Ch.4
Atomic Structure
How do we know atoms exist?
Picture, in your mind, what
you think an atom looks like.
Until recently, scientists had never seen an
atom.
Without actually seeing them, scientists
theorized the existence of atoms based on
their observations of the properties of
matter.
Greek philosophers proposed the
atomic theory.
Atomic theory- the theory that all matter
is composed of atoms.
Experimental results supporting the
existence of atoms did not appear until the
eighteenth-century.
• Early investigators noticed certain
characteristics shared by all chemical
compounds.
• These observation about compounds, and
the reactions they undergo, led to three
important laws: the law of definite
proportions, the law of conservation of
mass, and the law of multiple proportions.
The law of definite proportions
• The law of definite proportions states
that a given compound contains the
same elements in exactly the same
proportions by mass, regardless of the
size of the sample or the source of the
compound.
• This means that every molecule of a certain
compound has the same number and types of
atoms, regardless of how or where it was
produced.
Law of conservation of mass
• The law of conservation of mass states
that the mass of the products of a
reaction equals the mass of the
reactants.
• This law applies when two or more
elements combine to produce a
compound, when a compound
decomposes, or when the atoms in a
compound are rearranged. (Fig.3-3 p.76)
The law of multiple proportions
• The law of multiple proportions applies
to different compounds formed from
the same two elements.
• Example: there are three different compounds
made from the elements nitrogen and oxygen.
All three compounds are gases, but each has its
own physical and chemical properties (Nitrogen
dioxide, Nitrogen monoxide, Dinitrogen
monoxide).
Dalton’s atomic theory
Now that the laws had been established,
John Dalton used the Greek concept of
the atom and the
3 laws to give the atomic theory a
scientific basis.
Dalton believed that there were about a
hundred different kinds of atoms from
which all matter was formed.
• According to Dalton, elements were
composed of only one kind of atom and
compounds were built from two or more
kinds of atoms.
• Dalton proposed that the properties of
elements differ from one another because
their atoms differ.
• Dalton recognized that the laws of definite
composition, conservation, and multiple
proportions could be explained by the
existence of tiny particles that combine
with one another.
• Based on this, he proposed a theory about
atomic structure and matter.
A Summary of Dalton’s Atomic
Theory
1. All matter is made up of indestructible
atoms.
2. All atoms of a given element are identical.
3. Atoms of different elements are different.
4. Atoms of different elements combine in
simple-whole number ratios.
5. Chemical reactions consist of
combination, separation, or
rearrangement of atoms.
What is the internal structure of
atoms?
• Experiments by several scientists in the
mid-1800s led to the first modification of
Dalton’s theory.
• See Historical Perspective
• Atoms were found to be divisible after all.
Scientists discovered that the atom was
made of smaller particles, referred to as
subatomic particles.
• While many subatomic particles have
been discovered, only three are important
to your study of chemistry.
• Can you guess which three ?
• These particles are electrons, protons, and
neutrons
• Proton-particle with a positive charge
found in atomic nuclei.
• Neutron- particle with no electric
charge found in the nuclei.
• Both protons and neutrons are located
in the nucleus.
• Nucleus- dense central portion of the
atom that contains all of the atoms
positive charge and nearly all of its
mass.
• Electron- particle with a negative
charge found outside the nuclei.
• Because protons and electrons have equal
but opposite charges, a neutral atom must
contain equal numbers of protons and
electrons.
• The number of protons and atom
contains is known as the atomic
number.
• The number of electrons must equal the
number of protons.
• Atomic numbers are always whole
numbers.
• Atomic Mass number is equal to the total
number of particles in the nucleus, which is
the total number of protons and neutrons in
the nucleus of the atom.
number of protons and neutrons (mass number) = 11
minus the number of protons (atomic number) = - 5
number of neutrons = 6
Periodic Table
8
8 protons
O
Oxygen
15.9994
8 neutrons
8 electrons
Atomic structures can be represented by
symbols.
Atomic number and mass number are
sometimes written with an element’s
symbol. The atomic number always
appears to the lower left of the symbol.
• Mass numbers are written at the upper
left of the symbol.
• Both numbers may be written with the
symbol.
Mass number
Atomic number
4
2
He
Isotopes
• All atoms of an element have the same
atomic number and therefore the same
number of protons. However, for many
elements, the number of neutrons can
vary.
• Atoms of the same element with
different numbers of neutrons are
called isotopes.
6Li
7Li
3 p+
3 n0
2e– 1e–
3 p+
4 n0
2e– 1e–
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Review
1. What is atomic theory?
2. State the laws of definite proportions,
conservation of mass, and multiple
proportions.
3. What is the difference between an element
and a compound?
4. What type of electric charge does an electron
carry?
5. What three subatomic particles are important
to chemistry?
Models of the Atom
a Historical Perspective
Early Greek Theories
• 400 B.C. - Democritus thought matter
could not be divided indefinitely.
• This led to the idea of atoms in a void.
fire
Democritus
earth
Aristotle
air
water
• 350 B.C - Aristotle modified an earlier
theory that matter was made of four
“elements”: earth, fire, water, air.
• Aristotle was wrong. However, his
theory persisted for 2000 years.
John Dalton
• 1800 -Dalton proposed a modern atomic model
based on experimentation not on pure reason.
•
•
•
•
All matter is made of atoms.
Atoms of an element are identical.
Each element has different atoms.
Atoms of different elements combine
in constant ratios to form compounds.
• Atoms are rearranged in reactions.
• His ideas account for the law of conservation of
mass (atoms are neither created nor destroyed)
and the law of constant composition (elements
combine in fixed ratios).
Adding Electrons to the Model
Materials, when rubbed, can develop a charge
difference. This electricity is called “cathode rays”
when passed through an evacuated tube (demos).
These rays have a small mass and are negative.
Thompson noted that these negative subatomic
particles were a fundamental part of all atoms.
1) Dalton’s “Billiard ball” model (1800-1900)
Atoms are solid and indivisible.
2) Thomson “Plum pudding” model (1900)
Negative electrons in a positive framework.
3) The Rutherford model (around 1910)
Atoms are mostly empty space.
Negative electrons orbit a positive nucleus.
Ernest Rutherford (movie: 10 min.)
• Rutherford shot alpha () particles at gold foil.
Zinc sulfide screen
Thin gold foil
Lead block
Radioactive
substance path of invisible
-particles
Most particles passed through.
So, atoms are mostly empty.
Some positive -particles
deflected or bounced back!
Thus, a “nucleus” is positive &
holds most of an atom’s mass.
Bohr’s model
• Electrons orbit the nucleus in “shells”
• Electrons can be bumped up to a higher
shell if hit by an electron or a photon of light.
There are 2 types of spectra: continuous spectra &
line spectra. It’s when electrons fall back down that
they release a photon. These jumps down from
“shell” to “shell” account for the line spectra seen in
gas discharge tubes (through spectroscopes).