The Atom
The Roots of Atomic Theory
Science as we know it today did not exist several thousand years ago.
 There were no controlled experiments
 There were few tools for scientific exploration
Curiosity sparked the interest of many scholarly thinkers (philosophers)
 They speculated the nature of matter
 The formulated explanations based on their own life experiences
 Many concluded that matter was composed of:
Earth
Water
Air
Fire
 It was commonly accepted that matter could be endlessly divided into
smaller and smaller pieces.
Greek philosopher Democritus (460-370 BC)
 First person to propose the idea that matter was NOT infinitely divisible.
 Believed that matter was made up of tiny individual particles called atomos
 Believed that atoms could not be created, destroyed, or divided
 While his ideas don’t agree with modern atomic theory, his belief in the
existence of atoms was ahead of his time.
Here are his ideas about atoms:
 Matter is composed of atoms, which move through empty space.
 Atoms are solid, homogeneous, indestructible, and indivisible.
 Different kinds of atoms have different shapes and sizes.
 Size, shape, and movement of atoms determine the properties of matter.
Greek philosopher Aristotle (384-322 BC)
 One of the most influential Greek philosophers
 Rejected Democritus’s notion of atoms because it did not agree with his
ideas about nature
 He believed that empty space did not exist, therefore, atoms could not
move through it
 Because of his influence Democritus’s atomic theory was rejected
Here are his ideas about matter:
 Empty space cannot exist.
 Matter is made of earth, fire, air, and water.
John Dalton (1766-1844)
 Schoolteacher from England who revived and revised Democritus’s idea
based on scientific research
 He studied numerous chemical reactions, made careful observations and
measurements
 He was able to determine the mass ratios of the elements involved in
those reactions
Here are Dalton’s atomic theory ideas:
 Matter is composed of extremely small particles called atoms
 Atoms are indivisible and indestructible.
 Atoms of a given element are identical in size, mass, and chemical
properties.
 Atoms of a specific element are different from those of another element.
 Different atoms combine in simple whole-number ratios to form
compounds
 In a chemical reaction, atoms are separated, combined or rearranged.
Dalton’s atomic theory was a huge step toward the current atomic model of
matter. However, not all of his theory was accurate.
 He was wrong about atoms being indivisible. Atoms are divisible into
several subatomic particles.
 He was wrong all atoms of a given element having identical properties.
Atoms of the same element can have slightly different masses.
The Law of Conservation of Mass states that mass is conserved in any process such
as a chemical reaction.
 When atoms of two or more elements combine to form a compound, the
number of atoms of each element is conserved. The mass is conserved as
well.
What is an Atom?
 Suppose you decided to grind down a gold ring into a pile of dust. Each
fragment of that gold dust retains all the properties of gold. Even if you
could divide the gold dust into small particles, you would eventually get to
the point where you could not divide the particle any further. This smallest
particle would still retain all of the properties of gold.
 An atom is the smallest particle of an element that retains all the
properties of that element.
The Electron
Once scientists were convinced of the existence of atoms, a new set of questions
emerged:
 What is an atom like?
 What is the composition of an atom?
Scientists began to make connections between matter and electric charge.
 Think static electricity when you comb your hair
 With the help of a vacuum pump, scientists passed electricity through glass
tubes from which most of air had been removed. These tubes are called
cathode-ray tubes.
Sir William Crookes, English physicist
 While working with the cathode ray tubes, he discovered that there was a ray
going through the tube, originating at the cathode and traveling to the anode.
This ray was called a cathode ray.
Scientists continued research using the cathode ray tubes and by the end of the 1800s
were convinced of the following:
 Cathode rays were a stream of charged particles
 The particles carried a negative charge
Even after changing the metal that made up the electrodes, or varying the gases inside
the cathode ray tube, that did not change the cathode rays produced.
This led to the following conclusion:
 The negative particles found in the cathode rays are in all forms of matter
 The negative particles are now known as electrons.
Mass and Charge of an Electron

