Absolutely Small - Michael D. Fayer [63]
FIGURE 10.8. Schematics of the five hydrogen atom 3d orbitals, which are named in relation to their shapes. Each orbital has two angular nodes, as well as positive and negative lobes. The angular nodes are the planes in four of the diagrams and the cones and disk in the fifth diagram. When a nodal plane is crossed, the wavefunction changes sign. The lobes in each diagram show where most of the electron probability amplitude is located. Four of the orbitals consist of four lobes. The dz2 orbital has a different shape. It sill has two nodal surfaces, the xy plane and the conical surfaces. These probability amplitude waves decay smoothly to zero away from the nucleus (the proton) and do not stop abruptly as in the diagrams.
11
Many Electron Atoms and the Periodic Table of Elements
THE PROPERTIES OF ATOMIC and molecular matter are determined by the quantum mechanical details of the atoms that make up a substance. Common table salt is sodium chloride, NaCl. Na is the symbol for the sodium atom. Sodium’s atomic number is 11. The atomic number is the number of protons in the nucleus, that is, it is the positive charge on the nucleus. Na has 11 protons in its nucleus and 11 negatively charged electrons. Chlorine (symbol Cl) has an atomic number of 17. Cl has 17 protons in its nucleus and 17 electrons. When table salt, which is composed of little white crystals of NaCl, is put in water, it dissolves. In water, the Na becomes the positively charged sodium ion, Na+ (sodium has lost an electron), and the Cl becomes the negatively charged chloride ion, Cl- (chlorine has gained an electron). Sodium gives up an electron to chlorine to form the sodium cation (positively charged ion) and the chloride anion (negatively charged ion). The charges on the sodium cation and the chloride anion make these ions very soluble in water.
Methane is natural gas that we burn in our gas stoves, gas clothes dryers, and power plants. Its chemical formula is CH4. This means it is composed of one carbon atom (symbol C, atomic number 6) bonded to (attached to) four hydrogen atoms (symbol H, atomic number 1). Methane does not become ions when put in water. In fact, it does not dissolve in water. Unless it is raised to very high temperature as in a flame, it does not come apart at all. Why does NaCl become separated ions, Na+ and Cl-, when dissolved in water, while carbon always makes four chemical bonds and methane does not come apart in water to form ions? The answers to these questions and many of the properties of all of the atoms can be understood by looking at the nature of many electron atoms and the collection of systemic information about atoms contained in the Periodic Table of the Elements.
HYDROGEN IS DIFFERENT
The hydrogen atom is different from all other atoms, and the difference is highly significant. The hydrogen atom consists of a positively charged nucleus (a proton) and one negatively charged electron. The sole electrostatic interaction is the attraction of the electron to the proton because oppositely charged particles attract. The next simplest atom is helium. Helium consists of a positively charged nucleus, with a positive charge of +2 (symbol He, atomic number 2) and two electrons, each with a negative charge of -1. Each electron is attracted to the nucleus. In addition, the two electrons repel each other because like changes repel. The repulsion is referred to as electron-electron repulsion. Because a hydrogen atom has only a single electron, there is no electron-electron repulsion.
In the energy level diagram for the hydrogen atom (Figure 10.1), all of the orbitals with the same principal quantum number n have the same energy. So the 2s and the 2p orbitals have the same energy. The 3s, 3p, and 3d orbitals all have the same energy, and so on. The fact that