Sucrose is composed of the elements carbon,
hydrogen, and oxygen. This is a qualitative
expression of composition. A qualitative observation
is one that can be made without measurement.
In 100 g of sucrose contains 42.1 g of carbon, 51.4 g of
oxygen, and 6.5 g of hydrogen. This is a quantitative
expression of composition. A quantitative
observation is one that uses measurement. You make
quantitative measurements every day when you
answer such questions as What’s the temperature?
How long was the pass?
A substance is matter with the same fixed
composition and properties. A substance can be either
an element or a compound. Any sample of pure
matter is a substance.
Most of the matter you encounter every day is a
mixture. A mixture is a combination of two or more
substances in which the basic identity of each
substance is not changed. Unlike pure substances,
mixtures do not have specific compositions.
Physical properties are characteristics that a sample
of matter exhibits without any change in its identity.
Examples of the physical properties of matter include
its solubility, melting point, boiling point, color,
density, electrical conductivity, and physical state
(solid, liquid, or gas).
Chemical properties are those that can be observed
only when there is a change in the composition of the
substance. Chemical properties describe the ability of
a substance to react with other substances or to
decompose. Lack of reactivity is also a chemical
property.
•
•
The smallest particle of an element that
retains the properties of that element.
Idea of the atom was first suggested by
Democritus
•
first subatomic particle discovered
•
have a negative charge
•
equal to the number of protons and atomic
number
•
discovered by J. J. Thomson
•
found in the electron cloud
•
amu 1/1897
•
•
have a positive charge
equal to the atomic number of the
element
•
found in the nucleus of the atom
•
amu 1
•
have a neutral (no) charge
•
found in the nucleus of the atom
•
approximate amu 1
•
center of the atoms
•
composed of protons and neutrons
•
very dense
•
has a positive charge
•
discovered by Rutherford in his gold foil
experiment.
•
•
•
atoms that have the same number of
protons (thus the same element), but a
different number of neutrons.
alike chemically, but have different
masses
written with the name or symbol of the
element followed by the mass number
•
Carbon-12 or carbon-14
John Dalton (1766-1844)
- an English schoolteacher and chemist
The following statements are the main points of Dalton’s atomic theory.
1. All matter is made up of atoms.
2. Atoms are indestructible and cannot be divided
into smaller particles. (Atoms are indivisible.)
3. All atoms of one element are exactly alike, but they
are different from atoms of other elements.
An atom’s atomic number is the number of
protons in its nucleus. The number of electrons in
an atom equals the number of protons.
Atoms of the same element always have the same
number of protons and electrons but vary in the
number of neutrons.
protons = atomic number
electrons = protons
neutrons = atomic mass - protons
Atomic
Number
Mass
Number
4
9
# of
protons
# of
neutrons
10
# of
electrons
Element Name &
Symbol
Neon - Ne
23
Sodium – Na
7
9
59
10
Cobalt - Co
Atomic
Number
Mass
Number
# of
protons
4
9
4
# of
neutrons
10
# of
electrons
Element Name &
Symbol
Neon - Ne
23
Sodium – Na
7
9
59
10
Cobalt - Co
Atomic
Number
Mass
Number
# of
protons
# of
neutrons
4
9
4
5
10
# of
electrons
Element Name &
Symbol
Neon - Ne
23
Sodium – Na
7
9
59
10
Cobalt - Co
Atomic
Number
Mass
Number
# of
protons
# of
neutrons
# of
electrons
4
9
4
5
4
10
Element Name &
Symbol
Neon - Ne
23
Sodium – Na
7
9
59
10
Cobalt - Co
Atomic
Number
Mass
Number
# of
protons
# of
neutrons
# of
electrons
Element Name &
Symbol
4
9
4
5
4
Beryllium - Be
10
Neon - Ne
23
Sodium – Na
7
9
59
10
Cobalt - Co
Atomic
Number
Mass
Number
# of
protons
# of
neutrons
# of
electrons
Element Name &
Symbol
4
9
4
5
4
Beryllium - Be
10
Neon - Ne
23
Sodium – Na
7
9
59
10
Cobalt - Co
Atomic
Number
Mass
Number
# of
protons
# of
neutrons
# of
electrons
Element Name &
Symbol
4
9
4
5
4
Beryllium - Be
10
20
10
10
10
Neon - Ne
11
23
11
12
11
Sodium – Na
7
14
7
7
7
Nitrogen - N
9
19
9
10
9
Fluorine - F
27
59
27
32
27
Cobalt - Co
Each energy level can hold a limited number of
electrons. The lowest energy level is the
smallest and the closest to the nucleus. This
first energy level holds a maximum of two
electrons. The second energy level is larger
because it is farther away from the nucleus. It
holds a maximum of eight electrons. The third
energy level is larger still and holds a maximum
of 18 electrons.
The electrons in the outermost energy level are
called valence electrons.
A Lewis dot diagram illustrates valence
electrons as dots (or other small
symbols) around the chemical symbol of
an element. Each dot represents one
valence electron.
The maximum number of dots on a the
diagram is 8.
The period number tells the number of energy
levels of an atom.
Example sodium is in the _____ period, so it must
have _____ energy levels.
p+ = ______
_____
n0 = ______
Na
_____________
e- = ______
_______
Bohr Diagram
Lewis Structure
Na