Despite all of the cathode ray tube experiments, scientists could not succeed in
determining the mass of a single cathode ray particle.
J. J. Thompson (1856-1940)
 English physicist who set out to determine the ratio of a cathode ray’s charge to
mass
 Measuring the effects of both magnetic and electric fields, he was able to
determine the charge to mass ratio of the charged cathode ray particle
This is what he concluded:
 After comparing the ratio to other known ratios, Thompson concluded that
the mass of the charged cathode ray particle was less than that of a
hydrogen atom.
 This meant that there were particles smaller than an atom and Dalton’s
theory was wrong.
 He won a Nobel Prize in 1906 for discovering the electron.
Robert Millikan (1868-1953)
 American physicist, determined the charge of an electron using an oil drop
apparatus
1. Oil is sprayed into the top of the chamber
2. Droplets fall due to gravity, with a few falling through a hole in the
positively charged plate
3. The electric field is adjusted until a droplet hovers. The upward electric
force exerted on the droplet by the positively charged plate is exactly
balanced by the downward force of gravity exerted on the droplet.


He determined that the magnitude of the charge on each drop increased in
discrete amounts and determined that the smallest common denominator was
1.602 x 10-19 coulombs (A coulomb is a unit of electrical charge). He identified
this number as the charge of the electron.
Knowing the electron’s charge and using the known charge-to-mass ratio,
Millikan calculated the mass of an electron to be:
9.1 x 10-28g = mass of an electron
The Plum Pudding Model
 The existence of the electron and the knowledge of its properties let to more
questions about the nature of atoms.
 Since matter is neutral (no electric charge) and electrons are part of matter, how
can all matter be neutral? You know matter is neutral because you don’t go
around under everyday circumstances giving static shocks to people.
 Also, what accounts for the rest of the mass of a typical atom?
 J. J. Thompson proposed the Plum Pudding Model:
The Nucleus
Ernest Rutherford (1871-1937)
 Studied how positively charged alpha particles interacted with solid matter
 Conducted an experiment to see if alpha particles would be deflected as they
passed through a thin gold foil
 Based on the Plum Pudding Model, he expected the path of the alpha particles to
be only slightly altered by a collision with an electron.

Here’s what he found instead:
 A few of the alpha particles were deflected at large angles
 Several of the alpha particles were deflected straight back

This is what he concluded:
 The Plum Pudding Model was incorrect
 Based on the frequency of deflections, he concluded that the atom consisted
of mostly empty space through which the electrons move
 Almost all of the atom’s positive charge and almost all of its mass was
contained in a tiny, dense region in the center of the atom. This is what he
called the nucleus.
 He also concluded that negatively charged electrons are held within the atom
by their attraction to the positively charged nucleus.
To give you an idea of how small and dense the nucleus is, consider this:
 If the nucleus were the size of a dot in an exclamation point at the end of this
sentence, its mass would be approximately as much as 70 cars!
 An atom’s diameter is approximately 10,000 times the diameter of its nucleus. So
if the atom had a diameter of two football fields, its nucleus would be the size of a
nickel.
By 1920, Rutherford had refined his concept of the nucleus and concluded that the
nucleus contained positively charged particles called protons.
 A proton is a subatomic particle carrying a charge equal to but opposite that of
an electron.
 In 1932, Rutherford’s co-worker, English physicist, James Chadwick, showed
that the nucleus contained another subatomic particle called the neutron. A
neutron has a mass nearly equal to that of a proton, but has no charge.
Summary of the model of the atom
 All atoms are made up of three subatomic particles: electron, proton, and neutron
 Atoms contain a positively charged nucleus surrounded by one or more
negatively charged electrons
 The electrons are held within the atom by their attraction to the positively charged
nucleus
 The nucleus is composed of neutral neutrons and positively charged protons
 The nucleus contains more than 99.97% of the atom’s mass
 Because an atom is electrically neutral, the number of protons in the nucleus
equals the number of electrons surrounding the nucleus