Atoms Flashcards
(24 cards)
What is an atom?
An atom is the fundamental building block of matter, having a confined positively charged nucleus at the center, surrounded by negatively charged electrons revolving in fixed orbits. Every inorganic, organic, or even synthetic object is made up of atoms.
Write a short note on Dalton’s atomic theory.
POSTULATES :
- Atoms are the smallest constituents of matter and can’t be divided further(indivisible)
- Atoms belonging to same matter have similar characteristics and mass; atoms of different matter have different properties and mass
- Atoms are reoriented in a chemical reaction (not generated or destructed)
MERITS :
It demonstrated laws of mass conservation, fixed composition and multiple proportions
DEMERITS :
It was unable to demonstrate experiments of electrostatics (dry paper bits sticking to comb) that showed the existence of charge.
Write a short note on Thomson’s atomic model.
POSTULATES :
- Atom is like a sphere where positive charge has uniform distribution throughout
- Electrons are scattered inside in a way that most stable electrostatic arrangement is achieved, meaning that minimum possible energy of the system should is achieved
- Aka watermelon model / plum pudding model / raisin bread model
MERITS :
It positively illustrated the net neutrality (equal +ve & -ve charges = no net charge) of atom
DEMERITS :
It was inconsistent with the experiments conducted later and the discovery of neutron and proton.
Write a short note on the alpha-particle scattering Experiment.
- Hans Geiger and Ernst Marsden carried out few experiments on the advice of Rutherford.
- They directed a beam of 5.5 MeV α-particles emitted from a (214, 83) Bi radioactive source at a thin metal foil made of gold.
- Alpha-particles emitted by radioactive source were collimated into a narrow beam by their passage through lead bricks. The beam was allowed to fall on a thin foil of gold of thickness 2.1 × 10^(–7) m.
- The scattered alpha-particles were observed through a rotatable detector consisting of zinc sulphide screen and a microscope.
- The scattered alpha-particles on striking the screen produced brief light flashes or scintillations. These flashes may be viewed through a microscope and the scattered α-particles’ distribution as a function of scattering angle(θ) was analyzed and plotted as a graph.
What was inferred by the results of the alpha-particle scattering experiment?
- As the scattering angle(θ) got higher, count of scattered α-particles observed got lower, and vice versa.
- Almost all of the α-particles passed through the gold foil undeflected, and infinitesimally small number of α-particles got deflected
- This proved that almost all the space in an atom is empty and the positive charge is confined to an extremely small region (because very few of the positively charged α-particles were repelled by gold foil). This region was called nucleus.
- Electrons and protons are bound together by electrostatic forces of attraction and atom, as a whole is electrically neutral
- Atom has a structure analogous to our solar system where nucleus (like sun) is at center and all the electrons (like planets) revolving around it in a specified circular path at large speeds
- Rutherford’s experiments suggested the size of
the nucleus to be about 10^(–15) m to 10^(–14) m
What were the drawbacks of Rutherford’s nuclear model of atom?
- Accelerated charged particle produces electromagnetic waves (Maxwell Theory), so orbital radius of electron should go on decreasing and finally electron should fall into the nucleus. But atoms are stable in nature and this stability of atoms could not be clarified by Rutherford’s model
- According to the classical electromagnetic theory,
the frequency of the EM waves emitted by the revolving electrons is equal to the frequency of
revolution. As the electrons spiral inwards, their angular velocities and hence their frequencies would change continuously, and so will the frequency of the light emitted. Thus, they would emit a continuous spectrum, in contradiction to the line spectrum actually observed. - The model didn’t talk about electronic structure of atoms, viz. electrons orientation, their orbital motion and relative energy of electron in different orbits
- Dual character of electromagnetic radiation couldn’t be elaborated by the model
The trajectory of an alpha-particle can be computed employing Newton’s 2nd law of motion and the Coulomb’s law for electrostatic force of repulsion between the alpha-particle and the positively charged nucleus. The magnitude of this force is:
F = (1 / 4πƐo) × [ (2e)(Ze) / r^(2) ]
[where r is the distance between the α-particle and the nucleus.]
The force is directed along the line joining the α-particle and the nucleus.
The magnitude and direction of the force on an α-particle continuously changes as it approaches the nucleus and recedes away from it.
What is impact parameter?
The impact parameter is the perpendicular distance of the
initial velocity vector of the α-particle from the centre of the nucleus.
The trajectory traced by an α-particle depends on the impact parameter, b of collision.
- In case of head-on collision, the impact parameter
is minimum and the α-particle rebounds back (θ ≅ π).
- For a large impact parameter, the α-particle goes nearly undeviated and has a small deflection (θ ≅ 0)
Show that : Kinetic energy (K) = - (1/2) × Potential energy (U) Kinetic energy (K) = - Total energy (T) Potential energy (U) = 2 × Total energy (T)×
The electrostatic force of attraction (Fe) between the revolving electrons and the nucleus provides the requisite centripetal force (Fc) to keep them in their orbits. Thus, for a dynamically stable orbit in a hydrogen atom,
Fe = Fc
(1/4πε0) × (e^2 / r^2) = mv^2 / r
Thus the relation between the orbit radius and the electron
velocity is
r = e^2 / 4πε0mv^2
The kinetic energy (K) and electrostatic potential energy (U) of the electron in hydrogen atom are
K = (1/2)mv^2 and U = - e^2 / 4πε0r
(The negative sign in U signifies that the electrostatic force is in the -r direction.)
