1. (i) If the positive charge on the clay is less than the total negative charge of the beads, the atom will not be neutral. It will have a net negative charge.
2. (ii) If the clay itself carries a bit of negative charge, the total negative charge increases further. Hence, the model will again not represent a neutral atom.

1. α-rays are made up of helium ions with +2 charge.
2. β-rays are negatively charged particles and are fast moving electrons.
3. λ-rays are neutral but are high-frequency electromagnetic radiation.

1. The discovery of radioactivity showed that certain elements emit invisible radiations and particles spontaneously.
2. This proved that atoms are not indivisible as previously believed. Instead, atoms contain smaller subatomic components that can be emitted during radioactive decay, thereby contradicting the earlier concept of atoms being the smallest and indivisible units of matter.

1. Protium ₁H¹ — 1 proton, 0 neutron. Mass number: 1.
2. Deuterium ₁H² — 1 proton, 1 neutron. Mass number: 2.
3. Tritium ₁H³ — 1 proton, 2 neutrons. Mass number: 3.

1. According to Rutherford's atomic model, the positive charge and most of the mass of an atom are concentrated in a very small, dense central region called the nucleus.
2. This conclusion was based on observations from the gold foil experiment, where the scattering of alpha particles indicated a compact, heavy, positively charged centre in the atom.

1. Electrons do not fall into the nucleus because they revolve in fixed energy levels (shells) around the nucleus.
2. According to Bohr's Model of the Atom, each electron has a definite amount of energy. While moving in these orbits, electrons do not lose energy, so they do not spiral into the nucleus despite the electrostatic attraction between electrons and protons.

1. In 1911, Geiger and Marsden performed the gold foil experiment to test Thomson's atomic model.
2. They set up an experiment in which a narrow beam of positively charged alpha particles was directed towards a very thin sheet of gold foil. The paths of the particles after passing through or getting deflected were observed carefully.

1. Let percentage of X-16 = x. So percentage of X-18 = (100−x).
2. Average atomic mass: (x×16)/100 + [(100−x)×18]/100 = 16.2.
3. 16x + 1800 − 18x = 1620 ⇒ −2x = −180 ⇒ x = 90.

1. Number of protons = 20. Mass number = 41.
2. Number of neutrons = Mass number − Number of protons = 41 − 20 = 21.

1. In Bohr's model of an atom with three shells, the three stationary orbits are designated as K-shell (nearest to the nucleus), L-shell, and M-shell.
2. The atom with three shells can accommodate a maximum of 2, 8, and 18 electrons respectively (using the 2n² formula for n=1,2,3).

1. Total α-particles used = 1 mole = 6.022 × 10²³ particles.
2. Percentage deflected > 50° = 1% (given). Percentage deflected < 50° = 100−1 = 99%.
3. Actual number = (99/100) × 6.022×10²³ = 596.178×10²³/100 = 5.96 × 10²³ particles.

1. (a) Nucleus
2. (b) Electronic configuration
3. (c) Atomic number, mass number

1. The mass and charge of the nucleus of an atom of any element are found to be whole number multiples of the mass and charge of a proton.
2. Hence, protons are constituents of all atoms.

1. A simple arithmetic average atomic mass is calculated by adding the masses of all isotopes of an element and dividing by the number of isotopes, without considering how commonly each isotope occurs.
2. In contrast, a weighted average atomic mass takes isotopic abundance into account. Each isotope's mass is multiplied by its fractional abundance, and the results are added.
3. This gives a more accurate value that reflects the actual natural occurrence of isotopes in an element.

1. Rutherford's model failed to explain atomic stability because, according to classical physics, an electron moving in a circular orbit around the nucleus should continuously radiate energy.
2. As a result, it would lose energy, slow down, and eventually spiral into the nucleus. This would make the atom unstable, contradicting the fact that atoms are stable.
3. Bohr improved this model by proposing that electrons revolve in fixed energy levels (stationary orbits) where they do not radiate energy. Radiation is emitted or absorbed only when an electron jumps between these fixed orbits. Hence, Bohr's model successfully explained atomic stability.

1. (i) Sample A has more protons (17) than electrons (16). Hence, it is a cation.
2. (ii) Sample B (mass number = 18+19 = 37) and Sample C (mass number = 17+20 = 37) have the same mass number but different atomic numbers (18 and 17 respectively). Hence, B and C are a pair of isobars.
3. (iii) Samples C and D have the same atomic number (17) but different mass numbers (37 and 34). Hence, C and D are a pair of isotopes.

