What are canal rays?
Canal rays (also called anode rays or positive rays) are streams of positively charged particles.
They were discovered by E. Goldstein in 1886. In his experiment, he used a discharge tube with a perforated cathode (cathode with holes/canals). He observed that rays were produced that traveled in the opposite direction to cathode rays — moving from the anode toward the cathode and passing through the holes in the cathode.
How they form: The high-energy cathode rays (electrons) moving from cathode to anode knock electrons out of neutral gas atoms in the tube, creating positively charged ions. These positive ions are attracted toward the cathode and some pass through the holes (canals) in the cathode, forming the canal rays.
Properties of canal rays:
1. They carry positive charge.
2. The charge-to-mass ratio of particles depends on the type of gas used in the tube.
3. When hydrogen gas is used, the positive particles produced have the smallest mass — these are protons.
If an atom contains one electron and one proton, will it carry any charge or not?
The atom will not carry any charge — it will be electrically neutral.
An electron carries a negative charge of −1 (or −1.6 × 10−19 C).
A proton carries a positive charge of +1 (or +1.6 × 10−19 C).
Since the magnitudes of the charges are equal and opposite, they cancel each other perfectly. Therefore, an atom with 1 electron and 1 proton (which is a hydrogen atom, 1H) is electrically neutral with a net charge of zero.
On the basis of Thomson's model of an atom, explain how the atom is neutral as a whole.
According to J.J. Thomson's model (also called the "plum pudding model" or "watermelon model"), an atom consists of:
1. A uniform, spherical cloud (sphere) of positive charge spread throughout the entire atom.
2. Electrons (negatively charged particles) embedded within this positive sphere like plums in a pudding (or seeds in a watermelon).
Neutrality: Thomson proposed that the total amount of positive charge in the sphere is exactly equal to the total negative charge of all the electrons embedded in it. Since equal and opposite charges cancel each other, the atom as a whole is electrically neutral.
For example, in a carbon atom (according to Thomson's model), 6 electrons are embedded in a sphere of +6 positive charge, making the net charge = +6 − 6 = 0 (neutral).
On the basis of Rutherford's model of an atom, which subatomic particle is present in the nucleus of an atom?
According to Rutherford's nuclear model (1911), the nucleus of an atom contains protons (positively charged particles).
Rutherford's alpha-particle scattering experiment showed that most of the mass and all of the positive charge of an atom is concentrated in a very small, dense central region called the nucleus (of radius ~10−15 m). Electrons revolve around the nucleus in circular orbits at a relatively large distance.
Note: At the time of Rutherford's experiment (1911), neutrons had not yet been discovered. Neutrons were later discovered by James Chadwick in 1932. We now know that the nucleus contains both protons and neutrons (collectively called nucleons). In Rutherford's model, only protons were known to be in the nucleus.
Draw a sketch of Bohr's model of an atom with three shells. Briefly describe the model.
Bohr's Model: Proposed by Niels Bohr in 1913, this model improved upon Rutherford's model by introducing the concept of stationary energy levels (shells or orbits).
Description of Bohr's model with 3 shells (e.g., Sodium, Na with atomic number 11):
The atom has a nucleus at the centre containing protons (+) and neutrons. Around the nucleus are 3 concentric circular shells (orbits):
K shell (n=1, 1st orbit): Closest to nucleus; can hold maximum 2 electrons.
L shell (n=2, 2nd orbit): Second ring; can hold maximum 8 electrons.
M shell (n=3, 3rd orbit): Third ring; can hold maximum 18 electrons.
Electrons revolve around the nucleus in these fixed circular orbits without radiating energy. Energy is absorbed or emitted only when an electron jumps from one orbit to another.
Key postulates:
1. Electrons revolve in fixed circular orbits called energy shells or levels.
2. Each shell has a fixed energy; electrons in a shell have a definite energy.
3. An electron does not radiate energy while revolving in an orbit (stationary states).
4. Electrons can jump to higher shells by absorbing energy, and return to lower shells by emitting energy.
What would be the observation if the α-particle scattering experiment is carried out using a foil of a metal other than gold?
If the α-particle scattering experiment were carried out using a foil of any other metal (e.g., silver, platinum, copper), the observations would be qualitatively similar:
1. Most α-particles would pass straight through the foil undeflected (showing that most of the atom is empty space).
2. A small fraction would be deflected at small angles.
3. Very few would be deflected back at large angles (close to 180°).
These observations would still demonstrate the nuclear model of the atom — that the atom has a small, dense, positively charged nucleus with electrons far away.
Why gold was specifically chosen:
1. Gold is extremely malleable and can be beaten into a very thin foil (only a few hundred atoms thick). A thinner foil ensures α-particles encounter only one or a very few nuclei, giving cleaner results.
