1.
2.
3.
4.
The ancient Greek philosopher Democritus coined what word for a tiny piece of matter
that cannot be divided?
a) Element
b) Electron
c)
Atom
d) Molecule
Dalton’s theory (~1800; based on behavior of gasses) included all but one of the following
points. Which is not from Dalton?
a) All elements are composed of atoms.
b) Most of an atom’s mass is in its nucleus.
c)
Compounds contain atoms of more than one element.
d) In a specific compound, atoms of different elements always combine in the same
way.
J. J. Thomson’s gas tube electrical experiments (~1897) provided first evidence of:
a) Atoms
b) Nucleus
c)
Subatomic particles
d) Elements
Rutherford’s gold foil experiment (~1914) provided evidence for one of these:
a) Negative and positive charges are spread evenly throughout an atom.
b) Alpha particles have a positive charge.
c)
Gold is not as dense as previously thought.
d) There is a dense, positively charged mass in the center of an atom.
J.J. Thomson was the first to detect
subatomic particles (later named
electrons)
Electron:


›
›
›
›
Negatively charged subatomic particle.
Found in the space outside the nucleus.
Very light mass (~1/2000 of Hydrogen
atom)
Charge of 1-.
Thomson’s Model of the Atom
Thomson’s Experiments
In his experiments, J. J. Thomson used a sealed
tube containing a very small amount of gas.
Sealed tube
filled with gas
at low
pressure
Glowing beam
Positive plate
Metal disk
Metal disk
Source of
electric
current
Negative
plate
Metal disk
Source of
electric
current
Rutherford found positive charge
varies among the elements.

› Based on experiments with gold and
other elements.
Proton:

›
›
›
›
Positively charged subatomic particle
Found in the nucleus of an atom.
Charge of 1+.
Each nucleus contains at least one
proton.
Rutherford’s Atomic Theory
The Gold Foil Experiment
Deflected
particle
Undeflected
particle
Alpha
particles
Gold
atoms
Slit
Beam of
alpha
particles
Screen
Source of
alpha
particles
Alpha
particles
Nucleus
James Chadwick 1932 - the English physicist
carried out an experiment to show that
neutrons exist.
›
›
Radioactive decay produced particles about
the mass of a proton but without any charge.
Proved the particles produced were neutral
because a charged object did not deflect their
paths.
Neutron:
3)
›
›
›
Neutral (no charge) subatomic particle
Found in the nucleus of an atom.
Mass ~ equal to that of a proton.
Comparing Subatomic Particles
Everything we know about subatomic particles is
based on how the particles behave in
experiments.
› Scientists still do not have an instrument that can
show the inside of an atom.
Mass
• Protons and neutrons have almost the same
mass.
Mass of ~2000 electrons = the mass of one
proton.
Charge
• An electron charge (-1) is equal in size but the
opposite sign of a proton charge (+1).
• Neutrons have no charge.
Location in the atom
• Nucleus - Protons and neutrons
• Outside the nucleus - electrons
Atoms of different elements have different
numbers of protons.
 Atomic Number = Number of protons in an
atom of that element.

For any given element:
 All atoms have same atomic number.
› Hydrogen has 1 proton; atomic number = 1.
› Helium has 2 two protons; atomic number = 2
› Etc.
 Each
element has a unique atomic number.
Atomic Number and Mass Number
Each
element has a different atomic
number.
›A The atomic number of sulfur (S) is 16.
›B The atomic number of iron (Fe) is 26.
›C The atomic number of silver (Ag) is 47.
Atomic Number and Mass Number

Atoms are neutral when:
• Each positive charge in an atom is
balanced by a negative charge.
 i.e. # of protons = # of electrons
 Each proton has an electron to offset its charge

Examples:
• Hydrogen atomic number = 1
 Each neutral hydrogen atom has 1 proton & 1
electron.
• Sulfur atomic number = 16
 Each neutral sulfur atom has 16 protons & 16
electrons.
Atomic Number and Mass Number


Mass Number = Sum of protons +
neutrons.
To find # of neutrons (when mass # &
atomic # are known):
# of neutrons = mass # - atomic #
Example: Aluminum with mass # = 27
Mass # = 27
 Atomic # = 13.
 27 - 13 protons = 14 neutrons


An atom becomes charged when it
looses or gains an electron (e-).
› Positive Charge = loss of e-’s
 H becomes H+1
› Negative Charge = gaining of e-’s
 H becomes H-1

Electrons are the only subatomic particle
that can be gained or lost.
 Isotopes
= same element; different #
neutrons
 Isotopes
of an element:
1) Same # of protons
• i.e. same atomic number
2) Different
# of neutrons
• Different atomic mass because of the
neutrons

Isotopes are referred to by mass
number
› For example, oxygen has 3 isotopes:
 “oxygen 16”, “oxygen 17”, and “oxygen
18”
All
three oxygen isotopes have same
reactions
› Examples:
 All combine with hydrogen to form
water
 All combine with iron to form rust.
Isotopes

Most elements: Physical and chemical
properties of all isotopes are very similar.

Hydrogen is an exception.
›Hydrogen-1 no neutrons (most H is hydrogen-1)
›Hydrogen-2 has one neutron
›Hydrogen-3 has two neutrons.
 Because a hydrogen-1 atom has only one proton,
adding a neutron doubles its mass.
Isotopes
Heavy Water - hydrogen-2 atoms & oxygen-16
 Hydrogen-2 atoms twice the mass of Hydrogen-1

› Properties of heavy water are different from the
properties of ordinary water.
Assessment Questions
1)
Of the subatomic particles that form the
atom, the one with the smallest mass is:
a) Neutron.
b) Electron
c) Proton
d) Nucleus
Assessment Questions
2)
Of the subatomic particles that form the
atom, the one with the positive charge
is:
a) Neutron.
b) Electron
c) Proton
d) Nucleus
Assessment Questions
3)
Of the subatomic particles that form the
atom, the one with the neutral charge
is:
a) Neutron.
b) Electron
c) Proton
d) Nucleus
Assessment Questions
4)
Of the subatomic particles that form the
atom, which ones are found in the
nucleus?
a) Neutron.
b) Electron
c) Proton
d) None
Assessment Questions
5)
In which way do isotopes of an element
differ?
a. number of electrons in the atom
b. number of protons in the atom
c. number of neutrons in the atom
d. net charge of the atom