periodic table of Elements and isotopes) (Atomic
The modern view of an atom has come from many fields of chemistry and
physics. The idea of an atom came from ancient Greek science/philosophy and from
the results of 18th and 19th century chemistry:
- concept of the atom
- measurements of atomic mass
- repeating or periodic relationship between the elements
Concept of the Atom
From the ancient Greeks through today, we have pondered what ordinary matter is
made of. To understand the problem, here is a simple demonstration from a book
entitled "The Extraordinary Chemistry of Ordinary Things, 3rd Edition"
by Carl H. Snyder:
If you do the same thing with any element, you will reach an indivisible part
that has the same properties of the element, like the single paper clip. This
indivisible part is called an atom.
- Take a pile of paper clips (all of the same size and color).
- Divide the pile into two equal piles.
- Divide each of the smaller piles into two equal piles.
- Repeat step 3 until you are down to a pile containing only one paper clip.
That one paper clip still does the job of a paper clip (i.e., hold loose
- Now, take a pair of scissors and cut that one paper clip in half. Can half
of the paper clip do the same job as the single paper clip?
The idea of the atom was first devised by Democritus in 530 B.C. In
1808, an English school teacher and scientist named John Dalton proposed
the modern atomic theory. Modern atomic theory simply states the
Dalton's atomic theory formed the groundwork of chemistry at that time. Dalton
envisioned atoms as tiny spheres with hooks on them. With these hooks, one atom
could combine with another in definite proportions. But some elements could
combine to make different compounds (e.g., hydrogen + oxygen could make water or
hydrogen peroxide). So, he could not say anything about the numbers of each atom
in the molecules of specific substances. Did water have one oxygen with one
hydrogen or one oxygen with two hydrogens? This point was resolved when chemists
figured out how to weigh atoms.
- Every element is made of atoms - piles of paper clips.
- All atoms of any element are the same - all the paper clips in the
pile are the same size and color.
- Atoms of different elements are different (size, properties) - like
different sizes and colors of paper clips.
- Atoms of different elements can combine to form compounds - you can
link different sizes and colors of paper clips together to make new
- In chemical reactions, atoms are not made, destroyed, or changed -
no new paper clips appear, no paper clips get lost and no paper clips change
from one size/color to another.
- In any compound, the numbers and kinds of atoms remain the same -
the total number and types of paper clips that you start with are the same
as when you finish.
How Much Do Atoms Weigh?
The ability to weigh atoms came about by an observation from an Italian chemist
named Amadeo Avogadro. Avogadro was working with gases (nitrogen,
hydrogen, oxygen, chlorine) and noticed that when temperature and pressure was
the same, these gases combined in definite volume ratios. For example:
Avogadro said that at the same temperature and pressure, equal volumes of the
gases had the same number of molecules. So, by weighing the volumes of gases, he
could determine the ratios of atomic masses. For example, a liter of oxygen
weighed 16 times more than a liter of hydrogen, so an atom of oxygen must be 16
times the mass of an atom of hydrogen. Work of this type resulted in a relative
mass scale for elements in which all of the elements related to carbon (chosen
as the standard -12). Once the relative mass scale was made, later experiments
were able to relate the mass in grams of a substance to the number of atoms and
an atomic mass unit (amu) was found; 1 amu or Dalton is equal to
1.66 x 10-24 grams.
- One liter of nitrogen combined with three liters of hydrogen to form
- One liter of hydrogen combined with one liter of chlorine to make hydrogen
At this time, chemists knew the atomic masses of elements and their chemical
properties, and an astonishing phenomenon jumped out at them!
Fission and Fusion
Nuclear bombs involve the forces, strong and weak, that hold the nucleus of
an atom together, especially atoms with unstable nuclei. There are two basic
ways (splitting/binding) that nuclear energy can be released from an atom:
- Nuclear fission - You can split the nucleus of an atom into two
smaller fragments with a neutron. This method usually involves isotopes of
uranium (uranium-235, uranium-233) or plutonium-239.
- Nuclear fusion -You can bring two smaller atoms, usually hydrogen
or hydrogen isotopes (deuterium, tritium), together to form a larger one
(helium or helium isotopes); this is how the sun produces energy.