Structure of Atom 3 Flashcards
what is atomic number
no of protons in an atom of element and no of electrons in a neutral atom
what are nucleons
protons and neutrons are called nucleons
mass number (A) = number of protons (Z)
+ number of
neutrons (n)
= no of nucleons
what are isobars
atoms of different element that have the ame mass number but different atomic numbers.
Ar-40, Ca-40, K-40
N-14 C-14
what are isotopes
atoms of the same element that have same no atomic number but different mass numbers.
it is evident that difference
between the isotopes is due to the presence
of different number of neutrons present in
the nucleus. For example, considering of
hydrogen atom again, 99.985% of hydrogen
atoms contain only one proton. This isotope
is called protium( 1
1
H). Rest of the percentage
of hydrogen atom contains two other isotopes,
the one containing 1 proton and 1 neutron
is called deuterium (
1
2D, 0.015%) and the
other one possessing 1 proton and 2 neutrons
is called tritium (
1
3
T ). The latter isotope is
found in trace amounts on the earth.
do isotopes have different chemical properties
chemical properties
of atoms are controlled by the number of
electrons, which are determined by the
number of protons in the nucleus. Number of
neutrons present in the nucleus have very
little effect on the chemical properties of an
element. Therefore, all the isotopes of a given
element show same chemical behaviour.
isoelectronics what are they
they are species( atoms/ions) that have same number of electrons.
ex: O 2-, F-, Na+, Mg2+ Al 3+, Ne have 10 e-s.
what are postulates of bohr’s atomic model
i) The electron in the hydrogen atom can
move around the nucleus in a circular
path of fixed radius and energy. These
paths are called orbits, stationary states
or allowed energy states. These orbits are
arranged concentrically around the
nucleus.
ii) The energy of an electron in the orbit does
not change with time. However, the electron will move from a lower stationary
state to a higher stationary state when
required amount of energy is absorbed
by the electron or energy is emitted when
electron moves from higher stationary
state to lower stationary state (equation
2.16). The energy change does not take
place in a continuous manner
iii) The frequency of radiation absorbed or
emitted when transition occurs between
two stationary states that differ in energy
by ∆E, is given by :
ν = ΔE/h = E2-E1/h
Where E1
and E2
are the energies of the
lower and higher allowed energy states
r espectively. This expression is commonly known as Bohr’s frequency
rule.
iv)The angular momentum of an electron
in a given stationary state can be
expressed as in equation (2.11)
më v r= h/2 Pi n = 1,2,3….. (2.11)
Thus an electron can move only in those
orbits for which its angular momentum is
integral multiple of h/2π that is why only
certain fixed orbits are allowed.
whatare three takeway’s from bohr’s model of atom
a) The stationary states for electron are
numbered n = 1,2,3………. These integral
numbers (Section 2.6.2) are known as
Principal quantum numbers.
b) The radii of the stationary states are
expressed as :
rn = n
2 a0
(2.12)
where a0
= 52,9 pm. Thus the radius of
the first stationary state, called the Bohr
orbit, is 52.9 pm. Normally the electron
in the hydrogen atom is found in this
orbit (that is n=1). As n increases the
value of r will increase. In other words
the electron will be present away from
the nucleus.
c) The most important property associated
with the electron, is the energy of its
stationary state. It is given by the
expression.
2
1
R
= − En H
n
n = 1,2,3…. (2.13)
where RH
is called Rydberg constant and its
value is 2.18×10–18 J
what does negative energy for e- indicate
This negative sign means that
the energy of the electron in the atom is
lower than the energy of a free electron at
rest. A free electron at rest is an electron
that is infinitely far away from the nucleus
and is assigned the energy value of zero.
Mathematically, this corresponds to
setting n equal to infinity in the equation
(2.13) so that E∞=0. As the electron gets
closer to the nucleus (as n decreases), En
becomes larger in absolute value and more
and more negative. This is because when the e- becomes close to nucleus, it experience a force of attraction due to which energy is gven out. The most negative
energy value is given by n=1 which
corresponds to the most stable orbit. We
call this the ground state.
what is ionized hydrogen atom
When the electron is free from the influence
of nucleus, the energy is taken as zero. The
electron in this situation is associated with the
stationary state of Principal Quantum number
= n = ∞ and is called as ionized hydrogen atom
whta are hydrogen like species? is bohrs theory applicable
Bohr’s theory can also be applied to the
ions containing only one electron, similar
to that present in hydrogen atom. For
example, He+
Li2+ , Be3+ and so on. The
energies of the stationary states
associated with these kinds of ions (also
known as hydrogen like species) are given
by the expression
E= Rh x Z2/n2
radii
R= n2 x 52.9 pm/ Z
what are the limitationsof bohr atomic mdoel
- unable to explain the line spectra of nulti electronatoms. Bohr’s atomic theor was successful in explaining the line spectra of hydorgen and hydrogen like species. It failed to explain the line apectra of other atoms. When spectroscopes with high resolving power was introduced it was observed that even for hydrogen, each line was spilt into a number of closely spaced lines.
-unable to explain the splittting of spectral lines under the effect of magnetic field (zeeman effect) and electric field (stark effect)
-unable to explain the 3-dimensional model of atom
-unable to explain the ability of atoms to fomr molecules by chemicl bonds
