In a neutral atom, protons equal electrons, so there are 31 protons. Subtracting 31 protons from the mass number 70 gives 39 neutrons present in the nucleus of atom X.
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Element a is Lithium Li with 3 electrons, 1 valence electron, valency 1 and atomic number 3. Element b is Carbon C with 6 electrons, 4 valence electrons, valency 4 and atomic number 6. Element c is Phosphorus P with ...
Magnesium contains 12 protons, 12 neutrons and 12 electrons. The 12 electrons are distributed across shells in the electronic configuration 2, 8, 2, placing two in K-shell, eight in L-shell and two in M-shell.
The correct chronological order is (iv), (ii), (iii), (i). Dalton first proposed indivisible atoms, Thomson introduced embedded electrons, Rutherford discovered the central nucleus and Bohr explained stable quantized circular orbits with fixed energies.