Download Atomic Structure Complete Notes with models and their Experiments & Defects and more Study notes Chemistry in PDF only on Docsity!
Atomic Structure
Multiple Choice Questions
- In an atom, electrons:
1) Move around the nucleus in circular orbits.
Move around the nucleus in elliptical orbits
From diffused cloud around the nucleus
None of these
- Mass of the neutron is:
Same as proton 2) Much less than that of proton
Slightly less than that of proton 4) Slightly more than that of proton
- Positive charge in an atom is:
Concentrated in the nucleus 2) Scattered all over the atom
Revolve around the nucleus 4) None of these
- Nuclear model of the atom was proposed by:
Neil Bohr 2) Rutherford
Thomson 4) Moseley
- Neutron was discovered by:
Thomson 2) Chadwick
Bohr 4) Rutherford
- Mass number of an element represents sum of number of:
Protons and Neutrons 2) Protons and electrons
Electrons and neutrons 4) None of these
- Atomic number of atoms represents:
Protons and Neutrons 2) Protons only
Protons or electrons in a neutral atom 4) Electrons and neutrons
- According to Dalton's atomic theory, an atom:
Can be further divided 2) Cannot be subdivided
Contains neutrons, protons and electrons 4) None of the above
- Rutherford was awarded Nobel Prize in 1908 for his studies on:
Radicals 2) Nuclear model of atoms
Radioactive matter 4) Discovery of neutron
- Cathode rays consists of a stream of:
Protons 2) Neutrons
Electrons 4) Positrons
- Beta rays consists of a stream of:
Protons 2) Neutrons
Electrons 4) Positrons
- Cathode rays are deflected by:
A magnetic field only 2) An electric field only
Both electric and magnetic field 4) Neither electric nor magnetic
- The nature of anode rays depends on:
Nature of discharge tube 2) Nature of residual gas
All of these 4) None of the above
- Rutherford's experiments on scattering of particles showed for the first time that atom has:
Electrons 2) Positrons
Neutrons 4) Nucleus
- Line spectra are characteristic of:
Atoms 2) Molecules
Ions 4) Radicals
- In the discharge tube emission of cathode rays requires:
Low potential and low pressure 2) Low potential and high pressure
High potential and high pressure 4) High potential and low pressure
- When alpha - particles are sent through a thin metal foil, most of them go straight through the foil because:
alpha particles are much heavier then electrons 2) alpha particles are positively charged
Most part of atom has empty space 4) alpha particles have high velocity
- The number of electrons in an atom of an element is equal to its:
Atomic weight 2) Atomic number
Equivalent weight 4) Mass number
- The isotope C-12 is present in abundance of:
96.9% 2) 97.6%
99.7% 4) None of these
- Who discovered the proton?
Goldstein 2) J. J. Thomson
Neil Bohr 4) Rutherford
- In an atom number of protons and neutrons are added to obtain:
number of electrons 2) number of nucleons
atomic number of element 4) number of isotopes
- If proton number is 19, electron configuration will be:
2, 8, 9 2) 2, 8, 8, 1
2, 8, 1 4) 2, 8, 3
- If nucleon number of potassium is 39, number of neutrons will be:
39 2) 19
20 4) 29
- Electronic configuration is distribution of:
proton 2) neutron
electron 4) positron
- How many subshells in a L shell:
one 2) two
three 4) four
- De Broglie extend the wave particle duality to electron in:
1920 2) 1922
1923 4) 1925
- Name the material of screen which used in Rutherford atomic model:
Aluminum foil 2) zinc sulfide
sodium sulfide 4) Aluminum sulfide
- Which rays are used for sterilization of medical instruments:
alpha rays 2) beta rays
gamma rays 4) X-rays
Important Question Answers
Q.1: Explain the discovery of electron in discharge tube experiment? OR
Explain Cathode Rays Experiment.
Ans: Discharge Tube Experiment
J.J Thomson and William Crooks, in 1879, discovered the subatomic particle “electron” by a discharge tube experiment.
Construction
Two metal electrodes A discharge tube A vacuum pump High voltage source
Observation
At normal pressure, no conduction, but when the pressure is lowered by evacuating the air with a vacuum pump, conduction. Conduction takes place. When a very high voltage is applied across the electrodes at very low pressure, the rays appeared from the cathode and travelled towards the anode in a straight line, and are called Cathode rays.
