CHEM107_Exam3_Study Guide Answer

CHEM107_Exam3_Study Guide Answer - Texas A M University...

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Texas A & M University Department of Chemistry CHEM107 Spring 2011 Instructor: Masud Monwar, Ph.D. Study Guide for Exam 3 Posted on: 30 th March, 2011 Chapter 6: The Periodic Table and Atomic Structure 1. Why the members of the same group have similar chemical properties? Answer: The chemical properties of an element depend on its valence shell electrons. As the member of the same group has same number of valence electrons, they have similar chemical properties. 2. a. Why the atomic size increases down the group? Answer: As we are moving down the group, more and more energy shells added to the atom, and as energy shells are responsible for atomic size, the size of the atom increases down the group. b. Why the atomic size decreases across the period from left to right for main group elements? Answer: As we move across a period, the atomic number increases. The electrons enter into the same energy shell, but as the number of protons increases in the nucleus, it attracts the electrons with more power/strength and shrinks/contracts the atomic size . c. Compare the size of the following atoms: i) Li, C, O Answer: Li > C > O ii) Li, Mg, Rb Answer: Rb > Mg > Li iii) Li, P, S, Ca, Sr Answer: Sr > Ca > P > S > Li 3. a. Compare the size of the following pairs and briefly explain your answer: i) Li, Li + Answer: Li > Li + ; Li atom loses one electron (2s 1 ) and forms Li + ion. It therefore loses 2 nd energy shell and decreases in size. Further, as the nucleus have more protons than the electrons, it attracts electrons with more strength, and that causes a further contraction in size of Li + ion. © Masud Monwar 2011
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ii) Cl, Cl - Answer: Cl - > Cl ; Cl gains one electron to fill the octet in its outer shell and forms Cl- ion. Size of Cl- increases because…….a) electron crowding increases as the outer shell is filled and to reduce repulsions among the electrons the size increases and b) as number of protons are less than number of electrons in Cl-, attraction of nucleus on electron decreases, causing further increase in size. b. What is the trend in sizes of the ions N 3- , O 2- and F - ? Briefly explain your answer. Answer: N 3- > O 2- > F - . This trend follows from the ionic size trend : N > O > F. In addition, N, O and F gain 3, 2 and 1 electron respectively. As more electrons are gained, more is the electron-electron repulsion which causes the increase in size. 4. a. What is ionization energy? Answer: Ionization energy is the energy required to remove an electron from an atom in the gas phase. b. Consider the following data of ionization energy of Mg: Mg(g) ------- Mg + (g) + e ; IE 1 = 738 kJ/mol Mg + (g) ------- Mg 2+ (g) + e ; IE 2 = 1451 kJ/mol Mg 2+ (g) ------- Mg 3+ (g) + e ; IE 3 = 7733 kJ/mol i) Why the ionization energy increases with successive removal of electrons? Answer: As the electrons are removed successively, the number of protons in the nucleus increases compared to the number of electrons. So nucleus attracts the electrons with greater strength, and it becomes progressively difficult to remove electrons and requires more energy.
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