Unit-3 Atomic Structure
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Unit-3 Atomic Structure
Answer: Dalton gave the atomic theory in 1808; the other scientists worked on subatomic particles and atomic models.
Answer: Dalton stated that atoms of different elements combine in simple whole-number ratios to form compounds.
Answer: Dalton assumed all atoms of an element are identical, but isotopes are atoms of the same element having different masses.
Answer: J.J. Thomson discovered the electron in 1897 while studying cathode rays in a discharge tube.
Answer: Deflection by electric and magnetic fields showed cathode rays carry a negative charge, so they are streams of electrons.
Answer: E. Goldstein discovered canal rays in 1886 while studying the positive rays in a discharge tube.
Answer: Canal rays were found to be streams of positively charged particles, later identified as protons.
Answer: James Chadwick discovered the neutron in 1932.
Answer: Chadwick bombarded beryllium with alpha particles and detected a new neutral radiation, which he identified as neutrons.
Answer: A neutron has no charge and its mass is slightly greater than that of a proton.
Answer: An electron carries one unit of negative charge (-1).
Answer: An electron is about 1836 times lighter than a proton, so its relative mass is about 1/1836.
Answer: A proton carries one unit of positive charge (+1).
Answer: The atomic number equals the number of protons in the nucleus, so it is 11.
Answer: Mass number A = number of protons + number of neutrons (nucleons).
Answer: Equal positive charges (protons) and negative charges (electrons) cancel out, making the atom neutral.
Answer: Electrons revolve around the nucleus in fixed shells or energy levels.
Answer: The nucleus contains protons and neutrons, which are together called nucleons.
Answer: Electrons are found outside the nucleus in shells; protons and neutrons are inside the nucleus.
Answer: Since very few alpha particles were deflected, the nucleus is extremely small, occupying only a tiny fraction of the atom's volume.