• He also claimed that electrons were in orbit around the center

Rutherford

Key Points of this Theory

• Atoms were mostly empty space
• He claimed a small, positively charged nucleus contained protons.
• He claimed the negatively charged electrons orbited randomly around the nucleus.

Bohr Model

Key Points of this Theory

• Electrons move in definite levels or shells around the nucleus in a pattern.
• Electrons move from one shell to the next by giving off or absorbing energy.

Chadwick

Key Points of this Theory

• Discovered the neutron, a particle having no charge but the same mass as a proton.
• Helped explain why atoms were heavier than the total of protons and electrons.

The Electron Cloud Model

Key Points in this Theory

• Electrons not in set orbits, but have set distances from nucleus
• This creates a negatively charged cloud around the nucleus
• It is impossible to determine where any one electron is at any given time

What Are Molecules?

Many atoms cannot exist singularly and will, therefore, react with one another to form molecules. A molecule is defined as a collection of two or more atoms of the same or different element in a definite arrangement. The atoms in a molecule are held together by chemical bonds which can only be broken when a chemical reaction takes place. Molecules can be simple, consisting of only a few atoms or complex containing thousands of atoms. For example, the simple molecule oxygen is a gas found in the air we breathe. It consists of two oxygen molecules chemically bonded together (O₂). By contrast, the carbon atoms in a 1-carat diamond weighing 0.2g contain 10²² atoms.

Some other common examples of molecules you will meet in this course are:

• H₂O (water)
• CO₂ (carbon dioxide)
• N₂ (nitrogen gas)
• O₃ (ozone)
• CH₂COOH (ethanoic acid)

What Are Compounds?

Molecules which contain different types of atoms are called compounds. All compounds are considered molecules; however, not all molecules are compounds. For example, in the image below, Hydrogen gas (H₂) is a molecule, but not a compound because it is made of only one element. Water, on the other hand, is considered both a molecule and a compound as it contains more than one element - two atoms of hydrogen (H₂) and one of oxygen (O).

Counting Atoms in Molecules and Compounds

When working out how many atoms there are in a compound or molecule there are two numbers that must be considered:

1. Subscripts – the small numbers that tell you how many atoms there are of a specific type in the molecule. For example: In 3H₂O, the ₂ is the subscript. The subscript ₂ in the example above comes after the H. This means there are two H’s (hydrogen atoms) in each molecule of H₂

2. Coefficients – the regular-­sized numbers that tell you how many molecules you have. For example, in 3H₂O, the 3 is the coefficient. This shows that there are three H₂O molecules in total.

Therefore the number of hydrogen (H) atoms and the number of oxygen (O) atoms in 3H₂O is six hydrogen atoms and three oxygen atoms. To find out the number of atoms, multiply all the subscripts in the molecule by the coefficient. (This will give you the number of atoms of each element.)

To mathematically find the number of elements that make up 3H₂O, multiply the 2 by the coefficient 3 to find that there are 6 H’s. Then we multiply the 1 by the coefficient 3 to find that there are 3 O’s.

NOTE:

Although the 1 is usually not written, 3H₂O can be written as 3H₂ O₁. (In other words, 3H₂O and 3H₂O₁ is the same thing.)

How to Count Atoms in a Chemical Formula (5 Easy Steps)

Step 1: Write out the chemical formula.

Step 2: List all the atoms present.

Step 3: Count the number of atoms of each element in one molecule.

Step 4: Multiply the number of atoms of each element by the coefficient.

Step 5: Check that your answer makes sense.

Example Atoms and Molecules Problems:

1. How many atoms in gasoline (C₈H₁₈)?

2. How many oxygen atoms in C₁₀H₇O?

3. Count the number of hydrogen atoms in 2 molecules of vitamin C (2C₆H₈O₆).

4. Give the total number of atoms in 3 molecules of caffeine (C₈H₁₀N₄O₂).

Atoms and Molecules Lab Activity

Using the Particle Information below, complete each task.

Protons

Atoms are the smallest pieces of matter.  They consist of a central part called the nucleus and an area surrounding the nucleus called the electron cloud.  Atoms are also electrically neutral when all the particles are present.

