Atomic Theory
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The concept of atoms began almost 2500 years ago in ancient Greece.
Greek philosophers based their conclusions not on evidence but from philosophical reasoning.
The concept of atoms remained a philosophical belief until the discovery of two quantitative laws of chemical combination.
- Law of Conservation of Mass - No detectable gain or loss of mass occurs in chemical reactions. Mass is conserved.
- Law of Definite Proportions - In a given chemical compound, the elements are always combined in the same proportions by mass.
A early nineteenth century English scientist, John Dalton, reasoned that if atoms really exist, they must have certain properties to account for the two laws of chemical combination.
These properties are now called Dalton's atomic theory.
- Matter consists of definite particles called atoms.
- Atoms are indestructible. In chemical reactions, the atoms
rearrange but they do not themselves break apart.
- The atoms of one particular element are all identical in mass and other properties.
- The atoms of one particular element are all identical in mass and other properties.
- When atoms of different elements combine to form compounds, new and more complex particles form. However, in a given compound the constituent atoms are always present in the same fixed numerical ratio.
This theory is not perfectly correct.
We now know that atoms can be broken into smaller pieces (see Nuclear Reactions) and most elements occur as mixtures of two or more isotopes (atoms of an element with slightly different masses).
Modern Atomic Theory
It has been found that atoms are also made up of three subatomic particles--protons, neutrons, and electrons.
The atom has a nucleus made up of protons and neutrons with electrons "orbiting" around it.
The internal structure of the atom allows us to understand its properties.
Because protons and neutrons are in the nucleus, they are sometimes collectively called nucleons.
Protons have a mass of approximately 1 atomic mass unit (amu--1.67 x 10-24 g) and have a positive charge.
Neutrons have a mass of approximately 1 amu but has no charge (neutral).
Electrons have a mass of approximately 1/1836 amu and have a negative charge.
Protons repel each other and electrons repel each other because of identical charges, keeping them spread out throughout the volume of the atom.
So that the repulsion don't split the entire atom apart, the repulsions are also offset be nuclear forces involving other subatomic particles not studied here.
Protons and electrons have opposite charges, and therefore attract each other.
This attraction holds the electrons around the nucleus (see Atomic Structure and Bonding).
The atomic number or number of protons determine what element is.
For example, every element in the universe with 10 protons is neon, and every element with 79 protons is gold.
Isotopes are variations on atoms.
The properties are slightly different and may be radioactive.
The atomic masses of isotopes are different because the number of neutrons in the atom are different.
You can not have a different number of protons because then by definition it is a different atom.
The mass number is the sum of the protons and the neutrons.
In a neutral atom (an atom with no electrical charge), the number of electrons always equal the number of protons.
Isotopes are represented with a mass number as superscript and atomic number as subscript.
For example, an atom with 82 protons and 126 neutrons is written as 20882Pb.
Pb is the symbol for lead.
An atom with 82 protons is always lead.
