Atoms are submicroscopic particles that make up the matter we see around us.
Some 92 kinds of atoms exist in nature, such as hydrogen, oxygen, iron,
gold, etc. That is, gold is made of gold atoms, and iron of iron atoms.
Other substances are made of fixed combinations of atoms, called molecules.
For example, water is made of two hydrogen atoms linked to an oxygen atom,
hence its chemical formula, H2O.
Atoms are themselves made up of even smaller particles. Each atom consists of
a core, or more technically a nucleus around which yet smaller
particles called electrons orbit.
The nucleus is itself composed of smaller particles called protons and
neutrons. Protons, neutrons, and electrons differ in several ways, but most
importantly here in their electric charge. Protons carry a positive (+)
charge, neutrons carry no charge, and electrons carry a negative(-) charge.
A fundamental law of nature states that opposite electric charges attract and
like charges repel. Thus, two protons (both +) repel one another but a
proton and an electron (+ and -) attract. This attraction between the
positive charge of the proton and the negative charge of the electron holds
the electrons in orbit around the nucleus and thus holds the atoms
themselves together. (Protons and electrons generate a gravitational force
as well, but because their mass is so tiny, the gravitational force between
them is insignificant compared to the electric force.)
1. Does a neutron attract an electron electrically?
2. Could an atom contain only neutrons in its nucleus and have orbiting
One of the most important properties of atoms is that they can store energy in
their electron orbital motion. You are familiar with storing energy by other
means. For example, you can store energy in a rubber band by hooking one end
over your thumb and stretching the band. You can release that stored energy
if you let go of the stretched band so it shoots across the room.
Similarly, when an electron is moved to an upper orbit in atom, energy is
stored in the atom. That energy is released when the electron returns to a
lower orbit. We see the released energy as light.
The figure below shows a simple animation of this process. A particle
collides with an atom and gives it energy. The energy lifts the atom's
electron to an upper level. The electron then returns to its original orbit
(drawn inward by the electrical attraction of the nucleus). The stored
energy is released as light.
Animation of atom emitting light
The rubber band analogy of storing energy can help you understand another
feature of how atoms store energy. Just as stretching the band farther stores
more energy (you can shoot it farther), so moving an electron farther from
the nucleus stores more energy in the atom. That is, orbits near the nucleus
have less energy than orbits farther out.
3. Given that blue light is more energetic than red, other things being
equal, which electron jump in the figure will produce blue light?
In describing how electrons orbit in an atom we have omitted one of their most
Electrons can orbit at only certain discrete distances from the nucleus
For example, in a hydrogen atom, the electron can orbit at a distance of about
0.05 nm, 0.2 nm, 0.45 nm, etc, but NOT at 0.6 nm or 0.02 nm.
(Note: 1 nm = nanometer = 10-9 meters). Scientists describe this
limitation on the orbits by saying they are quantized.
Notice this is very different from the case of planets orbiting the
Sun. A planet can orbit at any distance from a star. An electron can orbit
an atomic nucleus at only certain distances.
The quantum nature of atomic orbits critically affects the interaction of
atoms with light. Because the orbit in which an electron moves determines the
energy stored in the atom and because the orbit size can only have certain
values, the energy stored can only have certain values. That is,
the energy stored in an atoms is quantized.
For more on atoms and light, see the
tutorial on light (not yet available).
We said above that some 92 kinds of atoms exist in nature. What makes one
kind different from another? The answer is simple: the number of protons it
contains in its nucleus. A nucleus containing one protons is a hydrogen
atom. A nucleus with two is helium; six is carbon, 26 is iron, etc.
The number of neutrons is generally about the same as the number of protons,
although the usual kind of hydrogen atom has only a single proton with no
neutrons. Atoms with the same number of protons but different numbers of
neutrons are called isotopes . For example, an atom with 1 proton and
1 neutron is an isotope of hydrogen, called deuterium, or heavy hydrogen.
The figure at the left shows sketches of several kinds of atoms. The red
spheres represent protons. The blue spheres represent neutrons.
4. Which atom represents hydrogen?
5. Which atom represents carbon?
6. Which atom represents a helium atom?
7. If you took three helium atoms and somehow "fused" them into a single
atom, what kind of atom would you have?
In the above discussion we have avoided most numbers. To better understand
atoms, a few numbers will help. For example, the diameter of a hydrogen atom
is about 10-10 meters, or roughly 4 billionths of an inch
8. How many atoms would fit side by side in a line across your little
fingernail (roughly 1 cm across)?
The protons and electrons are themselves even tinier. The diameter of an
electron is about 10-14 meters, roughly ten thousand times smaller
than a hydrogen atom.
Electrons are also much less massive than protons or neutrons. The electron's
mass is about 1800 times smaller than a proton's. Protons and neutrons, on
the other hand, have roughly the same mass and radius. The proton mass is
about 1.67x10-27 kg. A typical young adult has a mass of about
60 - 80 kg.
9. How many protons does it take to make up the mass of a typical young
adult (taken to be 60 kg)?