Bohr proposed that electrons revolve in certain allowed orbits with fixed energy, and they can jump between orbits by absorbing/emitting quanta of energy. This explained hydrogen spectrum. Electron cloud is quantum mechanical model; wave nature by de Broglie; uncertainty by Heisenberg.
Explanation:
Atomic radius decreases across a period due to increasing nuclear charge. Sodium (Na) is the first element in period 3, thus it has the largest atomic radius. Argon (noble gas) has the smallest (though noble gas radii are van der Waals and larger than covalent, but still trend is decreasing). For metals, Na > Mg > Al; non-metals further smaller.
Explanation:
Carbon's ability to form bonds with other carbon atoms (catenation) and its tetravalency (four valence electrons) allow it to form long chains, branches, and rings, leading to millions of organic compounds. The C-C bond is strong (bond energy ~348 kJ/mol). Silicon also shows catenation but to much lesser extent. Carbon also forms multiple bonds, enhancing diversity.
Explanation:
While Democritus proposed the philosophical concept of the atom in ancient Greece, John Dalton formulated the first scientifically based atomic theory in 1803. His theory stated that elements are composed of indivisible atoms, atoms of the same element are identical, and compounds form from atoms combining in fixed ratios.
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