20 important questions with answers for Class 9 Science Chapter: Atoms and Molecules.
1. What is an atom?
Answer: An atom is the smallest unit of an element that retains the properties of that element. It consists of a nucleus containing protons and neutrons, with electrons orbiting around the nucleus.
2. What are molecules?
Answer: Molecules are the smallest units of a chemical compound that can exist. They are made up of two or more atoms chemically bonded together.
3. Define the law of constant proportions.
Answer: The law of constant proportions states that in a chemical substance, the elements are always present in definite proportions by mass.
4. What is the atomic theory proposed by Dalton?
Answer: Dalton's atomic theory states that:
- All matter is made up of atoms.
- Atoms of the same element are identical in mass and properties.
- Atoms of different elements combine in fixed ratios to form compounds.
- Atoms cannot be created or destroyed in chemical reactions.
5. Define a molecule of an element and a molecule of a compound.
Answer: A molecule of an element contains two or more atoms of the same element chemically bonded together (e.g., O2, N2). A molecule of a compound contains two or more atoms of different elements chemically bonded together (e.g., H2O, CO2).
6. What is the significance of a chemical formula?
Answer: A chemical formula represents the composition of a molecule in terms of the symbols of the elements present and the ratio in which they combine. It provides information about the types and numbers of atoms in a molecule.
7. What is meant by the term "atomic mass"?
Answer: Atomic mass is the mass of an atom, typically expressed in atomic mass units (amu). It is approximately equal to the number of protons and neutrons in the atom.
8. How do you calculate the molecular mass of a compound?
Answer: The molecular mass of a compound is calculated by summing the atomic masses of all the atoms present in the molecule. For example, the molecular mass of H2O is (2 × 1) + 16 = 18 amu.
9. Define an ion.
Answer: An ion is an atom or group of atoms that has gained or lost one or more electrons, thus acquiring a positive or negative charge. Cations are positively charged ions, and anions are negatively charged ions.
10. What is a chemical equation?
Answer: A chemical equation is a symbolic representation of a chemical reaction, showing the reactants and products with their respective quantities. It uses chemical formulas and symbols to depict the reaction.
11. State the law of conservation of mass.
Answer: The law of conservation of mass states that mass is neither created nor destroyed in a chemical reaction. The total mass of the reactants equals the total mass of the products.
12. Differentiate between atoms and molecules with examples.
Answer: Atoms are the smallest units of an element (e.g., O, H). Molecules are groups of two or more atoms chemically bonded together (e.g., O2, H2O).
13. Explain Avogadro’s hypothesis.
Answer: Avogadro's hypothesis states that equal volumes of all gases, at the same temperature and pressure, contain an equal number of molecules.
14. What are isotopes? Give examples.
Answer: Isotopes are atoms of the same element with the same number of protons but different numbers of neutrons. Examples include Carbon-12 and Carbon-14.
15. Define molar mass.
Answer: Molar mass is the mass of one mole of a substance (element or compound). It is expressed in grams per mole (g/mol) and is numerically equal to the atomic or molecular mass in atomic mass units.
16. What is a mole? Explain its significance.
Answer: A mole is a unit that measures the amount of substance. It contains Avogadro's number (6.022 × 10^23) of atoms, molecules, or ions. It is significant in chemical calculations as it allows chemists to count entities by weighing them.
17. How do you balance a chemical equation?
Answer: To balance a chemical equation, adjust the coefficients of reactants and products so that the number of atoms of each element is the same on both sides of the equation.
18. What is the empirical formula?
Answer: The empirical formula is the simplest whole-number ratio of atoms of each element in a compound. It may or may not be the same as the molecular formula.
19. Define valency and give examples.
Answer: Valency is the combining capacity of an element, indicating the number of electrons an atom can lose, gain, or share to form a chemical bond. For example, the valency of hydrogen is 1, and the valency of oxygen is 2.
20. How do ions form ionic bonds? Give an example.
Answer: Ions form ionic bonds through the transfer of electrons from one atom to another, resulting in positively and negatively charged ions that attract each other. For example, in NaCl, sodium (Na) loses an electron to form Na+, and chlorine (Cl) gains an electron to form Cl-; the Na+ and Cl- ions attract each other to form the ionic bond in sodium chloride.
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