Sabtu, 30 Jun 2012


Atomic Structure and the Periodic Table

An atom is the smallest part of an element that retains the properties of that element. The concept of an atom goes a
long way back. It was first suggested by an ancient Greek named Democritus (the Greek word "atomos" means
indivisible). Democritus theorized that if you took an object and cut it in half again and again you would eventually
end up with some particle which could not be further divided.
In the early 1800's an English scientist by the name of John Dalton started relating what chemists could see to the
concept of the atom. He came up with an atomic theory which could be stated as follows:

o All elements are composed of tiny indivisible particles called atoms.
o Atoms of the same element are identical and differ from atoms of other elements.
o Atoms of different elements can combine together in simple whole number ratios to form
o Chemical reactions are the rearranging of the combinations of atoms of elements in compounds. The
atoms themselves remain unchanged

The electron was discovered by the English physicist Sir Joseph J. Thomson around 1897 with the use of a cathode ray
tube. A cathode ray tube is similar to your TV. It has an anode (negative electrode) and a cathode (positive electrode).
These are enclosed in an evacuated (air removed) glass container and when a charge is applied the electrons flow from
anode to cathode through the open space of the glass container. Thomson observed these particles and determined that
the particles:
· move at a very high speed
· have a negative charge
· have a mass of about 1/2000 of a hydrogen atom (smallest atom)
· were the same regardless of which gas was used in the container or the metal used as the electrode
The particles were eventually named "electrons." His model of the atom was the “plum-pudding” model. This
discovery shattered Dalton's notion of an atom. To Dalton atoms were tiny, solid particles, not containing smaller particles

~Atomic number - indicates the number of protons and defines the element (atomic number 6 is always carbon, atomic number
7 is always nitrogen etc.).

~Mass number - equals the total number of neutrons and protons in the nucleus of an atom for the most common isotope this
equals the atomic mass rounded off to the nearest whole number
~Atomic mass - the average mass of an atom of an element (in amu)
~Calculation of the number of particles in an atom of an element :
- number of protons equals the atomic number
- number of neutrons equals the mass number minus the atomic number (remember virtually all the mass is
from the neutrons and protons in the nucleus-each with an amu of 1)
- number of electrons equals the number of protons in a neutral atom

An isotope is a particular form of an atom of an element. Isotopes have a different number of neutrons and therefore differ in
mass (that is why there is a non-whole number atomic mass which is an average of the various isotopes). The number of
protons is always the same in isotopes since they are different forms of the same element (must be same atomic number).

Types of Elements

A. Metals
1. Luster
2. Good conductors of heat and electricity
3. Malleable
4. Ductile
5. High tensile strength
B. Nonmetals
1. Many nonmetals are gases at room temperature
2. Solid nonmetals tend to be brittle and non-lustrous
3. Poor conductors of heat and electricity
C. Metalloids
1. Some properties of metals and some properties of nonmetals
2. Solids at room temperature
3. Semiconductors of electricity
D. Noble Gases
1. All are gaseous members of group 18
2. Generally unreactive and stable