2. STRUCTURE OF ATOM
Gold foil experiment
Ruther ford conducted the experiment by allowing high speed alpha
particles to hit a very thin gold foil. Most of the alpha particles passed through it.
The path of some particles were seen to have slightly deviated. Some rare ones
come back in the opposite direction as if they hit at something. Rutherford found
out they hit at something. Ruther fold found out there things by analyzing the
impressions made by alpha particles on a screen placed behind the gold foil. He
arrived at some inferences on there observation.
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3. · Atom has a central part . He called it nucleus
· Nucleans has positive charge
· Compared with the total volume of an atom the volume of the nucleus is
very meager.
· The mass of an atom is concentrated in the nucleus
· Electrons are distributed in the space around the nucleus.
RUTHERFORD ATOM MODEL
Rutherford presented an atom model based on his inferences
· Fundamental particles present in an atom – electron, proton and neutron
· Charged, particles of the atom - Electron and proton
· Uncharged particle – Neutron
· Part of an atom contains positive particle – Nucleus.
The ideas of Rutherford about atoms are
· At the centre of the atom a very small positively charged nucleus is
seen
· The entire mass and positive charge of the atom is concentrated at the
nucleus
· Electrons revolve around the nucleus
· Protons and neutrons are seen in the nucleus
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4. · The number of negatively charged electrons and positively charged
protons are equal. Hence as atom is electrically neutral.
The structure of the atom presented by Rutherford is similar to that of the
sales system.
Universe Atom
Centre: Sun
Planets : Revolve around the sun
Orbit : Path through which planets
move
Centre : Nucleus
Electron : Revolve around nucleus
Orbit : Path through which electrons
move
BOHR MODEL OF ATOM
Unlike the solar sytem, electrons in the Rutherford model revolve around a
charged nucleus. James Clark Maxwell postulates that when one charge revolves
around another charge, it will lose energy and ultimately will come closer to the
nucleus. When this posed a challenge for the stability of the atom, there arose the
need for a better model of an atom.
Neils Bohr established that when electron revolves through certain definite
paths around the nucleus. It doesn’t suffer energy change. He called these paths
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5. orbits. He understood that a definite number of electrons are included in each
orbit.
Bohr proposed the following ideas
· Shells are the regions around the nucleus in the atom where electrons
are found.
· Each shell has a definite amount of energy. Hence shells are called
energy levels.
· The shell nearest to the nucleus increases as the distance increases.
· If the electrons do not gai or lose energy, they will remain in the same
shell.
· If energy equal to the difference in energy can be transferred from one
shell to another shell.
POINTS TO REMEMBER
· Atom : Atom is the smallest particle of an element which shows all its
properties
· Atom contains sub atomic particles like positively charged protons,
negatively charged electrons and charge less neutron.
· Protons and neutrons present in the nucleus of an atom
· Electron are revolving around the nucleus through orbits.
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6. orbits. He understood that a definite number of electrons are included in each
orbit.
Bohr proposed the following ideas
· Shells are the regions around the nucleus in the atom where electrons
are found.
· Each shell has a definite amount of energy. Hence shells are called
energy levels.
· The shell nearest to the nucleus increases as the distance increases.
· If the electrons do not gai or lose energy, they will remain in the same
shell.
· If energy equal to the difference in energy can be transferred from one
shell to another shell.
POINTS TO REMEMBER
· Atom : Atom is the smallest particle of an element which shows all its
properties
· Atom contains sub atomic particles like positively charged protons,
negatively charged electrons and charge less neutron.
· Protons and neutrons present in the nucleus of an atom
· Electron are revolving around the nucleus through orbits.
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