1. The number of valence electrons determines whether an atom will gain or lose electrons to achieve a stable octet.
2. Atoms having fewer than four valence electrons generally tend to lose these electrons to attain stability.
3. Atoms having more than four valence electrons usually tend to gain electrons to complete their octet. This behaviour influences the valency of the element and its ability to form chemical bonds with other atoms.

1. (i) The Mg²⁺ ion is shown in the given figure.
2. (ii) Electronic configuration of Mg²⁺ ion = 2, 8. Electronic configuration of ₁₂Mg atom = 2, 8, 2.
3. (iii) Number of protons in Mg atom = 2 + 8 + 2 = 12 (sum of electrons in each shell of the neutral atom, since protons = electrons for a neutral atom).

1. John Dalton described atoms as indivisible particles.
2. J.J. Thomson proposed the 'plum pudding' model in which electrons are embedded in a positively charged sphere.
3. Ernest Rutherford proposed that atoms have a small, dense, positively charged nucleus at the centre.
4. Bohr's Model of the Atom stated that electrons move in fixed orbits around the nucleus, each with definite energy.

1. The atomic number of an element is defined by the number of protons present in the nucleus of its atoms.
2. This number is unique for each element and is used to identify it. It also determines the position of the element in the periodic table.
3. Since protons carry a positive charge, the atomic number represents the total positive charge in the nucleus, making it a fundamental property for distinguishing different elements.

1. Atomic number = 26. Mass number (nucleons) = 56.
2. Number of protons = Atomic number = 26. Number of electrons = 26 (for a neutral atom) = 26.
3. Number of neutrons = Mass number − Atomic number = 56 − 26 = 30.

1. (i) James Chadwick discovered neutrons in 1932. Neutrons are neutral particles (no charge) and they are found in the nucleus of an atom.
2. (ii) The element shown in the figure is Nitrogen (electronic configuration 2,5).
3. (iii) Phosphorus has atomic number 15. So its K shell has 2 electrons, L shell has 8 electrons, and M shell has 5 electrons. Hence the electronic configuration of phosphorus is 2, 8, 5.

OR

1. The maximum number of electrons that can be accommodated in a shell is given by 2n². For the M shell, n=3, hence M can accommodate a maximum of 18 electrons.

1. (i) James Chadwick discovered the neutron in 1932. His discovery helped explain the previously unexplained mass of atoms and completed the understanding of the basic subatomic particles.
2. (ii) Neutrons are located in the nucleus of an atom along with protons. They do not carry any electric charge but contribute significantly to the atomic mass.
3. (iii) The discovery of neutrons was important because it explained why atomic masses were greater than the number of protons. It also enabled advancements in nuclear energy, nuclear reactions, and the development of atomic science.

OR

1. Neutrons contribute to the mass of an atom because they have nearly the same mass as protons. Their presence increases atomic mass without affecting the atom's charge, since they are electrically neutral.

1. (i) Isotopes are atoms of the same element that have the same atomic number but different mass numbers due to varying numbers of neutrons in their nuclei.
2. (ii) Isotopes have similar chemical properties because they have the same number of electrons and identical electronic configurations, which determine chemical behaviour.
3. (iii) Carbon-14 is an isotope used in archaeology to determine the age of fossils. It helps scientists estimate how old organic materials are based on radioactive decay.

OR

1. Isotopes differ in physical properties because they have different masses due to varying numbers of neutrons. This affects properties like density, boiling point, and melting point.

1. (a) Electrons
2. (b) Nucleus
3. (c) Atomic number, mass number
4. (d) Anode
5. (e) Nucleus

1. (i) In the gold foil experiment, the projectiles used were alpha particles. These are very small, positively charged particles emitted from certain radioactive substances. Each alpha particle consists of two protons and two neutrons and was directed toward the gold foil for observation.
2. (ii) The gold foil experiment was conducted by Geiger and Marsden. The experiment was carried out under the guidance and supervision of Ernest Rutherford, who designed and interpreted the results of the experiment to study the structure of the atom.
3. (iii) According to Thomson's plum pudding model, the atom was considered to have positive charge spread uniformly throughout it. Therefore, it was expected that the alpha particles would pass straight through the gold foil with little or no deflection, and only a few particles might show slight deviation.

