Structure of Atom 5 Flashcards
what is the principal quantum number
The principal quantum number ‘n’ is a
positive integer with value of n = 1,2,3……. .The principal quantum number determines the
size and to large extent the energy of the
orbital. For hydrogen atom and hydrogen like
species (He+
, Li 2+, …. etc.) energy and size of
the orbital depends only on ‘n’.
what is the shell
The principal quantum number also
identifies the shell. With the increase in the
value of ‘n’, the number of allowed orbital
increases and are given by ‘n
2
’ All the
orbitals of a given value of ‘n’ constitute a
single shell of atom and are represented by
the following letters
n = 1 2 3 4 …………
Shell = K L M N …………
why does the eneergy and size denoted by principal quantum nuomber
Size of an orbital increases with increase
of principal quantum number ‘n’. In other
words the electron will be located away from
the nucleus. Since energy is required in
shifting away the negatively charged electron
from the positively charged nucleus, the
energy of the orbital will increase with
increase of n
what is azimthal quantum number
Azimuthal quantum number. ‘l’ is also
known as orbital angular momentum or
subsidiary quantum number. It defines the
three dimensional shape of the orbital. For a
given value of n, l can have n values ranging
from 0 to n – 1, that is, for a given value of n,
the possible value of l are : l = 0, 1, 2, ……….
(n–1)
what are sub shells/ sub levels?
Each shell consists of one or more subshells or sub-levels. The number of subshells in a principal shell is equal to the value
of n.
Each sub-shell is assigned an azimuthal quantum number
(l). Sub-shells corresponding to different
values of l are represented by the following
symbols.
Value for l : 0 1 2 3 4 5 …………
notation for s p d f g h …………
sub-shell
what magnetic orbital quantum number
Magnetic orbital quantum number. ‘ml
’
gives information about the spatial
orientation of the orbital with respect to
standard set of co-ordinate axis. For any
sub-shell (defined by ‘l’ value) 2l+1 values
of ml
are possible and these values are given
by :
ml
= – l, – (l–1), – (l–2)… 0,1… (l –2), (l–1), l
what is the 4tth quantum number
George Uhlenbeck and Samuel
Goudsmit proposed the presence of the fourth quantum number known as the electron
spin quantum number (ms). An electron
spins around its own axis, much in a similar
way as earth spins around its own axis while
revolving around the sun. In other words, an
electron has, besides charge and mass,
intrinsic spin angular quantum number. Spin
angular momentum of the electron — a vector
quantity, can have two orientations relative
to the chosen axis.
what is the e- spin quantum no
These two orientations are
distinguished by the spin quantum numbers
ms
which can take the values of +½ or –½.
These are called the two spin states of the
electron and are normally represented by two
arrows, ↑ (spin up) and ↓ (spin down). Two
electrons that have different ms
values (one
+½ and the other –½) are said to have
opposite spins. An orbital cannot hold more
than two electrons and these two electrons
should have opposite spins.
sum up the quantum numbers
i) n defines the shell, determines the size
of the orbital and also to a large extent
the energy of the orbital.
ii) There are n subshells in the n
th shell. l
identifies the subshell and determines the
shape of the orbital (see section 2.6.2).
There are (2l+1) orbitals of each type in a
subshell, that is, one s orbital (l = 0), three
p orbitals (l = 1) and five d orbitals (l = 2)
per subshell. To some extent l also
determines the energy of the orbital in a
multi-electron atom.
iii) ml
designates the orientation of the
orbital. For a given value of l, ml
has (2l+1)
values, the same as the number of orbitals
per subshell. It means that the number
of orbitals is equal to the number of ways
in which they are oriented.
iv) ms
refers to orientation of the spin of the
electron.
what is an orbit
An orbit, as proposed by Bohr, is a circular path around the
nucleus in which an electron moves. A precise description of this path of the electron is impossible
according to Heisenberg uncertainty principle. Bohr orbits, therefore, have no real meaning and
their existence can never be demonstrated experimentally
what is an orbital/ or atomic orbital
An atomic orbital, on the other hand,
is a quantum mechanical concept and refers to the one electron wave function ψ in an atom. It is
characterized by three quantum numbers (n, l and ml
) and its value depends upon the coordinates
of the electron.
does psi have any phsyical meaning? then what has meaning
ψ has, by itself, no physical meaning. It is the square of the wave function i.e., |ψ|2
which has a physical meaning. |ψ|2 at any point in an atom gives the value of probability density
at that point. Probability density (|ψ|2) is the probability per unit volume and the product of |ψ|2
and a small volume (called a volume element) yields the probability of finding the electron in that
volume
why is volume element taken to be a very small volume?
the reason for specifying a small volume element is that |ψ|2 varies from one region to
another in space but its value can be assumed to be constant within a small volume element)
what is an orbital
the region arund the nucleus which represent the ee- density at different points
