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ChemistryClass 9CBSE

Why are noble gases generally unreactive?

Noble gases (Group 18 elements) are generally unreactive because they have a complete outermost electron shell (an octet, or a duet for Helium). This stable electron configuration makes them chemically inert, as they have little tendency to gain, lose, or share electrons to form chemical bonds.

ChemistryClass 9CBSE

Discuss the role of the nucleus in determining the identity and stability of an atom.

The nucleus determines the identity of an atom through its number of protons (atomic number), which uniquely defines the element. The number of neutrons in the nucleus influences the stability of the atom; certain proton-to-neutron ratios lead to stable isotopes, while others result in unstable, radioactive isotopes. The strong nuclear force within the nucleus holds the protons and neutrons together, preventing the nucleus from flying apart due to electrostatic repulsion between protons.

ChemistryClass 9CBSE

What would be the charge of an atom that has 12 protons, 12 neutrons, and 10 electrons? Identify the element.

The element is Magnesium (Mg) because it has 12 protons (atomic number 12). The charge would be +2, because it has 12 positive charges (protons) and 10 negative charges (electrons), resulting in a net charge of 12 - 10 = +2.

ChemistryClass 9CBSE

An element has an atomic number of 19. Predict its valency and explain your reasoning.

For an element with atomic number 19 (Potassium), the electron configuration is 2, 8, 8, 1. It has 1 valence electron. To achieve a stable octet, it is much easier for this atom to lose 1 electron than to gain 7. Therefore, its valency is 1.

ChemistryClass 9CBSE

Explain the concept of 'electron cloud' in the modern atomic model, contrasting it with Bohr's fixed orbits.

In the modern atomic model (quantum mechanical model), the 'electron cloud' represents the region around the nucleus where an electron is most likely to be found. It's a probabilistic model, meaning we cannot pinpoint an electron's exact location but can describe its probable distribution. This contrasts with Bohr's model, which depicted electrons orbiting the nucleus in fixed, well-defined circular paths or orbits.

ChemistryClass 9CBSE

What was Dalton's atomic theory, and which of its postulates have since been proven incorrect?

Dalton's atomic theory stated that: 1. All matter is composed of indivisible atoms. 2. Atoms of a given element are identical in mass and properties. 3. Compounds are formed by a combination of two or more different kinds of atoms. 4. A chemical reaction is a rearrangement of atoms. The postulates proven incorrect are: 1. Atoms are indivisible (they can be divided into subatomic particles). 2. Atoms of a given element are identical in mass (isotopes exist, which have different masses).

ChemistryClass 9CBSE

Why do different isotopes of the same element exhibit similar chemical properties but slightly different physical properties?

Isotopes have similar chemical properties because chemical reactions primarily involve the valence electrons, and isotopes of the same element have the same number of protons and thus the same electron configuration (and number of valence electrons). They have slightly different physical properties (like density or melting/boiling points) because their different number of neutrons leads to different atomic masses.

ChemistryClass 9CBSE

If an element has an atomic number of 15, how many electron shells would be occupied in its neutral atom, and how many electrons would be in its outermost shell?

For an element with atomic number 15 (Phosphorus), the electron configuration is 2, 8, 5. Therefore, three electron shells would be occupied, and there would be 5 electrons in its outermost shell.

ChemistryClass 9CBSE

What is the main difference between an atom and an ion? Provide an example of each.

An atom is an electrically neutral particle, meaning it has an equal number of protons and electrons. For example, a neutral Oxygen atom (O) has 8 protons and 8 electrons. An ion is an atom or molecule that has gained or lost one or more electrons, resulting in a net electrical charge. For example, an Oxide ion (O2-) has 8 protons and 10 electrons (gained 2 electrons), and a Sodium ion (Na+) has 11 protons and 10 electrons (lost 1 electron).

ChemistryClass 9CBSE

How does the concept of 'octet rule' help explain chemical bonding?

The octet rule states that atoms tend to gain, lose, or share electrons in order to achieve eight electrons in their outermost electron shell, making them stable like noble gases. This tendency drives the formation of chemical bonds (ionic or covalent) as atoms strive to reach this stable electron configuration, thus explaining why and how elements combine.

