ATOMIC THEORY OF MATTER
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| THE STRUCTURE OF THE ATOM |
1808,John Dalton gave
his atomic theory of matter. According to this theory, Atom is the smallest
indivisible particle of an element. Atom can neither be created nor be destroyed.
Atoms of different elements differ in mass, size and chemical properties. The
'number' and 'kinds ‘of atoms in a given compound is fixed. Atoms of the same
elements can combine in more then one ratio to form different compounds.
ATOM
Smallest particle of an
element that can take part in chemical reaction.Atoms are made up of
electrons,protons and neutrons.
Electron (-1e0)
has negative charge (discovered by JJ Thomson in 1897),charge on electron = 1.6✕10-19C. Mass of electron= 9.1✕10-28g. The magnitude of negative
charge on electron was determined by Millikan.
Proton (1H1)
has positive charge (discovered by E Goldstein). Charge on proton (1H1)
= 1.6✕10-19C. Mass
of proton (1H1) = 1.6✕10-24g.
Neutron (0n1)
has no charge (discovered by Chadwick). Mass of neutron = 1.6✕10-24g.
MOLECULE
It is the smallest
particle of a compound.
Molecules of a compound
contain two are more different types of atoms. e.g., ammonia (NH3),
water (H2O) etc.
PROPERTIES OF CATHODE RAYS
These travel in straight
line,cast shadows of object placed in their path.
These are negatively
charged and produce mechanical effects.
Charged particles
present in cathode rays are electron.
PROPERTIES OF ANODE RAYS
These travels in
straight line and can also produce mechanical effects.These are positively
charged.
NUCLEUS (DISCOVERED BY RUTHERFORD)
Nucleus is positively
charged and present at the centre of an atom.
Entire mass of atom is
concentrated in the nucleus.
ATOMIC NUMBER
It is the number of
protons in one atom of an element.
Number of protons =
number of electrons (in case of neutral atom).
MASS NUMBER
It is the total number
of protons and neutrons present in one atom of an element.
Mass number = number of
protons + number of neutrons
= atomic number +
number of neutrons
=number of
electrons +number of neutrons
VALENCE ELECTRONS
Valence electrons are
located in the outermost shell of an atom.Elements having same number of
valence electrons have similar chemical properties.
When an atom loses
electron,it gets converted into cation,i.e., carries one positive charge and if
it gains one electron,it carries one negative charge, i.e., converted into
anion.
Elements having 1,2 or 3
valence electrons in their atoms are metals while those having 4,5,6 or 7 are
non-metals.
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