Download Section 1.3: Atomic Theory What is a Theory?

Section 1.3: Atomic Theory
What is a Theory?
A theory is an explanation of observations that are supported by consistent,
repeated experimental results.
Some theories stand the test of time and become accepted. Others are discarded or
modified as new or conflicting evidence from experimental data or observations
are presented.
Ex. Theory of Evolution, Theory of Plate Tectonics
What is a Law?
A law is a description of events, patterns, or relationships that have been observed
over and over again-it does not provide explanations. Scientific laws enable
scientists to predict observations and events. A scientific law is unlikely to change.
Ex. Law of Gravity, Newton’s Laws of Motion, Law of Reflection of
Light and the Atomic Theory
For a theory to become accepted, it must be able to predict observations for novel
situations.
Atomic Theory
an attempt to describe the nature of matter and explain how matter
behaves.
Has undergone many modifications as new facts became available.
2500 years ago, (500 BC), Empedocles, a Greeks scientist and
philosopher claimed that matter was made up of four “elements” – earth,
air, wind and fire.
(460 BC) Democritus, a Greek scholar, countered this theory, reasoning
that matter could not be divided into smaller and smaller pieces forever.
Eventually the smallest possible piece would be obtained.
This piece would be indivisible. He named the smallest piece of matter
“atomos,” meaning “not to be cut.”
This theory was ignored and forgotten for more than 2000 years! WHY?
The eminent philosophers of the time, Aristotle and Plato, had a more
respected (and ultimately wrong) theory.
Aristotle and Plato favored the earth, fire, air and water approach to the
nature of matter. Their ideas held sway because of their eminence as
philosophers. The atomos idea was buried for approximately 2000 years.
Talk about Alchemists who work in Europe and the Middle East during the middle
ages.
Development of the Modern Atomic Theory
 There are four main people who contributed to our understanding of the
atom. They are:
 John Dalton (1766 – 1844) 1808
 J. J. Thomson (1866 – 1940) 1897
 Ernest Rutherford (1871 – 1937) 1911
 Niels Bohr (1885 – 1962) 1922
1. John Dalton (Billiard Ball Model)
suggested that the particles that make up matter are like small, hard
spheres that are different for different elements.
defined an atom as the smallest particle of an element.
Summary of his theory:
o All element are made up of small particles called atoms.
o Atoms cannot be created, destroyed, or divided into smaller
particles.
o All atoms of the same element are identical in mass and size,
but they are different in mass and size from atoms of other
elements.
( i.e. Atoms of the same element are exactly alike. Atoms of
different elements are different.)
o Compounds are created when atoms of different elements join
together in definite proportions.
2. J.J. Thompson (Raisin Bun Model)
determined that currents (electricity) were streams of negatively
charged particles, later called electrons.
found that all substances could produce electrons; therefore they must
all contain them.
Thomson proposed a “raisin bun” or “plum pudding” model of the
atom. It pictures a positively charged ball like a bun with negatively
charged particles embedded in it like raisins. Theory short-lived.
3. Ernest Rutherford
He is famous for his gold-foil experiment. He exposed a very thin
sheet of gold to a stream of high speed, heavy particles that had a
positive charge, called alpha particles. Alpha particles were like tiny
bullets.
He found that most of the alpha particles passed right through the gold
foil without being affected. However, some particles were deflected
off course and a few alpha particles rebounded from the foil (like a
ball rebounds from a wall)
He concluded that there must be a lot of empty space between and
within the gold atoms in the sheet. Atoms were not a pudding filled
with a positively charged material.
Because some of the alpha particles bounced back, he concluded that
an atom had a small, dense, positively charged center, called the
nucleus. A positively charged center would repel a positively charged
alpha particle.
Ten years later, he also established that there must be at least two
kinds of particles inside the nucleus- a proton which has a positive
charge and a neutron which has no electric charge.
4. Niels Bohr
A student of Rutherford
He studied the regions surrounding the nucleus which was known to
contain negatively charged electrons.
Bohr was the first to attempt to describe the way electrons are
distributed in an atom.
He proposed that electrons surround the nucleus in specific energy
“levels” or “shells”. This means that each electron has a particular
amount of energy.
There is a fixed number of energy levels
Each energy level is capable of holding a certain number of electrons
(more about this in chapter 2 section 3)
The location of an electron (with respect to the nucleus) is an
indication of the energy it contains (closer more – further away less)
The part of the energy level or region of space in which we find
electrons are also called orbitals.
These are not the fixed pathways we normally think of when
discussing orbits, but rather they are a region or area where the
electron would be located at some point in time.
Electrons can move from one energy “level” or “shell” to another.
Electrons gain energy as they move up to a higher level or orbital and
lose energy as they move to a lower level or orbital.