|Notes: 1 September
||Phone: x 5719
Atoms and Atomic Structure
Elements are substances which cannot be broken down
further into simpler substances.
Atoms are the smallest constituents of elements. The
first successful atomic theory was that of John Dalton (1803).
Dalton's atomic theory states:
- All matter is composed of ultimately small particles, called
- Atoms are permanent and indivisible - they can neither be
created nor destroyed.
- Elements are characterized by their atoms. All atoms of a
given element are identical in all respects. Atoms of different
elements have different properties.
- Chemical change consists of a combination, separation, or
rearrangement of atoms.
- Chemical compounds are composed of atoms of two or more elements
in fixed ratios.
All of these statements are close to reality, and nearly describe
chemical behavior. But here are exceptions. Thus atoms
can be created and destroyed via nuclear processes. They consist
of different forms called isotopes. Atoms are not the smallest
Atoms are now known to consist of three different types
of particles: electrons, protons and neutrons (the common form
of one very important atom, hydrogen, has only two kinds: a proton
and an electron). The protons and neutrons reside in a small inner
portion called the nucleus while the electrons reside in
a relatively large cloud centered on the nucleus. Important properties
of these particles are listed in the table below:
||9.11 x 10-28g
|Proton (p or H+)
||1.67 x 10-24g
|| 1.67 x 10-24g
Some important terms which you must know are:
- Atomic number (Z) - the number of protons in the nucleus.
This number is characteristic of a given element.
- Atomic mass number (A) - the sum of the protons and
neutrons in a given atom (p + n).
- Atomic mass - the actual mass of an average
atom in a sample. The characteristic atomic masses for Earth
are shown on periodic tables.
- Atomic Mass Unit: the atomic mass unit = amu is a
unit of mass for atoms. It is defined as 1/12 the mass of one
atom of 12C, where the mass of 12C is defined
as 12 exactly.
Isotopes are forms of elements which differ only in the number
of neutrons. This means different isotopes of the same element
have essentially the same chemical properties but slightly different
physical properties. They can also different substantially in
terms of their nuclear stability. Let's look at some examples
You should be able to fill
in the blanks in a table like this with, the aid of a periodic
table, on a quiz.
Look at the Periodic Chart on the wall. The pattern arises
due to a repetition or periodicity of chemical properties.
The vertical columns of the charts are called groups, while the
rows are referred to a periods.
Note the numbering of the groups. The numbers from 1 - 18 are
the internationally accepted numbers. We will also use the I -
VIII "American" numbering system. Note that the "tallest"
columns comprise what are referred to as the "representative
elements" (IA - VIIIA).
Let's look at some of the elements and see what their properties
2 Na + 2 H2O Æ
2 Na+ + 2 OH- + H2
- Group IA, on the left side of the chart, is known as the
alkali metals because the react with water to produce
strong bases (a base is alkaline). Note that all of them are
soft (cut with a butter knife), low density (Li floats on oil,
Na and K float on water), very reactive metals. All of them react
with water with Li<Na<K<Rb<Cs. in each case the metal
gives its electron to water leaving hydroxide ion (OH-
a base) and hydrogen gas. For example with sodium:
2 Na + Cl2 Æ
- Group VIIA, on the right side of the chart, is known as the
halogens. The halogens form acids with water, are gases
at the top of the Periodic Chart and high vapor pressure liquids,
then solid going down; exist as diatomic molecules (X2),
and are very reactive towards metals. For example sodium reacts
violently with chlorine gas to give table salt (NaCl):
- Look at the elements of Period 3 (Na - Cl) Note how their
properties change from metallic to non-metallic.
© R A Paselk
Last modified 2 September 2004