2. Gold is chemically inert — it does not react with air or the alpha particles, ensuring no chemical interference.
3. Gold has a high atomic number (Z = 79) and heavy nucleus, which gives stronger electrostatic repulsion and larger deflection angles that are easier to observe.
Name the three sub-atomic particles of an atom and give their properties.
The three sub-atomic particles of an atom are:
(i) Electron:
Discovered by: J.J. Thomson (1897). Symbol: e−. Charge: −1 (or −1.6 × 10−19 C). Mass: 9.1 × 10−31 kg (negligible compared to proton). Location: Outside the nucleus, in shells/orbits.
(ii) Proton:
Discovered by: E. Goldstein (identified via canal rays, 1886); established by Rutherford. Symbol: p+. Charge: +1 (or +1.6 × 10−19 C). Mass: 1.673 × 10−27 kg (~1 u). Location: Inside the nucleus.
(iii) Neutron:
Discovered by: James Chadwick (1932). Symbol: n0. Charge: 0 (electrically neutral). Mass: 1.675 × 10−27 kg (~1 u, slightly heavier than proton). Location: Inside the nucleus. (Absent in ordinary hydrogen atoms.)
Helium atom has an atomic number of 2 and a mass number of 4. How many electrons, protons, and neutrons does it have?
For Helium (42He):
Atomic number (Z) = 2 = number of protons
Number of Protons = 2
In a neutral atom, number of electrons = number of protons.
Number of Electrons = 2
Mass number (A) = 4
Number of Neutrons = Mass number − Atomic number = 4 − 2 = 2
Summary: Helium has 2 protons, 2 electrons, and 2 neutrons.
Electronic configuration: K shell = 2 (completely filled). Since the outermost shell is fully filled, helium is a noble gas and is chemically inert.
In the atom of an element X, 6 electrons are present in the outermost shell. If it is in the third period of the periodic table, write the atomic number and electronic configuration of the element.
Third period means the element has electrons in 3 shells (K, L, M).
The outermost (third) shell M has 6 electrons.
Electronic configuration:
K shell (1st) = 2 electrons (max capacity = 2, fully filled)
L shell (2nd) = 8 electrons (max capacity = 8, fully filled)
M shell (3rd) = 6 electrons (given)
Electronic configuration: 2, 8, 6
The element is Sulphur (S) with atomic number 16.
Sulphur has 6 valence electrons and needs to gain 2 more to complete its octet, so its valency is 2.
Write the electronic configuration of any one pair of isotopes and isobars.
Isotopes: Atoms of the same element with the same atomic number but different mass numbers (different number of neutrons).
Example: Carbon isotopes — 12C and 14C
Both have atomic number = 6 (6 protons, 6 electrons).
12C: 6 protons, 6 neutrons, 6 electrons. Electronic configuration: K(2), L(4) = 2, 4
14C: 6 protons, 8 neutrons, 6 electrons. Electronic configuration: K(2), L(4) = 2, 4
Both isotopes have the same electronic configuration and hence the same chemical properties.
Isobars: Atoms of different elements with the same mass number but different atomic numbers.
Example: Calcium and Argon — 40Ca (Z=20) and 40Ar (Z=18)
Both have mass number = 40.
40Ca: Electronic configuration: K(2), L(8), M(8), N(2) = 2, 8, 8, 2
40Ar: Electronic configuration: K(2), L(8), M(8) = 2, 8, 8
Isobars have different electronic configurations and hence different chemical properties.
The atomic number of oxygen is 8. How many electrons are there in an oxygen atom? What is its electronic configuration?
Atomic number of oxygen (O) = 8.
In a neutral atom, the number of electrons = atomic number.
Number of electrons in oxygen = 8
Electronic configuration:
K shell (max 2): filled with 2 electrons.
L shell (max 8): remaining 8 − 2 = 6 electrons.
Oxygen has 6 valence electrons (in the outermost L shell). It needs to gain 2 more electrons to complete its octet, so its valency is 2.
Number of valence electrons in magnesium atom is ____?
Atomic number of Magnesium (Mg) = 12.
Electronic configuration:
K shell: 2, L shell: 8, M shell: 2 (remaining 12 − 2 − 8 = 2)
The outermost shell (M shell) has 2 electrons.
Number of valence electrons in magnesium = 2
Since magnesium has 2 valence electrons, it tends to lose these 2 electrons to form Mg2+ ion, giving it a valency of 2.
If Z = 3, what would be the valency of the element? Also name the element.
Z = 3 means the element has 3 protons and 3 electrons (neutral atom).
Electronic configuration:
K shell: 2 electrons, L shell: 1 electron (remaining 3 − 2 = 1)
The outermost shell (L shell) has 1 electron.
