Structure of Matter
Development of an Atomic Model
A model is important
to scienctists because it explains observed facts and can be modified.
More than 2400 years ago, the Greek philosopher Democritus
theorized the existence of the atom, the smallest piece of matter.
John Dalton's atomic theory was based on experimental
evidence about the behavior of matter.
Dalton's theory states that all mater is made of indivisible particles, or atoms.
A Divisible Atom
The discovery
of the electron by JJ Thompson proved that the atom is divisible.
Thompson's model pictured the atom being made of a
positively charged pudding-like material with negative charges scattered throughout.
Rutherford's experiements with gold foil
and positively charged particles led him to conclude that an atom has a small, dense, positively charged nucleus surrounded
by negatively charged electrons.
The Bohr model of the atom pictured electrons as moving in definite orbits, or energy levels,
around the nucleus.
According to the theory of wave mechanics, electrons do not move about an atom in definite orbits. The
exact location of an electron in an atom is impossible to determine.
Subatomic Particles
Protons and neutrons are found
in the nucleus. The nucleus accounts for 99.9 percent of an atoms's mass.
Protons have a positive charge and a mass
of 1 amu.
Neutrons are electrically neutral and have a mass of 1 amu.
The number of protons in the nucleus of an atom is
called the atomic number.
Atoms of the same element that have the same number of protons but different numbers of
neutrons are called isotopes.
The mass number of an atom is the sum of the protons and neutrons in its nucleus.
The atomic mass
of an element is the average mass of all the naturally occurring isotpes of that element.
Electrons have a negative charge
and a mass of 1/1836 amu.
Within the electron cloud, electrons are arranged in energy levels.
Subatomic particles
in the nucleus are made up of quarks.
Forces Within the Atom
The forces that govern the behavior of subatomic particles are electromagnetic,
stong, weak, and gravity.
