SCC-CH110/UCD-CH41C
Chapter: 5
Instructor: J.T.
P1
Compund: a substance that can be decomposed by chemical
action into simpler substance. I.e. ; lime CaO
Element:
a substance that cannot be decomposed by
normal chemical action into simpler substance,
i.e. Na (sodium)
Molecule:
the smallest particles of a compund (i.e.: CO2) or
group of elements (i.e.: O2) that can exist by
itself.
Atom:
the smalest particle of an element that has the
properties of that element.
Subatomic: particles smaller than an atom, i.e.: proton,
electron, neutron.
+
12
6
Proton
Neutron
Electron
Nucleus of carbon-12 atom:
C
Z = atomic number = 6 protons
6 Neutrons
A= mass number = 12
SCC-CH110/UCD-CH41C
A
Z
Sy
1
1
H
3
2
He
4
2
He
Chapter: 5
Instructor: J.T.
A= mass number =
+
Z= atomic number =
+
+
+
+
+
+
+
P2
SCC-CH110/UCD-CH41C
Chapter: 5
Instructor: J.T.
P3
Alpha (α) particles: The nucleus of a helium atom.
+
Radioactivity:
+
The property of spontaneous emission
of rays and/or particles from an atomic
nucleus.
Scintillation
Source
α,γ
Radioactive
element
Detector Screen
A device consisting of a surface coated with zinc sulphide. When alpha particles hit the surface, it
scintillates.
Results of Rutherford (1911) experiments:
™Every atom contains an extremely dense nucleus.
™All of positive charge is concentrate in the nucleus.
™The nucleus is surrounded by a large volume of empty
space that makes up the rest of atoms.
SCC-CH110/UCD-CH41C
Chapter: 5
Instructor: J.T.
P4
™Electrons very thinly populate the space outside the
nucleus.
Dalton’s atomic theory (1803):
1) Each element is composed of extremely small particles
called atoms (indivisible).
2) All atoms of a given element are identical.
3) Atoms are neither created nor destroyed in chemical
reactions. [Low of conservation of matter]
4) A given compound always has the same relative number
and kind of atoms. [Low of constant composition]
A good theory should also predict new fact! Dalton used his
theory to deduce the low of multiple proportions:
If two elements A and B combine to form more than one
compound, the masses of B that can combine with a given
mass of A are in the ratio of small whole numbers.
Example: water & hydrogen peroxide
H2O :
H2O2
8.0 g of oxygen combines with 1.0 g of hydrogen.
8.0/1.0 = 8.0
16.0 g of oxygen combines with 1.0 g of hydrogen.
16.0/1.0=16.0
16.0 : 8.0 = 2:1
SCC-CH110/UCD-CH41C
Chapter: 5
Instructor: J.T.
P5
Using the atomic theory, we can conclude that hydrogen
peroxide contains twice as many atoms of oxygen per
hydrogen atom as does water.
Subatomic particles:
Following Faraday, Crookes studies about Cathode rays in
1897 Thomson’s paper is accepted as the discovery of
electron.
Proton was discovered in 1919 by Rutherford (α particles
scattering model).
J. Chadwick discovered neutron in 1932.
mp
me
= 1836
¾ The atom contains a small positively charged nucleus
surrounded by electrons.
¾ The nucleus is at least 100,000 times smaller than the
atom.
SCC-CH110/UCD-CH41C
Chapter: 5
Instructor: J.T.
P6
¾ The nucleus consists of protons and neutrons (exception
H atom).
¾ Atoms of a particular element that differ only in the
number of neutrons in the nucleus are called isotopes.
Isotopes:
Atoms of a particular element that differ only in
the number of neutrons in the nucleus are called isotopes.
Atomic Mass Units
Chemists use an arbitrary unit called the amu or the Dalton.
Mass of one carbon-12 is chosen as an arbitrary standard
reference mass, as exactly 12 amu.
Therefore:
1 amu = 1 Dalton = 1/12 mass of the 12C = 1.6606 x10-24 g
The atomic mass of an element: the average mass of all
isotopes of an element as they occur in nature.
SCC-CH110/UCD-CH41C
Chapter: 5
Instructor: J.T.
P7
Example: calculate the atomic mass of a Cl atom base on the
following information:
Isotope
amu
naturally occurring
34.96885272
75.78%
35
17
37
17
Cl
Cl
36.96590262
24.22%
0.7578 x 34.96885272 = 26.4993965912 amu = 26.50
0.2422 x 36.96590262 = 8.95314732564 amu = 8.953
1.000 average atom
35.45 amu