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Unit 8 –
Atoms and
The Periodic Table
A. Early Models
a. During the 5th century B.C. Democritus stated
that the differences in substances were the direct
result of differences in the size of tiny uncuttable
particles.
b. During the 4th century B.C. Artistole said NO
WAY! He thought that only 4 elements actually
exist: water, air, fire and earth
B. Dalton’s Atomic Theory (1808)
Stated that elements are made up of tiny
particles called Atoms and each chemical
has a unique combination of atoms
C. Thomson Model (1897)
Discovered that atoms contain negatively
charged particles called “corpuscles” later
called electrons
D. Other discoveries at the time
a.
Henri Becquerel – (1896) - discovered
radioactivity by mistake
- he was studying light emission when he discovered that
uranium emits energy by itself
b. Marie and Pierre Curie – (1898) – separated
radioactive elements and coined the term
“radioactive decay”
E. Rutherford – Bohr Model
(1899 – 1920)
Rutherford proposed that the atom consists of a tiny
positively charged nucleus surrounded by a cloud
of negatively charged electrons.
The nucleus contains almost all of the mass of the
atom and consists of protons and neutrons.
The number of electrons surrounding the nucleus,
equals the number of protons so as to make the
atom neutral.
Bohr (1913)
Electrons are in orbits in a fixed position
around the nucleus.
It was proven later that Bohrs
theory was wrong
F. Atomic Number and
Atomic Mass
Atomic Number – the number of Protons in
the nucleus
Protons have a +1 charge
Neutral atoms have an equal
number of protons and electrons
Ions
Formed when electrons are either added or
subtracted from the neutral atom
i. If electrons are added a negative charge
is formed
ii. If electrons are removed , a positive
charge is formed
Examples
Add a electron to Fluorine ( F )
Protons = 9
Total charge = +9
Electron = 9 + 1 Total charge = -10
Total charge on the atom = -1
shown as F-
Example 2
Remove 2 electrons from Barium (Ba)
Protons = 56
Total charge = +56
Electron = 56 - 2 Total charge = -54
Total charge on the atom = +2
shown as Ba+2
Atomic Mass
If protons and neutrons have a mass of
approx. 1 gram, then the total mass of the
atom is equal to the total number of
protons and neutrons
Atomic mass = neutrons + protons
Neutrons = atomic mass – atomic number
As shown on the periodic table
Atomic Number
11
Na
Atomic Mass
23
neutrons = 23 – 11 = 14