Properties of Cathode Rays
The properties of cathode rays are as follows:
- They travel in straight line
- They deflect towards the positive plate in the electric and magnetic field.
- The temperature of the object rises as they fall on that object.
- When an opaque object is placed in their path, they produce a sharp shadow.
In 1932, English physicist James Chadwick discovered a third type of fundamental particle of atomic structure through artificial radioactivity.
Artificial Radioactivity:
The emission of radiation from the nuclei of lighter atoms due to bombardment by particles is known as artificial radioactivity. When a light element is bombarded by a particle, these alpha particles leave the nucleus in an unstable, disturbed state which, on settling down to a stable condition, sends out radioactive rays.
Chadwick's experiment:
Chadwick identified a new particle obtained from the bombardment of beryllium by alpha particles and found that it gave highly penetrating radiation. The radiation knocked protons out of paraffin with great
force; it had a unit mass and carried no charge. It was named neutron.
Reaction:
2 He^4 +^4 Be^9 6 C^12 +^0 n^1 (𝛼-particles) (Beryllium) (Carbon) (Neutron)
Properties of Neutrons
1. The neutrons are neutral particles.
2. They have no charge.
3. The mass of a neutron is almost equal to the mass of a proton
4. These particles are most penetrating in matter.
Q.4: Describe Rutherford’s experiment. How was the nucleus discovered in
Rutherford’s experiment? Give the defects of the Rutherford Model.
Ans: Rutherford Experiment and Discovery of Nucleus
Introduction:
In 1911, Lord Rutherford conducted a series of experiments and proposed a new model for the atom.
Experiment:
Rutherford took a thin sheet of gold and bombarded it with alpha particles obtained from a radioactive element like polonium. These rays scattered through the sheet and produced illumination on the zinc sulfide screen.
Observation:
1. Most of the particles pass straight and undeflected through the sheet and produce illumination on
the zinc sulfide screen.
2. Very few particles undergo a small and strong deflection after passing through the sheet.
3. A very few particles, one out of eight thousand, retraced their path.
Conclusion:
- The atom consist of two parts: Nuclear part and extra nuclear part.
- The heavy, positively charged central part of the atom is called the nucleus.
- Nearly all of the mass of an atom is concentrated in the nucleus.
- The size of the nucleus is very small compared with the size of an atom.
- The electrons revolve around the nucleus in extra nuclear part called orbits, shells or energy levels.
Defects of Rutherford Model
The Rutherford model of an atom resembles our solar system. It has the following defects:
- According to classical electromagnetic theory, an electron, being a charged body, will continuously emit energy. Thus, the orbit of the revolving electron becomes smaller and smaller until it falls into the nucleus, and the atomic structure collapses.
- If a revolving electron emits energy continuously, then there should be a continuous spectrum, but a line spectrum is obtained.
Q.5: Describe the salient features of Neil Bohr’s model of an atom.
Ans: Nein Bohr Atomic Model:
After Planck and Einstein, Danish physicist Neil Bohr in 1913 explained line spectra. The important assumptions for atomic structure are given below:
Q.7: Explain Schrodinger Atomic Model:
Ans: The Schrodinger Model is just an improvement of Bohr’s atomic model. He took an atom of hydrogen because it has one proton and one electron. He proved mathematically that electrons can be found in different positions around the nucleus and are determined by probability.
The quantum mechanical model determines that an electron can be found in various
locations around the nucleus. He found that electrons are in orbit as an electron cloud.
Each energy subshell in an orbit have different shapes which determine the presence of
electrons.
Different subshells of orbitals are named as s, p, d, and f, with different shapes, as ‘s’ is
spherical, and ‘d ’ is dumbbell-shaped.
The numbers and kinds of atomic orbitals depend upon the energy subshell.
Q.8: Define Concept of Shells (K, L, M, N, & O):
Ans: Shell, or Orbit, or Energy level
The Shell, or Orbit, or Energy level are all possible paths on which electrons are revolving around the nucleus. Which shows by “n” these shells are named as K, L, M, N, & P with quantum numbers n= 1, 2, 3, 4, 5, respectively. These shells have a definite amount of energy by means of increasing order, as they move away from the nucleus.
Subshell:
A subshell is a subdivision of electron shells separated by electron orbitals. Subshells are labeled as s, p, d, and f in an electronic configuration.
Composition Of The Atoms Of The First Eighteen
Elements
Q.9: Define and explain isotopes. Discuss the structures of isotopes of some
elements.
