Atomic Structure Flashcards
The electrical
discharge through the gases could be
observed only at very low………and at
very high………..
pressures
voltages
The flow of current
from cathode to anode was further checked
by making a hole in the………and coating
the tube behind anode with phosphorescent
material……….
anode
zinc sulphide.
Statement 1: Cathode rays themselves are not visible
but their behaviour can be observed with help of fluorescent material
Statement 2: Television
picture tubes are cathode ray tubes
and television pictures result due to
fluorescence on the television screen
coated with certain fluorescent
Both statements are correct
In
1897, British physicist……….
measured the ratio of electrical charge (e) to
the mass of electron (m
e ) by using cathode
ray tube and applying electrical and magnetic
field perpendicular to each other as well as
to the path of electrons
J.J. Thomson
…………. devised method known as oil drop experiment (1906-14),
to determine the charge on the electrons.
R.A. Millikan
In this method, oil droplets in the form of mist, produced by the atomiser, were allowed to enter through a tiny hole in the upper plate of electrical condenser. The downward motion of these droplets was viewed through the telescope, equipped with a micrometer eye piece. By measuring the rate of fall of these droplets, Millikan was able to measure the mass of oil droplets. The air inside the chamber was ionized by passing a beam of X-rays through it. The electrical charge on these oil droplets was acquired by collisions with gaseous ions. The fall of these charged oil droplets can be retarded, accelerated or made stationary depending upon the charge on the droplets and the polarity and strength of the voltage applied to the plate. Millikan concluded that electric charge is quantised
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Positively charged particle proton was
characterised in……..
1919
With help of anode ray experiment or cathod ray experiment with perforated CATHODE
Chadwick (1932) discovered……..
by bombarding a thin sheet of beryllium
by α-particles.
neutron
The major
problems before the scientists after the
discovery of sub-atomic particles were: 4
account for the stability of atom
compare the behaviour of elements
understand the origin and nature of
the characteristics of electromagnetic
radiation absorbed or emitted by atoms
explain the formation of different
kinds of molecules
An important
feature of plum pudding model is
that the mass of the
atom is assumed to be uniformly distributed
Statement 1:X-rays are produced effectively
when electrons strike the dense metal anode,
called targets
Statement 2:These are not deflected in electric and magnetic field
Both statements are correct
Assertion: X rays are
used to study the interior of the objects.
Reason: very
high penetrating power through the matter
Statement1: γ-rays
are high energy radiations they are
neutral in nature and do not consist of
particles.
Statement 2: penetrating power,
α-particles is the most
Statement 1 is correct and 2 is incorrect
Penetrating power
Alpha<beta<gamma
Beta=100xalpha
Gamma=1000xalpha
Rutherford model postulates: 4 postulates
Most of the space in atom is empty
Positive charge and most of the mass is densely concentrated in very small volume aka nucleus.
Electrons and nucleus are held by electrostatic force of attraction
The radius of
the atom is about 10–10 m, while that of
nucleus is 10–15 m.
Statement 1:Isobars are the atoms with different mass
number butsame atomic number
Statement 2: Atoms with identical atomic number but
different atomic mass number are known
as Isotopes.
Statement I is incorrect and 2 is correct
Isobars are the atoms with same mass
number but different atomic number
Number of neutrons
present in the nucleus have very little effect
on the chemical properties of an element.
True or false
True
Calculations show
that it should take an electron only………… seconds
to spiral into the nucleus.
10–8 s
Two
developments that played a major role in the
formulation of Bohr’s model of atom are
Dual character of the electromagnetic
radiation
Experimental results regarding atomic
spectra.
…………….radiations
consist of electromagnetic waves of various
frequencies or wavelengths.
thermal
Statement 1:In vaccum all types of electromagnetic
radiations, regardless of wavelength, travel at
the same speed
Statement: wavenumber is defined as the number of wavelengths
per unit length.
