Various models of the atom
have been proposed by
eminent scientists over the
years.
These models have increased our understanding
of atomic structure..
Among
these
models,
Rutherford
suggested that electron revolve around the
nucleus in well defined orbits. But there was a
problem with this..
The motion of electrons in Rutherford’s
model was unstable because any charged
particle moving
in a circular path emits
electromagnetic radiation, thus the electrons
would lose energy and fall into the nucleus
making the atom highly unstable.
Then, which would be a better
model of atom?
To overcome objections against
Rutherford’s model of atom, Niels
Bohr in 1913 proposed a new atom
model.
In 1913,
a Danish
physicist
Niels Bohr
NIELS BOHR
proposed
atomic model
known as
Bohr’s
Atomic Model.
It first proposes that electrons
revolve around the nucleus in specific
orbits and these orbits are associated
with definite energies and are called
shells or energy levels.
These orbits or shells are
represented by the letters K, L, M, N
or by the numbers
n= 1, 2, 3, 4…..
The maximum number of
electrons that can be accommodated
in a particular orbit is 2n2 (2 into
the power of 2), where “n” is the
number of orbit;
Thus, K shell would have 2
electrons, L shell – 8 electrons, M
shell – 18 electrons, N shell would
have 32 electrons, and so on….
That said, what does this model next
propose?
It next proposes that the
maximum number of electrons that can
be accommodated in the outermost
level is 8.
And finally, it says that the
orbit closest to the nucleus has
minimum energy and orbit farthest has
maximum.
Well then, does an electron radiate
energy by itself?
Since electrons move in a particular
orbit, they do not radiate energy by
themselves.
What happens instead is, when an
atom absorbs energy, the electrons get
excited and jump into the next higher energy
level.
The electron can radiate energy and
returns to its original state or drop down to
the next energy level.
Bohr’s model works well
for simple atoms and is easy
to understand.
It is one among the
atomic structures still in use
today, the other being the
quantum mechanical model.
Things to remember:
1. In 1913, a Danish physicist Niels Bohr
proposed atomic model known as Bohr’s
Atomic Model.
2. In an atom, the electrons revolve
around the nucleus in definite energy
levels called orbit or shells.
3. The maximum number of electrons that
can be accommodated in a particular
orbit is 2n2.
4.The maximum number of electrons
that can be accommodated in the
outermost shell is 8.
5. The orbit closest to the nucleus
has minimum energy and the orbit
farthest has maximum energy.
6. Electrons are excited to the
higher energy levels by absorbing
energy and return to lower energy
levels by radiating energy.
What was Bohr's first hypotheses
about the atom?
ANSWER:
Electrons moves in circular
orbits about the nucleus.
What was Bohr's second hypotheses
about the atom?
ANSWER:
Electron has only a fixed set of
allowed orbits as long as an
electron remains in a given orbit,
its energy is constant and no
energy is constant and no energy is
emitted.
What was Bohr's third hypotheses
about the atom?
ANSWER:
An electron can pass from one
allowed orbit to another. In such
transitions, fixed discrete
quantities of energy (quanta) are
involved, in accordance with
Planck's equation, E = hv.
When the transition is from lower
to higher, energy is ...
ANSWER:
Absorbed
When the transition is from higher
to lower, energy is ...
ANSWER:
Emitted
When n increases, the electron
is _____ from the nucleus
ANSWER:
Farther
What did Bohr propose in his
model of the atom?
ANSWER:
Proposed that an electron is
found in specific circular
paths or orbits around the
nucleus.
Who was Niels Bohr?
ANSWER:
- A young Danish physicist
- Student of Rutherford
- Developed the new atomic
model
What did Bohr do?
ANSWER:
Changed Rutherford’s model to
incorporate new discoveries
about how the energy of an
atom changes when the atom
absorbs or emits light.
What kind of energy did each
possible electron orbit in Bohr's
model have?
ANSWER:
A fixed energy
A atom in Bohr model.
The red circles are
protons, the white
ones neutrons and the
blue ones electrons
on two orbits
What are the fixed energies an electron can have?
Energy levels
True or False} The energy levels in atoms are equally spaced and the higher energy
levels are spaced out farther apart
ANSWER: False, they're unequally spaced and the higher energy levels are closer
together
atomic number
ANSWER: number of protons in an atom
atomic mass
ANSWER: number of protons + neutrons in an atom
Nucleus
ANSWER:
Proton
ANSWER;
the center of an atom
A part of the atom with a positive (+) charge.
Located in nucleus.
neutron
electron
energy level
A part of the atom with no (0) charge. Located
in nucleus.
A part of the atom with a negative (-) charge.
Located in energy levels. Almost no mass.
Located outside of nucleus. Electrons are
located here.
number of neutrons
symbol
atomic mass - atomic number
abbreviation for an element.