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Part 3: Atoms

What is right in front of you and yet is impossible to see? The answer is atoms. An atom is the smallest particle of matter that can exist. Anything smaller than an atom is just energy, not matter.

Here are some of the people who helped figure out the atom: mid-1600s, Boyle: all matter is made of atoms. early 1800s, Avogadro: gases of equal volume under equal pressure have the same number of atoms. early 1800s, Dalton: elements combine to make compounds in fixed amounts. late 1800s, Mendeleyev: arranged the elements in order of their weight. late 1800s, Thomson: discovered electrons. Early 1900s, Rutherford: discovered the nucleus of the atom by shooting particles through gold foil.

How did people figure out atoms existed when they are so small they can't be seen? They used the scientific method. They came up with theories which explained the facts, and then tested the theories with experiments to see if they worked. Bit by bit, all the evidence piled up until the atomic theory was the only good way to explain how matter worked.

Atoms are so small that if you lined 50,000,000 (50 million) of them up in a row, they would reach about a centimeter, yet atoms are mostly made up of empty space. That is because the parts of an atom are tiny. The center part of the atom compared to the rest of the atom is so small it is like a single paper clip compared to the size of the whole school. How small is an atom? So small that light beams miss it. That's why you cannot see an atom, even under a regular-light microscope.

Though you cannot see an atom with regular light, you can take a picture of it using transmission electron microscopes, field-ion microscopes, and scanning tunneling electron microscopes. Yet we knew that atoms existed long before we could take pictures of them.

Parts of the Atom

All atoms are more or less the same size, but different atoms are made differently. The atom is made of tiny bits of energy called subatomic particles, and each type of atom has a different number of particles. These particles are organized inside the atom in a definite pattern.

Because the particles are not matter themselves, just energy, they don't behave like matter. Sometimes it is useful to imagine them like little balls, and often diagrams of the atom show them that way, but subatomic particles are definitely not little balls. Subatomic particles are truly weird. Yet the way they act explains a great many things about matter, such as compounds, elements, nuclear bombs, electricity, and how you digest your food, to name only a few.

Structure of the Atom

Around the outside of the atom there are tiny particles called electrons. Electrons move constantly. Each electron has a negative electrical charge. Electrons can move away from the atom sometimes. They can be shared between atoms, or they can go from one atom to another. Electrons can even move through matter, which is what causes electricity. Electrons are very light and incredibly small.

In the center of the atom (the nucleus) are the bigger, heavier parts of the atom. There are two types of particle in the nucleus. One of them is the neutron, a particle with no charge. The other type of particle is the proton, a particle with a positive charge.

Is this what an atom looks like? Well, no, not really.
Is it a diagram which shows some basic ideas about the atom? Yes.

Is this what an atom looks like? No.
Is it a way of showing that the electron is sort of a cloud around the nucleus? Yes.

Neutrons and protons are held together very tightly in the nucleus, and are very hard to break apart. If you do break a nucleus apart, you release a great deal of energy, because the atom is made of energy.

 

You might know the formula for this amount of energy: E=mc² Which means that the energy (E) in an atom is equal to the mass of the atom (m) times the speed of light (c, or 300,000 kilometers per second) times the speed of light again. (the little number 2 means "squared," or times itself). Or, to put it more simply, there is a whole lot of energy inside an atom. A bunch. Oodles of it.

If you break apart a large number of atomic nuclei (the plural of nucleus), what you have is a nuclear explosion.

An atom has about the same number of neutrons as it has of protons, and the same number of electrons as protons too.

 

Protons and neutrons are made of even tinier particles which nobody has ever seen. These tiny particles are called quarks. Electrons are not made of smaller particles. They are small enough already.

How Atoms Behave

Remember elements? It turns out that each element is made of only one kind of atom. Hydrogen, for instance, is made only of hydrogen atoms. Helium is made of helium atoms. And uranium, the heaviest element found in nature, is made of only uranium atoms.

An atom of an element always has the same number of protons. An atom of hydrogen always has one proton, for instance. An atom of carbon has six protons. An atom of uranium has 92.

An atom with a certain number of protons always acts the same. Oddly, an atom can have different numbers of neutrons and different numbers of electrons and stay the same kind of atom. But if you add or take away a proton you have a different element.

Electrons can move around. When elements combine to form compounds, they are sharing or lending electrons. Sharing or lending electrons between atoms makes a chemical bond. A molecule (MOL-uh-kyool) is two or more atoms held together by a chemical bond. Just as an element is made of only one kind of atom, a compound is made of only one kind of molecule.

Water is a compound made of hydrogen and oxygen.
Is this what a water molecule looks like? Of course not.
Is it a way to show there are two hydrogen atoms and an oxygen atom in a molecule of water?
Well, yes.

Chemical bonds are the heart of chemistry. Chemists like to take molecules apart, make new molecules, find out the parts of molecules, figure out new ways of joining atoms, and generally mess around with matter.

In the next part of this unit, you will learn more about atoms and elements and get to know the codes chemists use to talk about chemicals.

Summary

• Atoms are the smallest particles of matter.
• Atoms are extremely small, and mostly made of empty space. They have a dense central part called the nucleus.
• The parts of the atom are the electrons, which have a negative charge and orbit the nucleus, the neutrons, which have no charge and are found in the nucleus, and the protons, which have a positive charge and are found in the nucleus.
• The number of protons in an atom is about the same as the number of neutrons and the number of electrons.
• Every element is made of only one kind of atom.
• An atom can only be one kind of element
• What kind of element an atom is depends on how many protons there are in its nucleus.
• When atoms share or lend electrons with other atoms, they make a chemical bond.
• Two or more atoms held together by a chemical bond are called molecules.
• A compound is made of only one kind of molecule.

Homework

1. Explain briefly (in a sentence or so) how scientists could figure out that all matter is made of atoms, even though atoms are too small to see.
2. In a list, give the three main parts of the atom, their charge, and where they are found in the atom.
3. The element carbon has an atom with six protons and the element boron has an atom with five protons. Suppose an atom has five neutrons, six protons, and seven electrons. Is it an atom of carbon or an atom of boron?

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Go on to Part 4: Chemical Notation

This page last modified August 15, 2002

Questions? Send me a note at dturner@haverford.org