The document discusses several key concepts in chemistry including: - Laws of chemical combination and conservation of matter discovered by scientists like Lavoisier and Proust through quantitative experiments. - Proust's law of constant proportions which states that elements are always present in a definite proportion by weight in a compound. - Atoms being the fundamental units of matter and their structure including subatomic particles. Atoms are extremely small but their masses can be measured by comparison to hydrogen atoms. - Chemical symbols used to represent elements, including early systems and current IUPAC symbols. Molecules can be monoatomic or polyatomic. - Concepts of mole, molar mass, and Avogadro

1. COUNTING
MATTER
BY FURQAAN

2. INTRODUCTIONAt the end of 18th century, scientists proved
experimentally that matter exists in the atomic
state and molecular state and tried to understand
why they undergo some changes.

3. LAWS OF CHEMICAL COMBINATION
Calcium Chloride and Sodium Sulphate react to
form white calcium Sulphate and Sodium
Chloride.
300.23g 300.23g

4. LAWS OF CONSERVATION OF MATTER
On weighing the material before and after chemical
reactions Antonie Lavoisier concluded and
formulated that “Matter is neither gained nor lost
during a chemical reaction”.This is called ‘The Law
of Conservation of Matter’.Scientists observed that
when matter reacts it follows certain rule.By
carefully studying various examples, the laws were
stated in 1800.They were later call Laws of Chemical
Combination.

5. Like Lavoisier, Joseph Louis Proust along with other
contemporary scientists conducted quantitative
experiments.They collected the samples Copper
Carbonate, one prepared in the lab and the other
isolated from nature.It was observed that regardless
of the source, it always contained the same
proportions of copper, oxygen and carbon in 5:4:1
parts by weight.Similarly proportion of Calcium and
Oxygen in Quick Lime (Calcium Oxide) CaO is 5:2.
JOSEPH LOUIS
PROUST
Antonie Lavoisier

6. LAW OF CONSTANT PROPORTION
Proust Formulated ‘The Law of Constant
Proportions’:“In a compound, the constituent
elements are always present in a definite
proportion by weight”
“In a compound, the
constituent
elements are always
present in a definite
proportion by
weight”

7. • For example, proportion of hydrogen and oxygen in
the water by weight is 1:8 i.e. 1 gram of hydrogen
and 8 grams of Oxygen form 9 grams of water
irrespective of the source.Similarly in proportion of
Carbon and Oxygen by weight in carbon-di-oxide is
3:8 i.e. 12 grams of Carbon and 32 grams of Oxygen.

8. THE ATOM:ITS SIZE,Mass AND SYMBOLS
You already know how small the atom is and what
its sub-atomic particles are.Yet at times it is very
difficult to imagine how small an atom can be since
none of us can see it.

9. LETS REVIVE!Subatomic particle, also called elementary
particle, any of various self-contained units of
matter or energy that are the fundamental
constituents of all matter. Subatomic particles
include electrons, the negatively charged, almost
massless particles that nevertheless account for
most of the size of the atom, and they include the
heavier building blocks of the small but very
dense nucleus of the atom, the positively
charged protons and the electrically
neutral neutrons.

10. Atoms are so small that until
recently they couldn’t even be
seen with the world’s most
powerful microscope.However
it is easier to think about how
long a line of 1 million atoms might
be.
Take a page of a book and hold it between
your thumb and forefinger so that you get
an idea of how thick the paper is.One
million atoms lined up in row would roughly
equal the thickness of the page.
WORLD’S MOST POWERFUL MICROSCOPE

11. The finest of optical microscopes also cannot see an
atom.Only electron microscopes are powerful
enough to magnify an atom so it can be
seen.Following image will give you an idea how
atoms look through such machines.

12. WEIGHING ATOMS
SEEMS A DIFFICULT TASK?
Yes, but scientists successful in weighing such tiny
particles.
Hydrogen being the smallest atom is taken as
standard particle for comparing the masses of
atoms of the other substances. The diagram on the
next page will help us imagine how the mass of
Nitrogen is measured by comparing it with the mass
of an atom of Hydrogen.

13. O
ooooooo
ooooooo
^^
Two Pans1 Atom of Nitrogen 14 Atoms of Hydrogen
Mass of Nitrogen Atom is 14
Mass of H is 1 u.14 H atoms are required to
weigh 1 atom of N.Therefore, relative mass
of 1 atom of Nitrogen is 14.
In a similar manner the relative atomic
masses of following elements are obtained.

14. ATOMIC MASSES OF SOME COMMON
ELEMENTS:-

15. CHEMICAL SYMBOLS OF ELEMENTS
Elements were given symbols by which they could
be identified.
Dalton first used symbols for names of the
elements using circles. Some of the interesting
symbols were:-
Dalton

16. Today we use the symbols decided by IUPAC
(International Union of Pure and Applied
Chemistry).They are the official names and are used
all over the world for international trade. The
current system of chemical notation was invented
by Berzelius.

