General Chemistry I Lecture Notes PDF

General Chemistry I Dr. Mahmoud A. A. Ibrahim Chemistry Department, Faculty of Science, Minia University Email: [email protected] 1 Mahmoud A. A. Ibrahim...

General Chemistry I Dr. Mahmoud A. A. Ibrahim Chemistry Department, Faculty of Science, Minia University Email: [email protected] 1 Mahmoud A. A. Ibrahim CH101  Lecture No. 4 Atomic properties (II) 2 Mahmoud A. A. Ibrahim CH101 5. Atomic properties Sizes of atoms and ions Atomic size varies in a relatively systematic way in the periodic table, as shown in the following figure: 3 Mahmoud A. A. Ibrahim CH101 5. Atomic properties Sizes of atoms and ions Atoms generally become larger from top to bottom in a group and they become smaller from left to right in a period. There are two principal factors to consider herein, one is the value of the principal quantum number of the valence electrons and the other is the effective nuclear charge felt by the valence electrons. 4 Mahmoud A. A. Ibrahim CH101 5. Atomic properties Sizes of atoms and ions Going from top to bottom within a group in the periodic table, the effective nuclear charge felt by the outer electrons remains nearly constant while the principal quantum number of the valence shell increases. As we know, as the value of the n for the valence shell increases, the larger the value of n the larger the orbital. Therefore, the atoms become larger as we go down a group simply because the orbitals containing the valence electrons become larger. 5 Mahmoud A. A. Ibrahim CH101 5. Atomic properties Sizes of atoms and ions Moving from left to right across a period, electrons are added to the same shell. The orbitals holding the valence electrons all have the same value of n, so changes in the principle quantum number certainly can’t be responsible for the size changes that occur From another point of view, as we move from left to right across a period, the outer shells of the atoms become more populated, but the inner core remains the same. However, while the outer shell is becoming more populated, the charge on the nucleus is also increasing and the difference between the nuclear charge and the charge on the core also increases. 6 Mahmoud A. A. Ibrahim CH101 5. Atomic properties Sizes of atoms and ions In other words, as we go across the period, the effective nuclear charge felt by the valence electrons becomes larger. This causes the valence electrons to be drawn inward, and thereby causes the sizes of the atoms to decrease. Across a row of transition elements or inner transition elements, the size variations are less pronounced than they are among the representative elements. This is because the outer shell configuration remains essentially the same while an inner shell is filler. 7 Mahmoud A. A. Ibrahim CH101 5. Atomic properties Sizes of atoms and ions For atomic numbers 21 to 30, for example, the outer electrons occupy the 4s subshell while the 3d subshell is gradually completed. The amount of shielding provided by the addition of electrons to this inner 3d level is greater than the amount of shielding that would occur if the electrons were added to the outer shell, so the effective nuclear charge felt by the outer electrons increases more gradually. As a result, the decrease in size with increasing atomic number is also more gradual. 8 Mahmoud A. A. Ibrahim CH101 5. Atomic properties Sizes of atoms and ions For anion and its original atom Anion is greater than its original atom in size. Ex. Radius of Cl- is greater than radius of Cl. When electrons are added to an atom, the mutual repulsions between them increase, and this cause the electrons to push apart and occupy a larger volume. 9 Mahmoud A. A. Ibrahim CH101 5. Atomic properties Sizes of atoms and ions For cation and its original atom, Cation is smaller than its original atom in size. Ex. Radius of Fe2+ is smaller than radius of Fe. When electrons are removed from the valence shell, the electron-electron repulsions decrease, which allows the remaining electrons to be pulled closer together around the nucleus. 10 Mahmoud A. A. Ibrahim CH101 5. Atomic properties Ionization energy Ionization energy (IE) is the energy needed to remove an electron from an isolated, gaseous atom or ion in its ground state. X(g) X+(g) + e− In effect, the ionization energy is a measure of how much work is required to remove an electron, so it reflects how tightly the electron is held by the atom. It is given in units of KJ/mol. Atoms with more than one electron have more than one ionization energy. These energies correspond to the stepwise removal of electrons, one after the other. 