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| 1. What are the three main parts of an atom and what do they do? |  |
Ans. An atom contains a nucleus (protons and neutrons) at its centre and electrons orbiting around it. Protons carry positive charge, neutrons are neutral, and electrons carry negative charge. The nucleus is incredibly dense and contains most of the atom's mass, while electrons occupy the space around it in electron shells or orbitals.
| 2. How is the Bohr model different from the modern atomic structure model? | |
Ans. Bohr's model describes electrons moving in fixed circular orbits around the nucleus like planets around the sun. The modern quantum mechanical model suggests electrons exist in probability regions called orbitals where they're likely to be found. Modern atomic structure better explains chemical bonding and electron behaviour observed in experiments, making it more accurate than Bohr's earlier representation.
| 3. Why do atoms have the same number of protons and electrons? | |
Ans. Atoms are electrically neutral because protons (positive charge) equal electrons (negative charge) in number. When this balance breaks, the atom becomes an ion-positively charged if electrons are lost, negatively charged if gained. This charge balance is fundamental to atomic stability and determines how atoms interact chemically with other atoms.
| 4. What's the difference between atomic number and mass number in atomic structure? | |
Ans. Atomic number equals the total number of protons in an atom's nucleus and defines the element's identity. Mass number is the sum of protons and neutrons combined. For example, carbon-12 has atomic number 6 (six protons) and mass number 12 (six protons plus six neutrons). These numbers distinguish isotopes and determine element classification.
| 5. How do electron shells and the octet rule connect to CBSE Class 9 chemistry concepts? | |
Ans. Electrons occupy shells around the nucleus, with each shell holding a maximum number of electrons. The outermost shell's electrons determine chemical reactivity; atoms seek stable configurations by gaining, losing, or sharing electrons to fill their outer shell completely. Understanding electron shell arrangement explains why certain atoms bond with others and forms the foundation for studying chemical reactions and molecular structure.
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