ChemistryClass 9CBSE

Describe the electron configuration of an atom of Sodium (Na) with atomic number 11.

For Sodium (atomic number 11), the electron configuration is 2, 8, 1. This means there are 2 electrons in the first shell, 8 electrons in the second shell, and 1 electron in the outermost (valence) shell.

ChemistryClass 9CBSE

What are isobaric atoms? Provide an example.

Isobars are atoms of different elements that have the same mass number (same total number of protons and neutrons) but different atomic numbers (different number of protons). For example, Argon-40 (18 protons, 22 neutrons) and Calcium-40 (20 protons, 20 neutrons) are isobars.

ChemistryClass 9CBSE

Explain how the concept of 'quantization of energy' is applied in Bohr's model of the atom.

In Bohr's model, the 'quantization of energy' means that electrons can only exist in specific, discrete energy levels or orbits around the nucleus. They cannot occupy energy levels in between these allowed orbits. Electrons can move from one energy level to another by absorbing or emitting a specific quantum of energy (a photon) corresponding to the energy difference between the levels.

ChemistryClass 9CBSE

If an atom has an atomic number of 11 and a mass number of 23, how many protons, neutrons, and electrons does it have in its neutral state?

Protons = Atomic Number = 11. Electrons = Protons (in a neutral atom) = 11.Neutrons = Mass Number - Atomic Number = 23 - 11 = 12.

ChemistryClass 9CBSE

Why is the atomic mass unit (amu) used to express the mass of subatomic particles and atoms instead of grams?

The atomic mass unit (amu) is used because the masses of individual atoms and subatomic particles are extremely small when expressed in grams. Using amu provides more manageable and convenient numbers for calculations and comparisons at the atomic scale. 1 amu is defined as 1/12th the mass of a carbon-12 atom.

ChemistryClass 9CBSE

Describe the characteristics of alpha particles, beta particles, and gamma rays in terms of their composition, charge, and penetrating power.

Alpha particles (α) are composed of two protons and two neutrons (a helium nucleus), have a +2 charge, and low penetrating power. Beta particles (β) are high-energy electrons, have a -1 charge, and moderate penetrating power. Gamma rays (γ) are electromagnetic radiation (photons), have no charge, and very high penetrating power.

ChemistryClass 9CBSE

What is the significance of the number of valence electrons in determining an element's chemical properties?

The number of valence electrons (electrons in the outermost shell) is crucial because these are the electrons involved in chemical bonding. Atoms tend to gain, lose, or share valence electrons to achieve a stable electron configuration, typically an octet. This behavior directly dictates an element's reactivity, the types of bonds it forms, and its chemical properties.

ChemistryClass 9CBSE

How did J.J. Thomson's 'plum pudding' model describe the atom? What was its major flaw?

Thomson's 'plum pudding' model proposed that the atom was a sphere of uniformly distributed positive charge, with negatively charged electrons (like 'plums') embedded within it. Its major flaw was its inability to explain the results of Rutherford's gold foil experiment, particularly the large deflections of alpha particles, which indicated a concentrated positive nucleus.

ChemistryClass 9CBSE

Differentiate between atomic mass and mass number. Why are they usually different?

Mass number (A) is the total count of protons and neutrons in a specific atom, always a whole number. Atomic mass (or atomic weight) is the weighted average of the masses of all naturally occurring isotopes of an element, often a decimal number. They differ because atomic mass accounts for the varying abundances of isotopes and the slight mass defect from nuclear binding energy, whereas mass number is just a count of nucleons for a single isotope.

ChemistryClass 9CBSE

Explain why atoms are generally electrically neutral. What happens if an atom gains or loses electrons?

Atoms are generally electrically neutral because they contain an equal number of positively charged protons in the nucleus and negatively charged electrons orbiting the nucleus, balancing out the overall charge. If an atom gains electrons, it becomes a negatively charged ion (anion). If an atom loses electrons, it becomes a positively charged ion (cation).

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