1.1 Atoms, Electrons, and Orbitals even include and -signs of wave functions in our drawings but only when they are necessary for understanding a particular concept Instead of probability distributions, it is more common to represent orbitals by their boundary surfaces, as shown in Figure 1.3 for the ls and 2s orbitals. The boundary sur face encloses the region where the probability of finding an electron is high--on the order of 90-95%. Like the probability distribution plot from which it is derived, a pic ture of a boundary surface is usually described as a drawing of an orbital single electron of hydrogen occupies a Is orbital, as do the two electrons of helium. The respective electron configurations are described as s Helium: 152 In addition to being negatively charged, electrons possess the property of spin. The spin quantum number of an electron can have a value of either +2 or -2 According to the Pauli exclusion principle, two electrons may occupy the same orbital only when they have opposite, or"paired, " spins. For this reason, no orbital can contain more than two electrons. Since two electrons fill the is orbital. the third electron in lithium (z=3)must occupy an orbital of higher energy. After 1s, the next higher energy orbital is 2s. The third electron in lithium therefore occupies the 2s orbital, and the electron configuration of lithium is Li case of hydrogen and helium). Hydrogen and helium are first-row elements; lithium the inside back presenteyof The period (or row) of the periodic table in which an element appears corresponds to the principal quantum number of the highest numbered occupied orbital (n= 1 in the the elements (n= 2)is a second-row element. with beryllium(Z =4), the 2s level becomes filled, and the next orbitals to be occupied in it and the remaining second-row elements are the 2po 2py, and 2p2 orbitals These orbitals, portrayed in Figure 1. 4, have a boundary surface that is described as"dumbbell-shaped. "Each orbital consists of two"lobes, "that is spheres that touch each other along a nodal plane passing through the nucleus. The 2p x 2py, and 2p, orbitals are equal in energy and mutually perpendicular. The electron configurations of the first 12 elements, hydrogen through magnesium, are given in Table 1. 1. In filling the 2p orbitals, notice that each is singly occupied before any one is doubly occupied. This is a general principle for orbitals of equal energy kno FIGURE 1.3 Bound urfaces of a 1s orbital and a 2s orbital. The boundary surfaces enclose the volume where there is a 90-95% probability of finding an electron Back Forward Main MenuToc Study Guide ToC Student o MHHE Websiteeven include  and  signs of wave functions in our drawings but only when they are necessary for understanding a particular concept. Instead of probability distributions, it is more common to represent orbitals by their boundary surfaces, as shown in Figure 1.3 for the 1s and 2s orbitals. The boundary sur￾face encloses the region where the probability of finding an electron is high—on the order of 90–95%. Like the probability distribution plot from which it is derived, a pic￾ture of a boundary surface is usually described as a drawing of an orbital. A hydrogen atom (Z 1) has one electron; a helium atom (Z 2) has two. The single electron of hydrogen occupies a 1s orbital, as do the two electrons of helium. The respective electron configurations are described as: Hydrogen: 1s 1 Helium: 1s 2 In addition to being negatively charged, electrons possess the property of spin. The spin quantum number of an electron can have a value of either 1 2 or 1 2. According to the Pauli exclusion principle, two electrons may occupy the same orbital only when they have opposite, or “paired,” spins. For this reason, no orbital can contain more than two electrons. Since two electrons fill the 1s orbital, the third electron in lithium (Z 3) must occupy an orbital of higher energy. After 1s, the next higher energy orbital is 2s. The third electron in lithium therefore occupies the 2s orbital, and the electron configuration of lithium is Lithium: 1s 2 2s 1 The period (or row) of the periodic table in which an element appears corresponds to the principal quantum number of the highest numbered occupied orbital (n 1 in the case of hydrogen and helium). Hydrogen and helium are first-row elements; lithium (n 2) is a second-row element. With beryllium (Z 4), the 2s level becomes filled, and the next orbitals to be occupied in it and the remaining second-row elements are the 2px, 2py, and 2pz orbitals. These orbitals, portrayed in Figure 1.4, have a boundary surface that is usually described as “dumbbell-shaped.” Each orbital consists of two “lobes,” that is, slightly flattened spheres that touch each other along a nodal plane passing through the nucleus. The 2px, 2py, and 2pz orbitals are equal in energy and mutually perpendicular. The electron configurations of the first 12 elements, hydrogen through magnesium, are given in Table 1.1. In filling the 2p orbitals, notice that each is singly occupied before any one is doubly occupied. This is a general principle for orbitals of equal energy known 1.1 Atoms, Electrons, and Orbitals 9 z z x y y x 1s 2s FIGURE 1.3 Boundary surfaces of a 1s orbital and a 2s orbital. The boundary surfaces enclose the volume where there is a 90–95% probability of finding an electron. A complete periodic table of the elements is presented on the inside back cover. Back Forward Main Menu TOC Study Guide TOC Student OLC MHHE Website