This element has 1 valence electron. It will tend to lose this 1 electron to achieve the stable noble gas configuration of helium (2 electrons in K shell).
Valency = 1
The element is Lithium (Li), atomic number 3, located in Group 1 (alkali metals) of the periodic table.
Composition of the nuclei of two atomic species X and Y are given as: X — protons = 6, neutrons = 6; Y — protons = 6, neutrons = 8. Give the mass number of X and Y. What is the relation between the two species?
For species X: Protons = 6, Neutrons = 6
For species Y: Protons = 6, Neutrons = 8
Relation between X and Y:
Both X and Y have the same number of protons (atomic number = 6), which means they are atoms of the same element — Carbon (C). But they have different mass numbers (12 and 14 respectively).
Therefore, X (12C) and Y (14C) are isotopes of carbon.
(14C is radioactive carbon-14, used in radiocarbon dating.)
For the element with atomic number 16, give: (i) electronic configuration (ii) number of valence electrons (iii) valency.
Atomic number 16 — this is Sulphur (S).
(i) Electronic configuration:
K shell: 2, L shell: 8, M shell: 6 (16 − 2 − 8 = 6)
(ii) Number of valence electrons:
The outermost (M) shell has 6 electrons. So valence electrons = 6.
(iii) Valency:
Sulphur has 6 valence electrons. To complete its octet (8 electrons), it needs to gain 2 more electrons.
Sulphur forms S2− ion (gains 2 electrons), or shares 2 electrons in covalent bonds. Its valency is 2.
What are isotopes? Give two examples of isotopes of an element and state any two uses.
Isotopes are atoms of the same element that have the same atomic number (same number of protons) but different mass numbers (different number of neutrons).
Since they have the same number of electrons, they have the same chemical properties but different physical properties (like density and radioactivity).
Examples:
Isotopes of Hydrogen:
11H (Protium) — 1 proton, 0 neutrons (ordinary hydrogen)
21H (Deuterium, D) — 1 proton, 1 neutron
31H (Tritium, T) — 1 proton, 2 neutrons (radioactive)
Isotopes of Carbon:
126C — 6 protons, 6 neutrons (most abundant)
146C — 6 protons, 8 neutrons (radioactive)
Uses of isotopes:
1. 235U (uranium-235) is used as fuel in nuclear reactors and nuclear weapons.
2. 60Co (cobalt-60) is used in cancer treatment (radiotherapy) to kill cancer cells.
3. 14C (carbon-14) is used in radiocarbon dating to determine the age of fossils and archaeological specimens.
4. 131I (iodine-131) is used in diagnosis and treatment of thyroid disorders.
What are isobars? Give an example. How are they different from isotopes?
Isobars are atoms of different elements that have the same mass number but different atomic numbers (different number of protons and neutrons).
Example: Calcium (40Ca, Z=20) and Argon (40Ar, Z=18) are isobars. Both have mass number = 40, but calcium has 20 protons and 20 neutrons, while argon has 18 protons and 22 neutrons.
Difference between Isobars and Isotopes:
Isotopes: Same element (same Z), same number of protons, different mass number, different number of neutrons. Same chemical properties, different physical properties.
Isobars: Different elements (different Z), different number of protons, same mass number, different chemical properties (different electronic configurations).
What is the atomic number and mass number of an element? What information do they give us?
Atomic Number (Z):
The atomic number of an element is the number of protons present in the nucleus of its atom.
The atomic number uniquely identifies an element. All atoms of the same element have the same atomic number.
Mass Number (A):
The mass number of an atom is the total number of protons and neutrons (nucleons) in its nucleus.
Example: For sodium (2311Na): Z = 11 (11 protons, 11 electrons), A = 23, N = 23 − 11 = 12 neutrons. Electronic configuration: 2, 8, 1.
State the drawbacks of Rutherford's model of the atom.
Drawbacks of Rutherford's Model:
(i) Could not explain atomic stability: According to classical electromagnetic theory, a charged particle (electron) moving in a circular orbit must continuously emit electromagnetic radiation (because it is accelerating toward the nucleus). This would cause the electron to lose energy, spiral inward, and eventually fall into the nucleus. The atom would collapse in about 10−8 seconds. However, atoms are stable, which Rutherford's model could not explain.
(ii) Could not explain line spectra: If the electron spirals into the nucleus while losing energy continuously, it would emit a continuous spectrum of light. But experiments show that atoms emit specific discrete lines (line spectrum), not a continuous spectrum. Rutherford's model had no explanation for this.
(iii) No information about electron arrangement: Rutherford's model did not describe where electrons were located or how they were arranged around the nucleus.
What is meant by valency? How is valency determined from the electronic configuration?