Both statements are correct
Number of nodal planes in 4 dz2 is………
Zero
Electric conduction takes place in a discharge tube due to the movement of
(a) positive ions
(b) negative ions
(c) electrons
(d) protons
A B and C only
Properties of cathode rays
Invisible rays and produced glow when hit the ZnS screen
Travel in straight line with high speed
Negatively Charged Particles therefore earlier called ‘negatrons’
Rotated as light weight pin wheel placed in their path
Heat the object they fall on due to transfer of KE
Produced shadow of metallic objects placed on it path
Produced X rays on falling onto heavy metals
Independent of nature of gas and cathode material
Properties of anode rays
Invisible and produced glow
Traveled in straight line with high spped
Rays deflected in electric and magnetic field showing behavior opposite to the cathode rays
Consist of positively charged particles
Possess ionizing power
e/m ratio was dependent on gas taken in tube
And e/m value is very smaller than electron
Define Continuous and discontinuous spectrum
Continuous spectrum: spectrum of EMW in any region[eg. visible] without any gaps in colours or wavelengths
Discontinuous Spectrum:if any wavelength or colour is missing
Plank’s quantum theory
Planck assumed that radiation could be sub-divided into discrete chunks of energy.
He suggested that atoms and molecules could emit or absorb energy only in discrete quantities ie discontinuous manner and not in a continuous manner.
He gave the name quantum to the smallest quantity of energy that can be emitted or absorbed in the form of electromagnetic radiation
The energy (E ) of a quantum of radiation is proportional to its frequency
Black body radiation
Hot objects emit electromagnetic radiations over a wide range of wavelengths. At high temperatures, an appreciable proportion of radiation is in the visible region of the spectrum. As the temperature is raised, a higher proportion of short wavelength (blue light) is generated.
Demonstrate with an example that intensities of radiations of different wavelengths emitted by hot body depend upon its temperature.
when an iron rod is heated in a furnace, it first turns to dull red and then progressively becomes more and more red as the temperature increases. As this is heated further, the radiation emitted becomes white and then becomes blue as the temperature becomes very high. This means that red radiation is most intense at a particular temperature and the blue radiation is more intense at another temperature.
Black body and black body radiation
An ideal body, which emits and absorbs radiations of all frequencies uniformly, is called a black body and the radiation emitted by such a body is called black body radiation.
A black body is also a perfect radiator of radiant energy.
Black body in real life
In practice, no such body exists. Carbon black approximates fairly closely to black body.
A good physical approximation to a black body is a cavity with a tiny hole, which has no other opening. Any ray entering the hole will be reflected by the cavity walls and will be eventually absorbed by the walls.
Bohr model of atom was based on……….. and………
Quantum theory of radiation and classical laws of physics
It is also based on or valid for particle nature of electron
Bohr postulates
1: 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.
2: 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. The energy change does not take place in a continuous manner.
3: The angular momentum of an electron is quantised.
4: Thus an electron can move only in those orbits for which its angular momentum is integral multiple of h/2π. That means angular momentum is quantised. Radiation is emitted or obsorbed only when transition of electron takes place from one quantised value of angular momentum to another. Therefore, Maxwell’s electromagnetic theory does not apply here that is why only certain fixed orbits are allowed.
Limitations of Bohr model
(1) Bohr’s theory does not explain the spectrum of multi electron atom/ion.
(2) Why the Angular momentum of the revolving electron is equal to nh/2 has not been explained by Bohr’s theory.
(3) Bohr inter related quantum theory of radiation and classical laws of physics without any theoretical explanation.
(4) Bohr’s theory does not explain the fine structure of the spectral lines. Fine structure of the spectral line is obtained when spectrum is viewed by spectroscope of more resolution power.
(5) Bohr’s theory does not explain the splitting of spectral lines in the presence of magnetic field (Zeeman’s effect) or electric field (Stark effect)
Principal quantum number
The principal quantum number determines the size and to large extent the energy of the orbital.
The principal quantum number also identifies the shell. With the increase in the value of ‘n’, All the orbitals of a given value of ‘n’ constitute a single shell of atom
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.
Azimuthal 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.
l can have n values ranging from 0 to n – 1
Each shell consists of one or more sub-shells or sub-levels. The number of sub-shells in a principal shell is equal to the value of n. For example in the first shell (n = 1), there is only one sub-shell which corresponds to l = 0.
Magnetic 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.
Value of m ranges from -l to +l including the zero
The three quantum numbers labelling an atomic orbital can be used equally well to define its energy, shape and orientation. But all these quantum numbers are not enough to explain the line spectra observed in the case of multi-electron atoms
True or false
True
some of the lines actually occur in doublets (two lines closely spaced), triplets (three lines, closely spaced) etc. This suggests the presence of a few more energy levels than predicted by the three quantum numbers.
In 1925, 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. 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.
Features of quantum mechanical model
- The energy of electrons in atoms is quantized (i.e., can only have certain specific values), for example when electrons are bound to the nucleus in atoms.
