Absolutely Small - Michael D. Fayer [69]
The Properties of Atoms
To begin our discussion of the properties of atoms based on the Periodic Table, we start with hydrogen. As usual, hydrogen is special because it only has one electron and is the first element in the Periodic Table. For the first row of the periodic table, helium has a closed shell with two electrons in the 1s orbital. Hydrogen can obtain the helium closed shell configuration by sharing with another atom to pick up an electron. For example, one H atom can share electrons with a second H atom, to form the hydrogen molecule. The symbol for the hydrogen molecule is H2. The subscript tells how many of a given type of atom are in a molecule. Because of the sharing, each H atom feels as if it has two electrons, the helium closed shell configuration. As we will see, hydrogen can form other molecules, but because it only needs one electron to get to the helium closed shell configuration, it only makes one chemical bond. Helium has the closed shell. It does not make any chemical bonds. There are no molecules with a helium atom in them. Exactly why this is true is described in detail in Chapter 12. Helium completes the first shell.
The next atom is lithium, Li, which is directly below H on the periodic table. Li can obtain the helium closed shell configuration by giving up an electron. Therefore, Li forms +1 ions, Li+1. A solid piece of Li is a metal. Metals can conduct electricity, which means electrons can move easily from one atom to another. The nature of metals and electrical conductivity will be discussed in Chapter 19. Metals have the property that as single atoms they can easily give up one or more electrons. The electron Li loses has to go somewhere. It will go to another atom that wants an electron to form a negative ion. So to form Li+1, Li needs a partner (see the discussion below when we get to the other side of the Periodic Table). The next element is beryllium. Beryllium will give up two electrons to go back to the He closed shell configuration. Therefore, Be will form +2 ions, Be+2. Because Be can readily give up electrons, solid beryllium is a metal. The next element is boron. Boron can lose three electrons to go back to the He closed shell configuration. Therefore, it forms +3 ions, and it is a metal.
Now things change. The next element is carbon. Carbon would have to give up four electrons to go back to the He configuration, but it can also gain four electrons to go to the next closed shell configuration, that of neon. As shown in Figure 11.5, Ne has a closed shell configuration, with the second shell filled. It has two electrons in the 1s orbital and then has the n = 2 shell filled with two electrons in the 2s and six electrons in the three 2p orbitals. Rather than losing so many electrons to go back to the He configuration, C goes forward to the Ne configuration, picking up four electrons by making four covalent bonds. For example, methane (natural gas) is the molecule CH4. Each H is bonded to the central C. The carbon shares four electrons, one from each of the hydrogens, so C obtains the Ne closed shell configuration by sharing four electrons with the four H atoms. Each H shares one electron with the C, so each H picks up one electron to have the He closed shell configuration. This is very important. Through covalent bonds (electrons sharing), each atom obtains a closed shell configuration. Another exceedingly important fact is that C always makes four bonds because it needs to share four electrons to get to the Ne configuration. This fact is fundamental to organic chemistry and to the chemistry of life. The bonding and chemistry of carbon will be discussed extensively in subsequent chapters
The next element is nitrogen. N needs three electrons to get to the Ne configuration, so it makes three covalent bonds. For example, it will combine with H to make NH3, which is ammonium. Oxygen needs two electrons to get to the Ne closed shell configuration, so it forms two bonds and, for example, makes H2O, water. So from these