Download Lesson 3.1

Lesson 3.1 Introduction to Atoms
Atomic Theory
The smallest visible speck of dust
may contain about 10 million billion
atoms
Around 430 BC, a Greek philosopher
Democritus suggested that matter
was formed of small pieces that
could not be cut into smaller parts.
He used the word “atomos”
meaning “uncuttable,” for these
smallest possible pieces.
The atom is the smallest particle
that still can be considered an
element
The idea of atoms began to develop
again in the 1600s.
Atomic theory grew as a series of
models that developed from
experimental evidence. As more
evidence was collected, the theory
and models were revised.
Dalton’s Atomic Theory in 1803
He thought that atoms were like
smooth, hard billiard balls.
1. All elements consist of atoms that
cannot be divided
2. All atoms of the same element are
exactly alike and have the same
mass. Atoms of different elements
have different masses
3. An atom of one element cannot
be changed into an atom of a
different element by a chemical
reaction
4. Compounds are formed when
atoms of more than one element
combine in a specific ratio
Thomson’s Model in 1897
One important change to Dalton’s
model was that atoms have even
smaller parts. In 1897, JJ Thomson
discovered that atoms contain
negatively charged particles called
electrons. But scientists knew that
atoms had NO electrical charge, so
that meant that atoms must also
contain some sort of positive charge
to balance out the negative
electrons.
Rutherford’s Model in 1911
Ernest Rutherford was one of
Thomson’s students. His research
team aimed a beam of positively
charged particles at a thin sheet of
gold foil. They predicted that the
charged particles would pass
straight though the foil – and most
did. To their surprise, a few
particles were deflected back at
them and sideways. Based on these
results, Rutherford suggested that
the atom is mostly empty space, but
has a positive charge at its center.
Like charges repel, so the atom’s
positive charge must be packed at
the center, called a nucleus. Any
particle that was deflected back had
been repelled by the positive
nucleus. Later research suggested
that the nucleus had one or more
positively charged particles, and he
called them protons.
Bohr’s Model in 1913
Niels Bohr was one of Rutherford’s
students! Bohr revised the atomic
model and suggested that electrons
are found only in certain orbits, like
planets orbiting the sun.
Cloud Model in the 1920s
Scientists determined that electrons
do not orbit the nucleus like planets,
but move rapidly within a cloud-like
region around the nucleus. An
electron’s movement is related to its
energy level, or the specific amount
of energy it has. Electrons at
different energy levels are likely to
be found in different places. This
was proposed by Erwin Schrodinger
– he took Bohr’s idea and improved
upon it.
Modern Model of the Atom
In 1932, English scientist James
Chadwick showed that another
particle exists in the nucleus of the
atoms. This particle, called a
neutron, was hard to find because it
has no electric charge.
At the center of the atom is a tiny,
dense nucleus containing protons
and neutrons. Surrounding the
nucleus is a cloudlike region of
moving electrons.
Most of an atom’s volume is the
space in which the electrons move.
This space is huge compared to the
space taken up by the nucleus. If
the nucleus was the size of a green
pea in the middle of a football
stadium, the electrons would reach
as far away as the top row of seats!
Protons are shown with a plus sign +
Electons are shown with a negative
sign –
The number of protons equals the
number of electrons, so the charges
balance, making the atom neutral.
Neutrons don’t affect the charge of
an atom because they have a charge
of zero.
Comparing Particle Masses
Electrons are so tiny that it takes
almost 1,840 electrons to equal the
mass of one proton. A proton and a
neutron are about equal in mass.
Together, the protons and neutrons
make up almost all the mass of an
atom.
Atoms are too small to be described
by grams and kilograms, so scientists
use “atomic mass units” or “amu.” A
proton OR a neutron is equal to one
amu.
Atomic Number – The number of
protons in the nucleus of an atom is
the atomic number.
Isotopes – All atoms of an element
have the same number of protons,
but sometimes the number of
neutrons can vary. Atoms with the
same number of protons, but
different numbers of neutrons are
called isotopes.
An isotope is identified by its mass
number, which is the sum of the
protons and neutrons in the atom.