Valency is the combining capacity of an atom of an element. It represents the number of electrons an atom can lose, gain, or share to achieve a stable (noble gas) electronic configuration (usually 8 electrons in the outermost shell, or 2 for the first period).
Determination from electronic configuration:
Rule 1: If the number of valence electrons is 1, 2, or 3 (metals), the valency = number of valence electrons (tendency to lose these electrons).
Rule 2: If the number of valence electrons is 4, 5, 6, or 7 (non-metals), the valency = 8 − number of valence electrons (tendency to gain electrons).
Rule 3: If the outermost shell is completely filled (8 electrons, or 2 for K shell), valency = 0 (noble gases, chemically inert).
Examples:
Na (Z=11): config 2,8,1 → 1 valence electron → Valency = 1
Mg (Z=12): config 2,8,2 → 2 valence electrons → Valency = 2
N (Z=7): config 2,5 → 5 valence electrons → Valency = 8−5 = 3
O (Z=8): config 2,6 → 6 valence electrons → Valency = 8−6 = 2
Ar (Z=18): config 2,8,8 → 8 valence electrons → Valency = 0
Write the electronic configurations of the following elements: (i) Sodium (Z=11), (ii) Chlorine (Z=17), (iii) Calcium (Z=20), (iv) Neon (Z=10).
Shell filling rule: K(max 2), L(max 8), M(max 18), N(max 32). Fill in order.
(i) Sodium (Na), Z=11:
Valence electrons = 1, Valency = 1
(ii) Chlorine (Cl), Z=17:
Valence electrons = 7, Valency = 8−7 = 1
(iii) Calcium (Ca), Z=20:
Valence electrons = 2, Valency = 2
(iv) Neon (Ne), Z=10:
Valence electrons = 8 (complete octet), Valency = 0 (noble gas)
How many neutrons are there in the following atoms? (i) 12C (ii) 23Na (iii) 197Au (iv) 238U
Formula: Number of neutrons (N) = Mass number (A) − Atomic number (Z)
(i) 12C (Carbon-12): A=12, Z=6
(ii) 23Na (Sodium-23): A=23, Z=11
(iii) 197Au (Gold-197): A=197, Z=79
(iv) 238U (Uranium-238): A=238, Z=92
Compare the three atomic models — Thomson's, Rutherford's, and Bohr's — and state the limitation of each.
Thomson's Model (1904) — "Plum Pudding Model":
Description: Atom is a uniform sphere of positive charge with electrons embedded in it like plums in a pudding.
Limitation: Could not explain the results of Rutherford's alpha scattering experiment (the deflection of alpha particles back from the gold foil). It had no nucleus.
Rutherford's Model (1911) — "Nuclear Model":
Description: Atom has a tiny, dense, positively charged nucleus at the centre with electrons revolving around it in circular orbits (like planets around the sun).
Limitation: (i) Could not explain atomic stability — accelerating electrons should radiate energy and spiral into the nucleus. (ii) Could not explain the discrete line spectra of atoms.
Bohr's Model (1913) — "Planetary Model with Energy Levels":
Description: Electrons revolve in fixed circular orbits of definite energy (energy shells). Electrons do not radiate energy in these orbits. Energy is emitted or absorbed when electrons jump between shells.
Limitation: (i) Works well only for hydrogen (one electron) but fails for multi-electron atoms. (ii) Does not explain the fine structure of spectral lines. (iii) Does not account for the wave nature of electrons (later described by quantum mechanics).
An element has atomic number 15. Find: (a) its electronic configuration, (b) the number of valence electrons, (c) the valency, (d) identify the element.
(a) Electronic configuration (Z = 15):
(b) Number of valence electrons:
The outermost (M) shell has 5 valence electrons.
(c) Valency:
(The element needs to gain 3 electrons to complete its octet, or can share 3 electrons.)
(d) The element:
Element with atomic number 15 is Phosphorus (P). It is a non-metal in Group 15 (Group VA) of the periodic table. It forms compounds with valency 3 (like PH3) and also 5.
What is meant by the maximum capacity of a shell? What is the formula to find the maximum electrons a shell can hold?
The maximum capacity of a shell refers to the maximum number of electrons that can be accommodated in that shell.
The formula to calculate the maximum number of electrons in the nth shell is:
Where n = shell number (principal quantum number).
Applying the formula:
K shell (n=1): Maximum electrons = 2 × 12 = 2
L shell (n=2): Maximum electrons = 2 × 22 = 2 × 4 = 8
M shell (n=3): Maximum electrons = 2 × 32 = 2 × 9 = 18
N shell (n=4): Maximum electrons = 2 × 42 = 2 × 16 = 32
Important note: The outermost (valence) shell can hold a maximum of only 8 electrons, regardless of the formula above, as per the octet rule used in Class 9 NCERT.
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