2. Atom is regarded as the
smallest, basic unit of matter.
Even if it is the basic unit of
matter, it is still composed of
parts.
>electrons (-)
>protons (+)
>neutrons (0)
3. The subatomic particles are:
• Protons – found in the nucleus;
positive charge equal in magnitude
to the electron’s negative charge.
• Neutrons – found in the nucleus; no
charge; virtually same mass as a
proton.
• Electrons – found outside the
nucleus; negatively charged.
4. An atom has equal number
of protons and electrons.
This makes the atom
neutral.
5. Protons and neutrons are
relatively heavier than
electrons. They compose
the nucleus and collectively
called as nucleons. The
mass of an atom is mainly
determined by the mass of
the nucleus.
7. Several models have tried to
show how the subatomic
particles are arranged in an
atom. Thus far, the accepted
model places the protons and
neutrons in the center of the
atom; or the so-called, nucleus.
Moving rapidly around the
nucleus are the electrons.
8. Atomic number, or the
number of protons of an
atom distinguishes an
element from the others.
This number is the same for
all atoms of a particular
element.
9. While the number of protons is fixed
for an atom of an element, the
number of neutrons may vary.
Atoms having the same number of
protons but different number of
neutrons are referred as isotopes.
The isotopes are identified through
their mass number which is the sum
of the number of protons and the
number of neutrons in an atom.
10. Information on the subatomic
composition of an element
may be represented through
shorthand notations.
13. JJ Thomson’s Model (1898)
The English
scientist Joseph
John Thomson
provided the first
hint that an atom
is made of even
smaller particles.
He discovered
electrons.
14. An atom is a positively charged
sphere with electrons embedded in
it. His model could be likened to a
watermelon with electrons as the
seeds scattered in a positively
charged watermelon.
15. He proposed a model
of the atom that is
sometimes called the
“Plum Pudding”
model.
Atoms were made
from a positively
charged substance
with negatively
charged electrons
scattered about, like
raisins in a pudding.
16. Rutherford’s Gold Foil Experiment
The English
physicist Ernest
Rutherford was hard
at work on an
experiment that
seemed to have little
to do with unraveling
the mysteries of the
atomic structure.
17. Rutherford’s Gold Foil Experiment
Graduate student of
J.J. Thompson
Father of nuclear
physics
Rutherford and his
student (Chadwick)
discovered
neutrons
18. Rutherford’s Model (1911)
An atom is mostly
empty space with a
small positively
charged dense core
called the nucleus.
The electrons orbit
randomly around the
nucleus.
19. Rutherford’s experiment Involved
firing a stream of tiny positively
charged particles at a thin sheet of
gold foil (2000 atoms thick)
20. Most of the positively charged
“bullets” passed right through the
gold atoms in the sheet of gold
foil without changing course at
all.
Some of the positively charged
“bullets,” however, did bounce
away from the gold sheet as if
they had hit something solid. He
knew that positive charges repel
positive charges.
21.
22. Bohr’s Model (1913)
The Danish
scientist Niels Bohr
proposed an
improvement. In his
model, he placed
each electron in a
specific energy
level.
23. Bohr’s Model (1913)
His model showed
electrons moving in
specific shells or
energy levels.
Electrons absorb or
give off energy
when they move
from one energy
level to another.
25. Wave Model
According to the theory of wave
mechanics, electrons do not
move about an atom in a definite
path, like the planets around the
sun.
Today’s atomic
model is based on
the principles of
wave mechanics.
27. The Wave Model
In fact, it is impossible to determine
the exact location of an electron. The
probable location of an electron is
based on how much energy the
electron has.
According to the modern atomic
model, an atom has a small positively
charged nucleus surrounded by a
large region in which there are enough
electrons to make an atom neutral.
