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| The Atom
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| When scientists started exploring matter, they
realised that matter can be divided into smaller and still smaller particles.
What was the ultimate particle like? They discovered that the smallest particle
of an element that maintains its chemical identity through all chemical and
physical changes is called and 'atom'. John Dalton (1766 - 1844) can rightly be
called the father of the Modern Theory on Atoms. He proposed his Atomic Theory
in 1808, i.e., almost 200 years back. He did not have the help of sophisticated
instruments that are available today to the scientists. Hence, many of his
proposals, have been modified and updated. Over the years, substantial changes
have taken place regarding the atomic theory, yet some of the assumptions that
Dalton made are still held valid.
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| Atom was derived from the Greek word 'atoms',
meaning 'indivisible'. Thus, it was considered to be indivisible. Molecules
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| Dalton's Atomic Theory
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| John Dalton provided a simple theory of matter to
provide theoretical justification to the laws of chemical combinations in 1805.
The basic postulates of the theory are:
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All substances are made up of tiny, indivisible particles called atoms.
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Atoms of the same element are identical in shape, size, mass and other
properties.
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Each element is composed of its own kind of atoms. Atoms of different elements
are different in all respects.
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Atom is the smallest unit that takes part in chemical combinations.
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Atoms combine with each other in simple whole number ratios to form compound
atoms called molecules.
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Atoms cannot be created, divided or destroyed during any chemical or physical
change.
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| Nature of Atom
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| At present we know that the atom is the smallest
particle of an element, which may or may not have independent existence. It is
made up of sub-atomic particles like electrons, protons and neutrons. Atoms of
one type of element differ from those of the other due to different number of
sub-atomic particles.
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| Representation of an Atom by a Symbol
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| A symbol signifies a shorthand representation of
an atom of an element. The symbol of any element is based on the English name
or Latin name (written in English alphabets) and is represented by using only
its first letter or the first letter and another letter.
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The first letter is written in capital and the second letter in small.
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The second letter can be any letter in the name of the element, which is
internationally accepted.
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| Symbols of Common Metals
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| Symbols of Chemically Active Non-metals
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| Symbols of Chemically Inactive Non-metals or
Noble Gases
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| The names of the elements had originated
from different sources in different ways. Some of them are discussed here. For
example,
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Hydrogen was named by Lavoisier using the Greek words 'Hydro' ie., water and
'Genes' meaning forming.
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Chlorine has its origin from Greek word - 'Chloros' - meaning greenish yellow,
'Bromos' means stench in Greek and 'Iodes' in Greek means violet.
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The word nickel has originated from a German word meaning 'Satan' or 'Old
nick'. Cobalt comes from a German word - 'Globlin' or 'Evil Spirit'.
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Argon comes from a Greek word meaning 'Inactive'.
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Gold is an Anglo-Saxon word - 'Aurum'.
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| Elements Named After Places
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| Scandium (Sc) - found in Scandinavia
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| Thulium (Tm) - named after Thule the earlier name
of Scandinavia
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| Europium (Eu) - after the continent Europe
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| Polonium (Po) - named after the Curies after
their home town in Poland
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| Named After Planets
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| Selenium (Se) - 'Seles' Greek name for the moon
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| Plutonium (Pu) - Neptunium (Np) Uranium (U)
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| Mercury was named after a planet but derives its
symbol Hg from the Latin word 'Hydragyrum' meaning liquid silver
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| Named After Scientists
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| Curium (Cm) after Pierre and Marie Curie
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| Fermium (Fm) after Enrico Fermi
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| Einsteinium (Es) after Albert Einstein
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| Mendelevium (Md) after Dimitri Mendeleev
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| Significance of a Symbol of an Element
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| Qualitative
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It represents a specific element.
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It represents one atom of an element.
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| Quantitative
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It represents how many times an atom is heavier than one atomic mass unit.
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It represents one-gram atom of an element.
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It represents how many atoms are present in one-gram atom (1 mole)
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| For example, 12 grams of carbon has 6.023 x 1023
atoms in it.
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| Size of the Atom Elements
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| Atoms are infinitesimally small. Smaller than any
thing we can imagine. One hydrogen atom, the smallest atom known, is
approximately 5 x 10-8 mm in diameter. To put that in perspective,
it would take almost 20 million hydrogen atoms to make a line as long as this
dash ' - '.
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| The extremely small sizes of atoms are
exceedingly impractical to work with; special measuring units have been devised
to determine the dimension of atoms in terms of mass. The mass or weight of an
atom can be computed in the following ways.
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| Mass of the Atom
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| Experiments have determined that the mass of an
atom is very small, ranging from 1.7x10-24 g to about 4.0x10-22
g. It is very difficult to accurately weigh a bunch of atoms, as they are
extremely light, small, and invisible; they weigh too little for us to express
their masses in grams or kilograms. As a consequence, it is necessary to reduce
the atomic masses into simple figures.
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| Atomic Mass
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| An indirect method to determine the absolute mass
of an atom has been devised. The mass of a particular atom is taken as a
standard unit and the masses of other atoms are related to this standard.
Hydrogen being the lightest element and being the smallest atom was chosen and
assumed to have a mass of 1. An atom of hydrogen was assigned an atomic mass
equal to one atomic mass unit (a.m.u). The number does not signify the mass of
an atom in grams. It is just a pure number. The masses of atoms of other
elements were compared to that of hydrogen, in order to find their atomic mass
relative to it. If one atom of sulphur weighs as much as 32 atoms of hydrogen,
then the relative atomic mass of sulphur is 32 a.m.u.
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| This way of defining the mass of one atom of
hydrogen has its difficulties. While the mass of one atom of hydrogen is
considered as 1 atomic mass unit, hydrogen gas in its natural state has 3
isotopes of atomic mass 1, 2 and 3 respectively. Thus average mass works out to
be 1.008 a.m.u rather than 1 a.m.u. This in turn complicates the atomic masses
of all other elements. Later on, an atom of oxygen was preferred as standard by
taking its mass as 16 units. In 1961, the International Union of Chemists
selected the most stable isotope of carbon (C-12 isotope) as standard for
comparison with other atoms or molecules.
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| Atomic Mass Unit
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| The mass of 1/12th 12C
isotope of carbon is now the standard used for defining one atomic mass unit
(a.m.u). It is equal to 1.66 x 10-24 g.
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| Atomic Mass of an Element or Relative Atomic Mass
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The atomic mass of an element is a relative
quantity and is the mass of one atom of the element relative to 
the mass of one carbon-12 atom. Thus,
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Relative atomic mass (RAM) of an element is
the number of times one atom of an element is heavier than 1/12 the mass of an
atom of carbon [C-12]. 
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For example, if 1 atom of Na weighs as much as 23
parts of 
isotopes, then the atomic mass of sodium is 23 a.m.u.
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| Atomic Mass Unit Scale
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| The scale in which the relative atomic masses of
different atoms are expressed is called atomic mass unit scale.
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Atomic masses are not expressed as whole numbers
because natural elements are mixtures of two or more isotopes. The atomic
weight of chlorine is 35.43 amu. Chlorine exists as two isotopes 
in the ratio 3:1
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| Therefore average atomic masses are not whole
numbers.
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| Gram Atomic Mass (GAM)
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| The atomic mass of an element expressed in grams
is called the gram atomic mass of an element. It is also called gram atom.
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| Example: The gram atoms present in 46 grams
of sodium
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| Gram atomic mass is the relative atomic mass of
an element expressed in grams.
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