- The existence of quantized electronic energy levels is a direct result of the wave like properties of electrons and are allowed solutions of Schrödinger wave equation.
- Both the exact position and exact velocity of an electron in an atom cannot be determined simultaneously (Heisenberg uncertainty principle). The path of an electron in an atom therefore, can never be determined or known accurately. That is why, as you shall see later on, one talks of only probability of finding the electron at different points in an atom.
- An atomic orbital is the wave function ψ for an electron in an atom. Whenever an electron is described by a wave function, we say that the electron occupies that orbital. Since many such wave functions are possible for an electron, there are many atomic orbitals in an atom. These “one electron orbital wave functions” or orbitals form the basis of the electronic structure of atoms. In each orbital, the electron has a definite energy. An orbital cannot contain more than two electrons. In a multi-electron atom, the electrons are filled in various orbitals in the order of increasing energy. For each electron of a multi-electron atom, there shall, therefore, be an orbital wave function characteristic of the orbital it occupies. All the information about the electron in an atom is stored in its orbital wave function ψ and quantum mechanics makes it possible to extract this information out of ψ.
- The probability of finding an electron at a point within an atom is proportional to the square of the orbital wave function i.e., |ψ|2 at that point. |ψ|2 is known as probability density and is always positive. From the value of |ψ|2 at different points within an atom, it is possible to predict the region around the nucleus where electron will most probably be found.
Aufbau Principle
The word ‘aufbau’ in German means ‘building up’.
The building up of orbitals means the filling up of orbitals with electrons. The principle states : In the ground state of the atoms, the orbitals are filled in order of their increasing energies.
The number of electrons to be filled in various orbitals is restricted by the exclusion principle, given by the Austrian scientist…………….
Wolfgang Pauli (1926).
Pauli’s exclusion principle
According to this principle : No two electrons in an atom can have the same set of four quantum numbers. Pauli exclusion principle can also be stated as : “Only two electrons may exist in the same orbital and these electrons must have opposite spin.”
The restriction imposed by Pauli’s exclusion principle on the number of electrons in an orbital helps in calculating the capacity of electrons to be present in any subshell.
Hund’s Rule of Maximum Multiplicity
This rule deals with the filling of electrons into the orbitals belonging to the same subshell (that is, orbitals of equal energy, called degenerate orbitals).
It states : pairing of electrons in the orbitals belonging to the same subshell (p, d or f) does not take place until each orbital belonging to that subshell has got one electron each i.e., it is singly occupied.
Bohr bury rule
Applied only on species that aren’t like hydrogen
For hydrogen like species energy is proportional to ‘n’
(n+l) for different values of this sum the energy is high for the one with high value of sum
For (n+l) same then compare with ‘n’ then energy is proportional to ‘n’
Statement 1: Absorption spectrum consist of some bright lines separated by some dark spaces
Statement 2: The emission spectra consists of dark lines
Both statement are incorrect
Absorption spectrum consist of some dark spaces separated by some bright lines
Emission spectra consists of bright lines always
What is an atomic spectrum?
It is the pattern of light emitted or absorbed by an atom due to electron transitions between energy levels.
What are the two types of atomic spectra?
- Absorption Spectrum – Dark lines on a bright background (electrons absorb energy).
- Emission Spectrum – Bright lines on a dark background (electrons release energy).
What is an atomic absorption spectrum?
It consists of dark lines in a continuous spectrum where specific wavelengths are absorbed by electrons moving to higher energy levels.
What are the characteristics of an atomic absorption spectrum?
✔ Unique for each element.
✔ Dark lines on a bright background.
✔ Absorbed wavelengths match those in the emission spectrum.
What is an atomic emission spectrum?
It consists of bright lines on a dark background, produced when electrons fall back to lower energy levels, releasing energy as light.
What are the characteristics of an atomic emission spectrum?
✔ Unique for each element.
✔ Bright lines on a dark background.
✔ Represents specific electron transitions.
How does a continuous spectrum differ from a line spectrum?
✔ Continuous Spectrum – All wavelengths present (e.g., sunlight).
✔ Line Spectrum – Only specific wavelengths appear (e.g., hydrogen emission spectrum).
What is the hydrogen emission spectrum?
The hydrogen spectrum consists of five series of spectral lines corresponding to electron transitions to different energy levels.
Why does each element have a unique spectrum?
✔ Each element has unique energy levels.
✔ The possible electron transitions differ for each element.
✔ This results in unique absorption and emission lines.
