the presentation gives the basic knowledge about atoms and molecules.

1. Atoms and Molecules
made by: hritik agarwal , class: 9th
Guided by: ma’am sameeksha
tyagi

2. preface
ancient Greek philosophers – Democritus and
Leucippus suggested that if we go on dividing
matter, a stage will come when particles obtained
cannot be divided further. Democritus called these
indivisible particles atoms (meaning indivisible). All
this was based on philosophical considerations
and not much
experimental work to validate these ideas

3. Acknowledgement
Special thanks to st. joseph’s hi-tech
gurukul and ma’am sameeksha tyagi for
their guidence and all the management
for their support.

4. Introduction
ATOM: an atom is the smallest unit of matter that has the
properties of an element. It is composed of a dense core
called the nucleus and a series of outer shells occupied by
orbiting electrons. The nucleus, composed of
protons and neutrons, is at the center of an atom.
MOLECULE: a molecule is in general a group of two or
more atoms that are chemically bonded together, that is,
tightly held together by attractive forces.

5. Our Aims
 to describe the law of conservation of
mass.
 to describe the law of constant
proportions.
 atomic mass.
To describe some terms like
molecules of elements, molecules of
compound , molecular mass and ions.

6. The law of conservation of mass
The law of conservation of mass,
or principle of mass conservation, states
that for any system closed to all transfers of
matter and energy (both of which have
mass), the mass of the system must remain
constant over time, as system mass cannot
change quantity if it is not added or
removed. Hence, the quantity of mass is
"conserved" over time. The law implies that
mass can neither be created nor destroyed.

7. The law of constant proportions
the law of constant
proportions, sometimes
called Proust's Law, states that
a chemical compound always
contains exactly the same proportion
of elements by mass.

8. Atomic Mass
Atomic mass or atomic weight is the
average mass of atoms of an element ,
calculated using the relative abundance
of isotopes in a naturally-occurring element.
Also Known As: Atomic Weight.
Examples: The atomic mass of carbon is
12.011; the atomic mass of hydrogen is
1.0079

9. Molecules of elements
The molecules of an element are constituted by
the same type of atoms. Molecules of many
elements, such as argon (Ar), helium (He) etc. are
made up of only one atom of that element. But this
is not the case with most of the nonmetals. For
example, a molecule of oxygen consists of two
atoms of oxygen and hence it is known as a
diatomic molecule, O2. If 3 atoms of oxygen unite
into a molecule, instead of the usual 2, we get
ozone. The number of atoms constituting a
molecule is known as its atomicity.

10. Molecules of compounds
Atoms of different elements join together in
definite proportions to form molecules of
compounds. For example:
Water=Hydrogen + Oxygen= 1 : 8

11. IONS
Compounds composed of metals and nonmetals
contain charged species. The charged species
are known as ions. An ion is a charged particle and
can be negatively or positively charged. A
negatively charged ion is called an ‘anion’ and
the positively charged ion, a ‘cation’.

12. Molecular mass
The molecular mass of a substance is
the sum of the atomic masses of all the
atoms in a molecule of the substance. It
is therefore the relative mass of a
molecule expressed in atomic mass units
(u).

13. Thanks for watching the
presentation.