# 708.docx - 2.1(a What is an isotope Answer The atoms of an...

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2.1 (a) What is an isotope? Answer: The atoms of an element which have the same atomic number but different atomic masses or mass number. OR The atoms of elements which have the same number of protons and different number of neutrons are called isotopes. Isotopes have same chemical properties (because they have electronic configuration) and different physical properties. Example: Carbon has three isotopes, C-12, C-13, C-14. (b) Why are the atomic weights of the elements not integers? Cite two reasons. Answer: The atomic weights of the elements not integers: 1, The atomic weight of elements is usually taken as average atomic mass of all the possible isotopes pf that element. 2, In most of chemical reactions the sample of an element taken is so large that it contain all possible isomers of that element that is why we take average atomic mass. 2.2 Cite the difference between atomic mass and atomic weight . Answer: Atomic mass is the mass of an individual atom, whereas atomic weight is the average (weighted) of the atomic masses of an atom's naturally occurring isotopes. 2.3(a) How many grams are there in 1 amu of a material? Solution: Since, 1 amu = 1 carbon atom 12 ……………….. (1) And 1 mole ; 6.022 10 23 carbonatoms = 12 grams 1 carbon atom= 12 6.022 10 23 g = 1.9926 10 23 …………….. (2) Putting mass of one carbon atom in equation (1) 1 amu = 1.9926 10 23 12 g =0.6605× 10 23 g = 1.661× 10 24 g 2.3 (b) Mole, in the context of this book, is taken in units of gram-mole. On this basis, how many atoms are here in a pound-mole of a substance? (b) Since there are 453.6 g/lbm, 1 lb - mol = (453.6 g/lb m) (6.022 × 10 23 atoms/g - mol) = 2.73 × 10 26 atoms/lb-mol 2.4 (a) Cite two important quantum-mechanical concepts associated with the Bohr model of the atom. Answer: Two important quantum-mechanical concepts associated with the Bohr model of the atom are (1) that electrons are particles moving in discrete orbitals, and (2) electron energy is quantized into shells.
(b) Cite two important additional refinements that resulted from the wave-mechanical atomic model. Answer: Two important refinements resulting from the wave-mechanical atomic model are (1) that electron position is described in terms of a probability distribution, and (2) electron energy is quantized into both shells and subshells—each electron is characterized by four quantum numbers.

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