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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).

ChemistryClass 9CBSE

Consider an atom with 17 protons, 18 neutrons, and 17 electrons. Identify the element, determine its atomic number and mass number, and state whether it is a neutral atom or an ion.

The element is Chlorine (Cl) because it has 17 protons (atomic number = 17). Its atomic number is 17. Its mass number is 17 (protons) + 18 (neutrons) = 35. Since the number of protons (17) equals the number of electrons (17), it is a neutral atom.

ChemistryClass 9CBSE

Draw a simple diagram representing the atomic structure of an oxygen atom (atomic number 8, mass number 16). Label the nucleus, protons, neutrons, and electrons.

The diagram should show a central nucleus with 8 protons and 8 neutrons. The first electron shell should contain 2 electrons, and the second electron shell should contain 6 electrons. All components should be clearly labeled.

ChemistryClass 9CBSE

Define valency and explain how it is related to the electron configuration of an atom.

Valency is the combining capacity of an element, indicating the number of electrons an atom will gain, lose, or share to achieve a stable electron configuration (usually a full outermost shell). It is determined by the number of electrons in the outermost shell (valence electrons); atoms tend to react to achieve an octet (or duet for hydrogen/helium).

ChemistryClass 9CBSE

Discuss the limitations of Rutherford's nuclear model of the atom that led to the development of Bohr's model.

Rutherford's model could not explain why electrons orbiting the nucleus didn't continuously lose energy and spiral into the nucleus, causing the atom to collapse. It also failed to explain the discrete line spectra observed for elements, suggesting electrons could only exist in specific energy levels.

ChemistryClass 9CBSE

How does the atomic number differ from the mass number? What information does each provide about an atom?

The atomic number (Z) represents the number of protons in an atom's nucleus and uniquely identifies an element. The mass number (A) represents the total number of protons and neutrons in an atom's nucleus. The atomic number tells us the element and the number of electrons in a neutral atom, while the mass number indicates the atom's total nuclear mass.

ChemistryClass 9CBSE

Explain the concept of electron shells (or energy levels) and how electrons are distributed within them according to Bohr's model.

Bohr's model proposes that electrons orbit the nucleus in specific, fixed energy levels called electron shells. Each shell can hold a maximum number of electrons: 2 in the first shell, 8 in the second, and 18 in the third (for the first few elements). Electrons fill the lowest energy shells first.

ChemistryClass 9CBSE

What is an isotope? Provide an example to illustrate your explanation.

Isotopes are atoms of the same element that have the same number of protons but different numbers of neutrons. This means they have the same atomic number but different mass numbers. For example, Carbon-12 (6 protons, 6 neutrons) and Carbon-14 (6 protons, 8 neutrons) are isotopes of carbon.

ChemistryClass 9CBSE

Describe Rutherford's Gold Foil Experiment and explain how its results led to the proposal of the nuclear model of the atom.

Rutherford's Gold Foil Experiment involved firing alpha particles at a thin sheet of gold foil. Most alpha particles passed straight through, suggesting atoms are mostly empty space. However, a small fraction were deflected at large angles, and some even bounced back, indicating the presence of a small, dense, positively charged nucleus at the center of the atom.

ChemistryClass 9CBSE

Explain the fundamental differences between protons, neutrons, and electrons in terms of their charge, mass, and location within an atom.

Protons have a positive charge (+1), a mass of approximately 1 atomic mass unit (amu), and are located in the nucleus. Neutrons have no charge (neutral), a mass of approximately 1 amu, and are also located in the nucleus. Electrons have a negative charge (-1), a very small mass (approximately 1/1836 amu), and orbit the nucleus in electron shells.

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