Defining the Atom
• All matter is composed of atoms
• Atoms are the smallest particle of an element
that retains its identity in a chemical reaction.
• Atom comes from the Greek word atomos
meaning indivisible.
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Democritus
• Democritus (460 BC – 370 BC) was among
the 1st to suggest the existence of atoms.
• Democritus believed that atoms were
indivisible and indestructible.
• His approached was not based on scientific
method and was not accepted until later
scientific theory.
Dalton’s Atomic Theory
• The modern atomic thought began with John
Dalton (1766-1844)
• Dalton used experimental method to
transform Democritus’s idea of atoms into a
scientific theory.
• Dalton studied the ratios in which elements
combine and the result was Dalton’s atomic
theory.
Dalton’s Atomic Theory
• The ideas of Dalton’s Theory are:
– All elements are composed of tiny particles called atoms.
– Atoms of the same element are identical. Atoms of
different elements are different.
– Atoms of different elements can physically mix together
or chemically combine in simple whole number ratios to
form compounds.
– Chemical reactions occur when atoms are separated,
joined, or rearranged. Atoms of one element are never
changed.
Sizing Up Atoms
• ATOMS ARE TINY!!!!!!
• A pure copper coin the size of a penny
contains 2.4 x 1022 copper atoms. There are
only about 6 x 109 people on Earth!!!
• Individual atoms are observable with
instruments such as scanning tunneling
microscopes.
Subatomic Particles
• Atoms are now know to be broken down into
smaller, more fundamental particles called
subatomic particles.
• There are 3 kinds of subatomic Particles.
– Electrons
– Protons
– Neutrons
Electrons
• Discovered in 1897 by J.J. Thomson (18561940) an English physicist.
• They are negatively charged subatomic
particles.
• Thomson performed experiments using a
sealed glass tube with gases in it. He passed
an electric current through the tube and the
result was a cathode ray.
Electrons Continued
• U.S. physicist Robert A. Millikan (1868-1953)
carried out experiments to find the quantity of
charge carried out by an electron.
• He calculated the mass of the electron; that
is very similar to the excepted mass given
today.
Protons
• Eugen Goldstein (1850-1930) observed in
1886 that in a cathode-ray tube there were
rays going in the opposite direction. He
concluded they were positively charged
particles.
• Protons are positively charged subatomic
particles.
• Each proton has a mass about 1840 times
that of an electron.
Neutrons
• In 1932 English physicist James Chadwick
(1891-1974) confirmed the existence of
another subatomic particle.
• Neutrons are subatomic particles with no
charge but a mass nearly equal to the
proton.
Rutherford’s Gold-Foil
Experiment
• This experiment change the prevailing
thought of the structure of the atom.
• Because of this experiment Rutherford
concluded that most the of alpha particles
pass through b/c the atom is mostly empty
space, and the reason that some of the
particles were deflected were due to the
concentration of the positive charge in the
atom which is now known as the nucleus.
Structure of the atom
• The nucleus of the atom is the tiny central
core of an atom and is composed of protons
and neutrons.
• Rutherford’s model is the nuclear atom: In
the nuclear atom, the proton and neutron are
located in the nucleus. The electrons are
distributed around the nucleus.
• This will later be revised, let’s sneak peak @
page 128.
Distinguishing Among Atoms
• We learned yesterday that atoms are
composed of protons, neutrons, and
electrons.
• Protons and Neutrons are in the nucleus and
electrons surround the nucleus.
• So how do elements differ if they all contain
atoms?
Atomic Number
• Elements are different because they contain
different numbers of protons.
• The atomic number of an element is the
number of protons in the nucleus of an atom
in that element.
– Ex: The atomic number of Hydrogen (H) is 1, so
all hydrogen atoms have 1 proton.
– Ex: All Oxygen atoms (O) have 8 protons, so the
atomic number of Oxygen is 8.
Atomic Number
• Remember all atoms are electrically neutral.
• Therefore; the number of Protons equal the
number of Electrons.
• Meaning the number of negatively charged
particles must equal the number of postively
charged particles.
Mass Number
• Most of the mass of an atom is concentrated
in its nucleus.
• The mass number of an atom is the total
number of protons and neutrons in an atom.
– Ex: Helium (He) has 2 protons and 2 neutrons:
its mass number is 4.
– Ex: Carbon (C) has 6 protons and 6 neutrons: its
mass number is12.
Mass Number
• If you know the atomic number and mass
number of an atom, you can determine its
composition.
• The number of neutrons in an atom is the
difference between the mass number and
atomic number.
• # of neutrons = Mass # - Atomic #
Mass Number
• The composition of an atom can be written in
Shorthand notation using atomic number and
mass number.
– Ex: Look at Figure 4.8
– The atomic number is the subscript and the
mass number is the superscript.
– How many electrons does a gold atom have?
– How many neutrons does a gold atom have?