Protons are one of the particles that make an atom.  They are found in the nucleus of the atom.  Protons have a positive charge.  1 proton carries a positive charge of 1. It is often written like this:  ⁺1

Protons also have mass.  Called an AMU (atomic mass unit), the mass of 1 proton is used as a unit of measurement of mass in the super small world of atoms.  1 proton = 1 AMU.

All the atoms of a particular element have the same number of protons.  The number of protons is used to identify the element, and it is the top number (atomic number) in the element box on the periodic table.

Task 1:  Complete the following on the Atom Template.

1. With a red map pencil, draw 5 small circles close together in the center.  Put small plus signs (+) inside the 5 small circles.
2. Draw a line to label the circles protons
3. Draw another line with a bracket to label the center area nucleus
4. Add a note in the margin:  Protons have mass, 1 AMU

Neutrons

Neutrons are one of the particles that make an atom.  They are found in the nucleus of the atom.  Neutrons do not have a charge.  They are electrically neutral, or 0.

Neutrons also have mass.  A neutron has the same mass as a proton.  Called an AMU (atomic mass unit), the mass of 1 proton is used as a unit of measurement of mass in the super small world of atoms.  Since protons and neutrons have the same mass, 1 neutron = 1 AMU.

Each atom has an atomic mass, which is the combined mass of all that atom’s protons and neutrons.  Protons + neutrons = atomic mass.  This number is the bottom number in each element box on the periodic table.

Task 2: Complete the following on the Atom Template.

1. With a blue map pencil, draw 5 small circles close together in the center.  Do not color them in.
2. Draw a line to label the circles neutrons
3. Draw another line with a bracket to label the center area nucleus
4. Add a note in the margin: Neutrons have mass, 1 AMU

Electrons

Electrons are one of the particles that make an atom.  They are found in the electron cloud area surrounding the nucleus.  Electrons have a negative charge.  1 electron carries a negative charge of 1. It is often written like this:  ^-1

Electrons are so small they do not have enough mass to count.  Electrons are not part of the atomic mass number of an element because they are so small.

Since atoms are electrically neutral, the number of protons and the number of electrons are the same.  For example if the atom has ⁺5 protons, it will have ^-5 electrons.

Task 3: Complete the following on the Atom Template.

1. With a black map pencil, draw 5 tiny circles spread out on the rings. Place a negative sign (-) next to each of the tiny circles.
2. Draw a line to label the circles electrons
3. Draw another line with a bracket to label the rings area electron cloud
4. Add a note in the margin: Electrons do not have enough mass to measure

Atoms and Molecules Summary Questions

Use your atom template to answer the following questions:

1. Where are the protons located in an atom? ____________________________________
2. Do protons have a charge? ____________________________________ If so, what charge? ____________________________________
3. Do protons contribute to the atom’s mass? ­____________________________________
4. Where are the neutrons located in an atom? ___________________________________
5. What other particle are they with? ____________________________________
6. Do neutrons have a charge? ____________________________________ If so, what charge? ­____________________________________
7. Do neutrons contribute to the atom’s mass? ___________________________________
8. Where are the electrons located in an atom? ___________________________________
9. Do electrons have a charge? ­____________________________________ If so, what charge? ____________________________________
10. Do electrons contribute to the atom’s mass? ___________________________________
11. For an atom to be neutral, 2 particles have to be equal in number.  Which 2 particles? ­____________________________________
12. Complete the chart below:

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Draw the following models:

13. Atom 1:    2 protons, 3 neutrons, 2 electrons
14. Atom 2:    3 protons, 3 neutrons, 3 electrons
15. Atom 3:    4 protons, 5 neutrons, 4 electrons
16. Atom 4:    6 protons, 6 neutrons, 6 electrons

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Unit 1 – Structure and Properties of Matter

• 1-1 Atoms and Molecules

• 1-2 The Kinetic Molecular Theory and Properties of Matter

• 1-3 Models of an Atom

• 1-4 Atomic Structure

• 1-5 Isotopes

• 1-6 Electron Configuration

• 1-7 The Periodic Table of Elements