CHEMISTRY OF LIFEChemistry is the study of matterCONCEPTUAL LIFE SCIENCECHEMISTRY OF LIFEChemistry is the study of matterLife science involves the study of living things. Since living things are made of matter, knowledge of simple chemistry will help you to understand the principles underlying the structure and activities of living things.ATOMSThe atom is the smallest quantity of an element that still possesses the properties of the element. The nucleus of the atoms contains the protons and the neutrons. The electrons are found in shells that surround the nucleus. Table III-1 contains information about the fundamental particles of the atom.Table III-1. Fundamental particles of the atom.Examples of atomic structureHydrogenHydrogen is the first and simplest element. In the nucleus it has one proton. In the electron shell is one electron. For hydrogen the atomic number is 1 and the mass is 1.3-13-2Figure 3-1. Formula and structure of hydrogen. The atomic number is represented by thelower number and the atomic mass is represented by the upper number.HeliumHelium is the second element. In the nucleus it has two protons and two neutrons.There are two electrons in the electron shell. For helium, the atomic number is 2 and the mass is 4.Figure 3-2. Formula and structure of helium.LithiumLithium is the third element. Its nucleus has three protons and four neutrons. There are two electrons in the first shell and one electron in the second shell. This is because the first shell only holds two electrons. The capacity of the second shell is eight electrons.3-3Figure 3-3. Formula and structure of lithium. Lithium has two electron shells because the first shell only holds two electrons.CarbonCarbon is element number 6. It has six protons and six neutrons. There are six electrons, two in the first shell and four in the second shell. Its atomic number is 6 and itsmass is 12.Figure 3-4. Formula and structure of carbon.OxygenOxygen is element number 8. It has eight protons and eight neutrons in its nucleus. It has eight electrons, two in the first shell and six in the second shell. Its atomic number is 8 and its mass is 16.3-4Figure 3-5. Formula and structure of oxygen. Note that the electrons in the outer shell are distributed unevenly. There are two pairs of electrons and two single electrons. This distribution is important in the way that oxygen bonds to other atoms.Dot structuresThe outer electrons are all that matter in chemical bonding. It is customary to represent them using dots. Figure 3-6. Dot structures. The electrons in the outer shell are represented using dots. These dot structures assist in understanding chemical bonding.ElementsElements are chemical substances composed of only one type of atom. There are over 100 elements. The elements are organized into the Periodic Table of the Elements.3-5Table III-2. Principal elements in protoplasm.Element Symbol PercentageHydrogen H 63Oxygen O 25.5Carbon C 9.5Nitrogen N 1.4Phosphorus P < 1Sulfur S < 1MoleculesMolecules are electrically neutral aggregates of atoms bonded together. “Electrically neutral” means that the number of protons equals the number of electrons.CompoundsCompounds are substances composed of a single kind of molecule. They can be broken down into simpler substances. For example, water (H2O) can be decomposed intohydrogen and oxygen. Sodium chloride (NaCl) can be decomposed into sodium and chlorine atoms.CHEMICAL BONDSWe are concerned with two kinds of bonds: ionic bonds and covalent bonds. Atoms behave in a way that gives them a complete outer shell. These bonding methods achieve that goal.Ionic bondsFigure 3-7. Formation of an ionic bond.3-6Ionic bonds are formed by complete transfer of one or more electrons from one atom to another. An example is the formation of sodium chloride (table salt). Covalent bondsFigure 3-8. Formation of a covalent bonds. Covalent bonds are formed by sharing of electrons between atoms. A bond is formed when a pair of electrons is shared. Each atom gets to share enough electrons to give it a complete outer shell.Figure 3-8. Some biologically important compounds.Water and salts are examples of inorganic compounds.3-7THE pH SCALEIonizationMolecules of ionic compounds contain ions. Ions are formed by complete transfer of electrons from one atom to another. You will recall that in the formation of sodium chloride (NaCl), one electron was transferred from sodium to chlorine. The result was formation of a sodium ion (Na+) and a chloride ion (Cl-).When salt is dissolved in water, the ions separate from each other. The result is that the positive sodium ions and the negative chloride ions move around among the water molecules as separate charged particles.Water can ionize. Despite being a covalent compound, water can dissociate to a slight extent to form ions. The result is that the water molecule (H2O) forms hydrogen ions (H+) and hydroxide ions (OH-).NeutralizationAcids form hydrogen ions. When an acid compound ionizes, it will form hydrogen ions. For example, hydrochloric acid (HCl):HCl  H+ + Cl-Bases form hydroxide ions. When a basic compound ionizes, it will form hydroxide ions.For example, sodium hydroxide (NaOH):NaOH  Na+ + OH-Neutralization is a reaction between an acid and a base. When an acid and a base react, the hydrogen and hydroxide ions combine to produce water. The other ions combine to produce a salt. A salt is formed by the positive ion of the base and the negative ion of the acid. For example:NaOH + HCl  HOH + NaClNeutralization has some important applications. You use vinegar (a weak acid) or boric acid to partly neutralize oven cleaner (a strong base). It can be used if lye (NaOH) is spilled on the skin while cleaning a drain. Farmers will add limestone to the soil if it is too acidic. Many over-the-counter products contain sodium bicarbonate (a weak base) to neutralize “excess stomach acid”. Sodium bicarbonate is produced in the digestive system for the same reason.3-8The pH scaleThe pH scale shows the strength of an acid or a base. The pH scale ranges from 0 to 14 as shown in figure 3-9. The number 7, in the middle, represents the neutral point betweenan acid and a base. Here, at pH 7, the concentration of hydrogen ions (H+) equals the concentration of hydroxide ions (OH-).Figure 3-9. The pH scale.The pH 7 value is the neutral point. The smaller than 7 a number is, the more acid there is