So the total energy E of the electron in a hydrogen atom is
E = K + U = (e^2 / 8πε0r) - (e^2 / 4πε0r) = - e^2 / 8πε0r
Write about the charge on electron.
The total energy of the electron is negative. This implies the fact that the electron is bound to the nucleus. If E were positive, an electron will not follow a closed orbit around the nucleus.
Hydrogen atom has ground state energy of -13.6eV. What are the kinetic and potential energies of atom at this state?
Given: Total energy = -13.6eV
Kinetic energy(K) = -Total energy(T) ∴ K = - (-13.6eV) = 13.6eV
Potential energy(U) = 2 × Total energy(T) ∴ U = 2 × (-13.6eV) = -27.2eV
What is atomic spectra?
It is the spectrum of frequencies of electromagnetic radiation emitted or absorbed during transitions of electrons between energy levels within an atom. Each element has a characteristic spectrum by which it can be recognized.
What are spectral lines and what is spectroscopy?
Spectral lines are the bright and dark line series that constitute the spectrum associated with an atom.
Spectroscopy is the learning and examination of emission and absorption spectra associated with an atom to determine its properties.
What is Hydrogen spectral series?
The spacing between lines within certain sets of the hydrogen spectrum decreases in a regular way. Each of these sets is called a spectral series.
Hydrogen spectra is constituted of 5 series of spectrum named after their discoverer (Lyman, Balmer, Paschen, Bracket and Pfund series)
What is Balmer formula?
Experimentally, Balmer found that the spectral lines could be expressed mathematically in the form of wavelength as:
1 / λ = R ( 1 / 2^2 - 1 / n^2 )
where
λ = wavelength
R = Rydberg constant = 1.097 × 10^7 m^(-1)
n = 3, 4, 5… (integral value)
What are the formulae for the different spectral series?
Lyman series :
1 / λ = R ( 1 / 1^2 - 1 / n^2 ) n = 2,3,4…
Balmer series :
1 / λ = R ( 1 / 2^2 - 1 / n^2 ) n = 3,4,5…
Paschen series :
1 / λ = R ( 1 / 3^2 - 1 / n^2 ) n = 4,5,6…
Brackett series :
1 / λ = R ( 1 / 4^2 - 1 / n^2 ) n = 5,6,7…
Pfund series :
1 / λ = R ( 1 / 5^2 - 1 / n^2 ) n = 6,7,8…
What regions do the different spectral series lie in?
UV -> Lyman series
Visible -> Balmer series
Infrared -> Paschen, Brackett and Pfund series
State the postulates of Bohr’s atomic theory.
(i) An electron in an atom could revolve in certain stable orbits without the emission of radiant energy
(ii) Electron revolves around the nucleus only in those orbits for which the angular momentum is some integral multiple of h/2π where h is the Planck’s constant [= 6.6 × 10^(–34) Js]
∴ L = nh/2π = MVnRn
(iii) An electron might make a transition from one of its specified non-radiating orbits to another of lower energy. When it does so, a photon is emitted having energy equal to the energy difference between the initial and final states. The frequency of the emitted photon is then given by h = Ei – Ef (where Ei & Ef are the energies of the initial & final states and Ei > Ef)
What is Bohr’s radius?
The radius on which electrons move around the nucleus in the orbit described by the Bohr’s model is known as Bohr’s radius.
Bohr’s radius = ao = h^2Ɛo / πme^2
Substitution of values of h, m, Ɛo and e gives
ao = 5.29 × 10^(–11) m
The total energy of the electron in the stationary states of the hydrogen atom is
E = - 13.6 / n^2 eV
The negative sign of the total energy of an electron moving in an orbit means that the electron is bound with the nucleus.
Energy will thus be required to remove the electron from the hydrogen atom to a distance infinitely far away from its nucleus (or proton in hydrogen atom).
Velocity of electron in an orbit is __________
v = e^2 / 2nhƐo
What is energy of an orbit?
The orbital energy of orbiting electron in the discrete energy levels in the Bohr’s model is called as the energy of orbits.
We already know, from Rutherford’s Model that total energy(T) is given by
T = -Ze^2 / (8πƐor)
Putting the value of Bohr’s radius, we get
T = -me^4 / 8π(Ɛo)^2(n)^2(h)^2
Putting in the values, we get
T = (-13.6/n^2) eV
[For the energy of innermost stationary orbit, put n=1]
What were the drawbacks of Bohr’s model?
- It was primarily for hydrogen atom
- It couldn’t elaborate spectra of multi-electron atoms
- Wave nature of electron was not justified by the model (inconsistent with the de Broglie’s hypothesis of dual nature of matter)
- It didn’t illustrated molecules making process of chemical reactions
- It violated Heisenberg’s Principal [Δx × Δp ≥nh/(2π)] which said that it was impossible to evaluate the precise position and momentum of electron (and other microscopic particles) simultaneously. Only their probability could be estimated.
- Zeeman effect (spectral lines variation due to external magnetic field) and Stark Effect (spectral lines variation due to external electric field) couldn’t be described by the model.
Write a short note on De Broglie’s Hypothesis.
- De Broglie’s Hypothesis showed the wave particle duality of matter
- It showed that, like photons, electrons must also have mass or momentum and wavelength(λ), given by the equation (here c = speed of light in air, v = frequency)
p = mv = h/λ = h/(c/λ) - It holds only for the subatomic (microscopic) particles like electron, proton etc. where mass is very small, so wavelength are large enough to be experimentally observable
- It does not hold for the macroscopic particles since mass there is very large, making wavelength too small to be experimentally observable
