Transcript ATOMS The discovery and structure of atoms.

ATOMS
The discovery and structure of
atoms.
1. Elements are composed of extremely small
particles called atoms. All atoms of a given
element are identical. The atoms of one element
are different from the atoms of all other elements.
2. Compounds are composed of atoms of more
than one element. The relative number of atoms
of each element in a given compound is always
the same.
3. Chemical reactions only involve the rearrangement
of atoms. Atoms are not created or destroyed in
chemical reactions.
2.1
Dalton
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The theory that atoms
are the fundamental
building blocks of matter
reemerged in the early
19th century,
championed by John
Dalton
Each element is composed of extremely
small particles called atoms.
All atoms of a given element are identical to
one another in mass and other properties,
but the atoms of one element are different
from the atoms of all other elements.
Discharge Tubes

Discharged tubes produced rays that
seemed to come from the cathode. By
studying the properties of the rays, they
were found to consist of negatively
charged particles of low mass. These
particles are known as electrons.

Crookes modified the gas discharge tube
to include a small pin wheel. When the
cathode rays struck the pin wheel, it
rotated, suggesting that the particles had
mass. Thomson subjected the cathode
rays to electric and magnetic fields,
causing the rays to bend, therefore
inferring the particles were of small mass
and negatively charged.
The Electron
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Streams of negatively charged particles were
found to emanate from cathode tubes.
J. J. Thompson is credited with their
discovery (1897).
Radioactivity:
The spontaneous emission of radiation by
an atom.
 First observed by Henri Becquerel.
 Also studied by Marie and Pierre Curie.
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Roentgen discovered X-rays coming from
cathode ray tubes. Bacquerel found that
similar rays came from uranium samples,
but reasoned they could not be the X-rays
he had been looking for because they
seemed to be self-generated (*remember
the film was exposed in the dark drawer
even though there was no exposure to
ultra violet light). Bacquerel’s rays turned
out to be radioactivity.
Radioactivity

Three types of radiation were discovered by
Ernest Rutherford:
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 particles
 particles
 rays
Discovery of the Nucleus
Ernest Rutherford
shot  particles at a
thin sheet of gold foil
and observed the
pattern of scatter of
the particles.
Rutherford found that most of the alpha
particles went straight through the gold
foil. A few were deflected at varying angles
and some were deflected backward
towards the source.
 Rutherford suggested that the bulk of the
mass and all of the positive charge was
located in a tiny, dense central core called
the nucleus. The negative charges
(electrons) were in circular motion around
the nucleus. The majority of the atom was
thought to be empty space.
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Rutherford’s calculations of the mass of
gold did not agree with the mass of its
protons. The electromagnetic theory
suggests that orbiting electrons should
emit electromagnetic waves and lose
energy. As a result, the electrons should
spiral into the nucleus. Rutherford’s model
of the atom did not explain why the
electrons did not spiral inward towards the
nucleus.
The Nuclear Atom
 Rutherford
postulated a very small,
dense nucleus with the electrons
around the outside of the atom.
 Most of the volume of the atom is
empty space.
Protons
were discovered by
Rutherford in 1919.
Neutrons were discovered
by James Chadwick in 1932
Subatomic Particles
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Protons and electrons are the only particles that
have a charge.
Protons and neutrons have essentially the same
mass.
The mass of an electron is so small we ignore it.
Symbols of Elements
Elements are symbolized by one or two letters.
Atomic Number
All atoms of the same element have the same
number of protons:
The atomic number (Z)
Atomic Mass
The mass of an atom in atomic mass units (amu)
is the total number of protons and neutrons in
the atom.
Isotopes:

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Atoms of the same element with different masses.
Isotopes have different numbers of neutrons.
11
C
6
12
C
6
13
C
6
14
C
6
Atomic number (Z) = number of protons in nucleus
Mass number (A) = number of protons + number of neutrons
= atomic number (Z) + number of neutrons
Isotopes are atoms of the same element (X) with different
numbers of neutrons in the nucleus
Mass Number
A
ZX
Atomic Number
1
1H
235
92
2
1H
U
Element Symbol
(D)
238
92
3
1H
(T)
U
2.3
2.3
Do You Understand Isotopes?
How many protons, neutrons, and electrons are in 146
C?
6 protons, 8 (14 - 6) neutrons, 6 electrons
How many protons, neutrons, and electrons are in 116
C?
6 protons, 5 (11 - 6) neutrons, 6 electrons
2.3
Now Lets Talk About Charges!
An ion is an atom, or group of atoms, that has a net
positive or negative charge.
cation – ion with a positive charge
If a neutral atom loses one or more electrons
it becomes a cation.
Na
11 protons
11 electrons
Na+
11 protons
10 electrons
anion – ion with a negative charge
If a neutral atom gains one or more electrons
it becomes an anion.
Cl
17 protons
17 electrons
Cl-
17 protons
18 electrons
2.5
Lets Do Some Examples!
• 7Li
35Cl
3
17
7Li
+
3
35Cl
17
As 3Ag 1+
-
Do You Understand Ions?
+
27
3
How many protons and electrons are in 13 Al ?
13 protons, 10 (13 – 3) electrons
2- ?
Se
How many protons and electrons are in 78
34
34 protons, 36 (34 + 2) electrons
2.5
Charges on Atoms
The number of electrons is the same as the
number of protons. When there is a
charge present that indicates either more
electrons (a negative charge) or less
electrons (a positive charge).
REFERENCES:
Wolf, Clancy, Jasper, Lindenberg, Lynn,
Mustoe, Smythe, (1999). SciencePower 9
(Atlantic Edition). McGraw-Hill Ryerson
Limited.
 http://science.pppst.com/index.html
modified
