Ordinary matter (as opposed to dark matter) is mostly composed of atoms.
Atoms…
are discrete entities.
A discrete system is composed of distinct individual parts. A discrete system is separable into pieces.
The opposite of discrete is continuous. A continuous system forms an unbroken whole without interruption. One region blends seamlessly into another.
can only be found in a limited number of basic types called elements (short for chemical elements).
There are 90 naturally occurring elements found on Earth.
An additional 28 synthetic elements have been produced artificially in laboratories on Earth.
Some of the synthetic elements have been found to occur naturally in small quantites on Earth and on stars other than the Sun.
can be stable or unstable.
Unstable atoms have a finite existence.
There is a statistical probability that an unstable atom will decay into an atom of a different element.
Stable atoms are eternal.
The stable atoms of everyday existence are several billion years old.
Nearly all of the hydrogen and helium in the universe was created in the first three minutes of the universe's existence (13.8 billion years ago).
Nearly all of the elements heavier than helium found on the Earth were created many millions of years before the solar system formed (4.5 billion years ago).
Stable atoms can be used over and over again (recycled) in different combinations and will never "wear out".
can be "seen" only with great difficulty.
Atoms are effectively invisible.
Atoms are on the order of 10−10 m in size.
Light waves are on the order of 10−6 m in size.
Since light is 10,000 times larger than atoms, atoms are
too small to be "seen" with light. (No optical
device can ever be used to image atoms.)
Atoms can be inferred to exist through…
the chemical laws of definite and multiple proportions.