1. (i) James Chadwick is the scientist responsible for resolving the atomic mass puzzle. He discovered the neutron in 1932, which helped explain the extra mass present in atoms without affecting their charge.
2. (ii) Atomic mass cannot be determined only by the number of protons because there is another subatomic particle in the nucleus called the neutron. Neutrons contribute significantly to the mass of the atom but do not affect its charge. For example, helium has more mass than hydrogen even though proton difference alone cannot explain it.
3. (iii) The neutron has a neutral electrical charge, meaning it carries no charge at all. However, its mass is nearly equal to that of a proton. Because it has no charge, it cannot be detected easily through charge-based experiments, even though it plays an important role in contributing to atomic mass.

1. (i) Atomic number: It is the number of protons present inside the nucleus of the atom. It is represented by (Z). E.g. Atomic number of hydrogen Z = 1, because the number of protons in a hydrogen atom is 1.
2. (ii) Mass number: It is the total number of protons and neutrons (nucleons) present inside the nucleus of an atom. Represented by A. A = Z + N where Z is the atomic number and N is the number of neutrons. E.g. Mass number of carbon is 12 (6 protons + 6 neutrons).
3. (iii) Isotopes: Atoms of the same element having the same atomic number (Z) but different mass numbers/atomic mass (A). E.g. Carbon has 2 isotopes, ¹²₆C and ¹⁴₆C.
4. (iv) Isobars: Atoms of different elements having the same mass number (A) but a different atomic number (Z). E.g. Calcium (Z=20) and Argon (Z=18) — both have mass number 40, but different numbers of protons; the number of neutrons in the atoms of this pair is different.

Two uses of isotopes

1. An isotope of uranium is used as a fuel in nuclear reactors.
2. An isotope of cobalt is used in the treatment of cancer.

Rutherford's α-particles scattering Experiment

1. In 1911, Rutherford performed the gold foil experiment. He bombarded a stream of α-particles on a gold foil, a thin sheet which was 0.00006 cm thick, in an evacuated chamber.
2. An α-particle is a positively charged helium ion (He²⁺). A simplified picture of this experiment is shown below.

Gold foil experiment apparatus: radioactive source, lead plate, thin gold foil, and movable zinc sulphide screen

Observation

1. Most of the α-particles passed straight through the foil without any deflection.
2. A few α-particles were deflected through a small angle and few through larger angles.
3. The number of α-particles which bounced back was very small.

Paths of alpha particles: passing straight, large deflection, and turning back

Conclusion

1. Most of the space inside of an atom is empty.
2. The heavy positively charged 'core' is present at the centre of the atom, named the nucleus.
3. The volume of the nucleus is very small in comparison to the total volume of the atom.

Structure of atom

1. On the basis of the gold foil experiment, Rutherford concluded that an atom consists of a nucleus which has positive charge and is surrounded by electrons moving around the nucleus.
2. The number of electrons and protons are equal, and the entire mass of the atom is concentrated at its nucleus. He compared this model with the solar system, where the sun (nucleus) is at the centre and planets (electrons) revolve around it.

Rutherford's model showing electron spiral path collapsing into the nucleus

Drawbacks in the Rutherford's model

1. According to classical electromagnetic theory, a moving charged particle such as an electron, under the influence of attractive force, loses energy continuously in the form of radiation. As a result, the electron should lose energy and move in even smaller orbits, ultimately falling into the nucleus. But this collapse does not occur in reality.
2. Rutherford did not specify the number of orbits and the number of electrons in each orbit.

1. (i) A major limitation of Rutherford's model was that it could not explain why atoms are stable. According to classical physics, an electron revolving around the nucleus should continuously lose energy and eventually spiral into the nucleus, which would make the atom collapse. However, atoms are actually stable in nature, which made this idea incorrect.
2. (ii) In the gold foil experiment, most alpha particles passed straight through the foil, but some were deflected at large angles and a few even bounced back. This showed that most of the atom is empty space, while a small, dense, positively charged region exists at the centre, leading Rutherford to propose the nuclear structure of the atom.
3. (iii) In Thomson's model, the positive charge was thought to be uniformly spread throughout the entire atom like a "pudding-like" sphere. In Rutherford's model, the positive charge is concentrated in a very small and dense central region called the nucleus, with electrons revolving around it outside this central core.

1. (i-a) As most of the α-particles passed straight through the gold foil, this proved that most of the space inside the atom is empty.
2. (i-b) A few of the α-particles, which are positively charged, got deflected due to the positive charge of the nucleus — confirming the nucleus is positively charged.
3. (ii-a) Number of neutrons = mass number − atomic number = 32 − 16 = 16.
4. (ii-b) Electronic configuration: K=2, L=8, M=6. Hence, the number of electrons in the outermost shell (M) is 6.
5. (iii) According to Rutherford's model of an atom, positively charged protons are present in the nucleus of an atom.