17. MOLECULES OF ELEMENTS AND COMPOUNDS
Atoms of some elements exist independently. In
some cases, two or more atoms of elements
combine to form ‘Molecules'. So some elements
can exist in the form of a single atom (mono atomic)
while some molecules may form diatomic
molecules like:-
Hydrogen (H2) Bromine (Br2)
Nitrogen (N2) Iodine (I2)
Oxygen (O2) Chlorine (Cl2)
Fluorine (F2)

18. When atoms of different
elements combine with each
other, they form compounds.
Molecules of these
compounds are formed by the
chemical attraction between
elements.

19. FINDING MOLECULAR MASS
Read this statement thrice to understand it:-
‘The sum of the atomic masses of the atoms present
in a given molecule is the molecular mass of the
substance’.
Molecular mass of Water:H2O can be calculated as
follows:

20. MOLECULAR MASS AND MOLE CONCEPT
When elements or compounds
react with one another, it
becomes necessary to know how
many atoms/molecules are taking
part in the chemical reaction.
For this, a new concept of ‘Mole’
was introduced.

21. WHAT IS A MOLE?
1 Mole of a substance is equal to its atomic
mass/molecular mass expressed in terms of grams.
Avogadro's number and the mole are very
important to the understanding of atomic structure.
The Mole is like a dozen. You can have a dozen
guitars, a dozen roosters, or a dozen rocks. If you
have 12 of anything then you would have what we
call a dozen.
Avogadro

22. The concept of the mole is just like the
concept of a dozen. You can have a mole of
anything. The number associated with a mole
is Avogadro's number. Avogadro's number is
602,000,000,000,000,000,000,000 (6.02 x
1023). A mole of marbles would spread over
the surface of the earth, and produce a layer
about 50 miles thick.

23. A mole of sand, spread over the United States,
would produce a layer 3 inches deep. A mole of
dollars could not be spent at the rate of a billion
dollars a day over a trillion years. This shows you
just how big a mole is. This number is so large that
it is usually only represented in scientific notation.

24. Avogadro experimentally found that 1
mole of any atom /molecule always
contains 6.022 x 1023 atom/molecules.
This number is also called the Avogadro
number denoted by N0.
Avogadro defined a mole as a collection
of 6.022 x 1023 number of
atoms/molecules just as in daily life we
use the words like dozen (12), century
(100), gross (144), etc.

25. ALWAYS REMEMBER
When molecular mass is expressed in grams,
it is called gram molecular mass.
Similarly, when atomic mass is expressed in
grams, it is called gram atomic mass of gram
atom.

26. VALENCYJust like elements have symbols, compounds
too have a symbolic representation known as
its formulae. Before writing formulae, it is
necessary to know how elements combine
with each other to form compounds. For this,
let us define a new concept called valency.
Valency is the combining
capacity of an element.

27. VALENCE AND VALENCY
Valence electrons are electrons in the outermost
shell.
Valency is the number of valence electrons an
atom uses to form a chemical bond.

28. All elements after atomic number 3 tend to
complete the octet of electrons in their outermost
orbit, i.e. a stable when there are 8 electrons in
their outermost orbit.This they do by donating,
accepting or sharing electrons of the outermost
orbit with electrons of other elements.In doing so,
they form chemical bonds with the other elements
leading to formation of compounds.

29. Let us take some examples:-
• Hydrogen is an exception to the rule. It has only one
valence electron (Valence electrons are electrons in
the outermost shell).Hence it shows a tendency to
either give its electron or share it. Therefore, its
valency is one (Valency is the number of valence
electrons an atom uses to form a chemical bond).
• The elements having atoms with full electrons shells
(Helium, Neon, Argon) are chemically inert. These
atoms are stable because they do not tend to give,
accept or share electrons.
Inert:Non-reactive

30. MORE ABOUT IT
Sometimes groups of atoms also give up or accept
electrons forming positive or negative groups.Such
atoms or groups of atoms having a positive of
negative charge are known as radicals.Sometimes
positive or negative sign is written before their
valency to indicate whether they donate or accept
electrons.
The number of electrons which any atom denotes
or accepts is also indicated through symbols.

31. Do you know what are ions?
Read this statement below 5 times to understand it.
An ion is an atom or molecule in which the total
number of electrons is not equal to the total
number of protons, giving the atom a net positive
or negative electrical charge. Ions can be created
by both chemical and physical means.
For Example: Sodium ion is written as Na+ and
choride ion as Cl-.
Magnesium ion is written as Mg2+ indicating that
the valency of Mg is +2 and that it tends to donate
two electrons while forming a compound.

32. IONS AND RADICALS
Some elements show variable valency.The alternate
valency shown by elements is indicated in brackets
by the side of their ions in Roman figures.
Following is the list of some commonly used ions
and radials. Knowledge of ions and radicals is useful
in writing molecular formula of a compound as well
as to understand and to write balanced chemical
equations.

34. CHEMICAL FORMULAE OF COMPOUNDS
Here are some simple rules to be followed while
making chemical formulae:-
For eg: Formula of Potassium Chloride:-
1. Write the symbols of Potassium and Chlorine:
Cl
2. Write their valencies below the symbols:
K Cl
+1 -1
K Cl

35. 3. Cross multiply the valencies in the manner
shown below:
K Cl
+1 -1
Solution: K x 1 , Cl x 1
=KCl.
.. The formula of Potassium Chloride is KCl..