23.1. The Bohr Model of the Atom http://www.ck12.org
a philosophical statement that there are atoms. Dalton’s theory proposed a number of basic ideas: 1) All matter is
composed of indivisible, tiny particles called atoms, 2) atoms can neither be created nor destroyed, 3) all atoms of the
same element are identical (have the same mass), 4) elements differ from one another because they have different
types of atoms (different mass), 5) compounds are composed of combinations of different elements because the
atoms of different elements are bonded to one another, and 6) chemical reactions occur when atoms in compounds
are rearranged.
Dalton’s theory did not convince everyone immediately. It did convince a number of chemists right away but several
decades were required for all opposition to cease. Dalton’s theory underwent a number of modifications in the next
150 years but many of its ideas are still part of the modern atomic theory. In 1869, Dmitri Mendeleev ascertained
that there were groups of elements which had the same (or very similar) chemical properties, the same valence, and
similar physical properties and showed that the chemical and physical properties of the elements were periodically
repeated. His periodic table remains a major idea in chemistry today.
Discovery of Electrons and Radioactivity
During the investigation of the flow of electric current through gases, scientists discovered rays that were emitted
from a cathode discharge tube and had the property of being deflected in electric and magnetic fields. It was
determined that these rays consisted of rapidly moving, negatively charged particles called electrons. It was also
determined that upon heating or illumination by light, metals emitted electrons. Logically, neutral atoms containing
electrons must also contain positively charged particles. Thus, the indivisibility of the atom was disproved. This
was further emphasized by Marie Curie’s discoveries that through alpha and beta decay, atoms of one element
could transmute into other elements. Curie’s discoveries also determined that atoms of the same element may have
different masses (isotopes) and thus the idea that all the atoms of the same element are identical was also lost.
Rutherford’s Planetary Model of the Atom
Now that it was known that the atom had component parts, a new model was needed. In the model proposed by J.
J. Thomson in 1903, the atom was represented as a positively charged sphere with the negatively charged electrons
distributed around the exterior. This was the so-called “plum-pudding” model with the positive charge playing the
role of the pudding and the electrons playing the role of the plums. The next big step in the development of the
model of the atom occurred in 1911 with Rutherford’s gold foil experiment. In the Thomson model of the atom, the
density of the atom was necessarily small. The mass of the atom divided by the volume of the atom and assuming
an even distribution of mass resulted in a low density for the atom.
In 1896, Henri Becquerel discovered that uranium compounds emitted penetrating rays, some of which were mas-
sive, high speed, positively charged particles which were later named alpha particles (α). These alpha particles
would be detected by a zinc sulfide coated screen (scintillation counter) that emitted a small flash of light every
time an alpha particle hit it. Rutherford used these alpha particles and the zinc sulfide coated screen in his gold
foil experiment. The alpha particles were fired at a very thin sheet of gold foil. It was thought that all of the alpha
particles would pass straight through the gold foil with no deflection. This was because the alpha particles were
known to be very dense and, due to the “plum pudding” model of the atom where the mass of the atom was spread
out evenly over the volume of the atom, the atoms of gold were thought to have very low density.
While the great majority of the alpha particles did pass straight through the foil with no deflection, to everyone’s
surprise, some alpha particles were deflected. In fact, some alpha were bounced almost straight backward by the
foil. Rutherford, using Coulomb’s law and Newton’s laws found that the results could be explained only if all the
positive charge of the atom were concentrated in a tiny, central core, now called thenucleus. Rutherford’s model of
the atom is therefore, called thenuclear modelof the atom. All of the positive charge and essentially all of the mass
of the atom are in its nucleus. The atom is 10,000 times as large as the nucleus and is mostly empty space. It was
known that electrons are outside the nucleus but how the electrons were arranged in an atom was still a mystery.