11 Mahmoud A. A. Ibrahim CH101 5. Atomic properties Ionization energy Li, for example, has three ionization energies because it has three electrons. Li(g) Li+(g) + e− (IE=520 kJ/mol) Li+(g) Li2+(g) + e− (IE=7,297 kJ/mol) Li2+(g) Li3+(g) + e− (IE=11,810 kJ/mol) In general, successive ionization energies always increase because each subsequent electron is being pulled away from an increasingly more positive ion and that required more work. 12 Mahmoud A. A. Ibrahim CH101 5. Atomic properties Ionization energy Ionization energy generally increases from bottom to top with an group and increases from left to right within a period. 13 Mahmoud A. A. Ibrahim CH101 5. Atomic properties Ionization energy It is helpful to mention that ionization energy is correlated to the atomic size, the trend of the IE is the opposite of the trend in the atomic size within the periodic table, i.e. when size increases, the IE decreases. The same factors that affect atomic size also affect ionization energy. As the value of n increases by going down a group, the orbitals become larger and the outer electrons are farther from the nucleus. Electrons farther from the nucleus are bound less tightly, so IE decreases by going from top to bottom in the periodic table. 14 Mahmoud A. A. Ibrahim CH101 5. Atomic properties Ionization energy There is a gradual overall increase in IE as we move from left to right across a period. The reason is the increase in effective nuclear charge felt by the valence electrons as we move across a period. This draws the valence electrons closer to the nucleus and to be held more tightly which makes it more difficult remove them. The results of these trends place elements with the largest IE in the upper right-hand corner of the periodic table. Noble gases have very much IEs, due to the huge energy required to break the noble gas electron configuration. 15 Mahmoud A. A. Ibrahim CH101 5. Atomic properties Ionization energy There are some irregularities in the horizontal trends in IE. For example, there is an irregularity occurs between nitrogen and oxygen. For nitrogen, the electron that is removed comes from a single occupied orbital, while for oxygen, the electron is taken from an orbital that already contains an electron. This can be explained as follows, for oxygen, repulsions between the two electrons in the p orbital that is about to lose an electron helps the electron leave. This “help” is absent for the electron that is about to leave the p orbital of nitrogen. 16 Mahmoud A. A. Ibrahim CH101 5. Atomic properties Electron affinity Electron affinity (EA) is the potential energy change associated with the addition of an electron to a gaseous atom or ion in its ground state. X(g) + e− X−(g) For nearly all the elements, the addition of one electron to the neutral atom is exothermic, and the EA is given as a negative value. This is because the incoming electron experiences an attraction to the nucleus which causes the potential energy to be lowered as the electron approaches the atom. 17 Mahmoud A. A. Ibrahim CH101 5. Atomic properties Electron affinity However, when a second electron must be added, work must be done to force the electron into an already negative ion. O(g) + e− O−(g) EA= −141 kJ/mol O−(g) + e− O2−(g) EA= +844 kJ/mol O (g) + 2e− O2−(g) EA= +703 kJ/mol Overall, the formation of an isolated oxide ion is endothermic. 18 Mahmoud A. A. Ibrahim CH101 5. Atomic properties Electron affinity Overall, electron affinity becomes more exothermic from left to right across a period and from bottom to top in a group. This trend is parallel to that for ionization energy. This is because a valence shell that loses electrons easily (low IE) will have little attraction for additional electrons (small EA). On the other hand, a valence shell that holds electrons tightly will also trend to bind an additional electron tightly. 19 Mahmoud A. A. Ibrahim CH101 5. Atomic properties Electron affinity There are irregularities in the periodic trends for electron affinity. For example, the Group IIA elements have little tendency to acquire electrons because their outer shell s orbitals are filled. The incoming electron must enter a higher energy p orbital. In Group VA, the EAs are either endothermic or only slightly exothermic. This is because the incoming electron must enter an orbital already occupied by an electron. 20 Mahmoud A. A. Ibrahim CH101  End of Lecture 4 21 Mahmoud A. A. Ibrahim CH101