Molecules and Compounds PDF

## 2.2 Molecules and Compounds ### Learning Outcomes Upon completion of this section, you should be able to: - Describe how elements are combined into molecules and compounds. - List the different types of bonds that occur between elements. - Compare the relative strengths of ionic, covalent, and h...

## 2.2 Molecules and Compounds ### Learning Outcomes Upon completion of this section, you should be able to: - Describe how elements are combined into molecules and compounds. - List the different types of bonds that occur between elements. - Compare the relative strengths of ionic, covalent, and hydrogen bonds. ### Figure 2.4 High levels of radiation. - a. Radiation kills bacteria and fungi. Irradiated peaches spoil less quickly and can be kept for a longer length of time. - b. Physicians use radiation therapy to kill cancer cells. Atoms, except for noble gases, routinely bond with one another. **A molecule is formed when two or more atoms covalently bond together.** For example, oxygen does not exist in nature as a single atom, O. Instead, two oxygen atoms are joined to form a molecule of oxygen, O₂. **When atoms of two or more different elements bond together, the product is called a compound.** Water (H₂O) is a compound that contains atoms of hydrogen and oxygen. We can also speak of molecules of water because a molecule is the smallest part of a compound that still has the properties of that compound. Electrons possess energy, and the bonds that exist between atoms also contain energy. Organisms are directly dependent on chemical-bond energy to maintain their organization. When a chemical reaction occurs, electrons shift in their relationship to one another, and energy may be given off or absorbed. This same energy is used to carry on our daily lives. ### Ionic Bonding In an electrically neutral atom, the positive charges of the protons in the nucleus are balanced by the negative charges of electrons moving about the nucleus. **Ions form when electrons are transferred from one atom to another.** For example, sodium (Na) tends to be an electron donor (Figure 2.5a). Chlorine (Cl), on the other hand, tends to be an electron acceptor. When a sodium atom and a chlorine atom come together, an electron is transferred from the sodium atom to the chlorine atom. This electron transfer causes a charge imbalance in each atom. The sodium atom has one more proton than it has electrons. Therefore, it has a net charge of +1 (symbolized by Na⁺). The chlorine atom has one more electron than it has protons. Therefore, it has a net charge of -1 (symbolized by Cl^-). **Such charged particles are called ions**. Sodium (Na⁺) and chloride (Cl^-) are not the only biologically important ions. Some, such as potassium (K⁺), are formed by the transfer of a single electron to another atom. Others, such as calcium (Ca²⁺), are formed by the transfer of two electrons to another atom.