1 Chapter 2 Introduction o Chemicals are the raw materials that make up our bodies the bodies of other organisms and the physical environment o Life s chemistry is tied to water Life first evolved in water All living organisms require water Cells consist of about 75 water BIG IDEAS I Element Atoms and Compounds 2 1 2 4 II Chemical Bonds 2 5 2 9 III Water s Life Supporting Properties 2 10 2 16 II ELEMENT ATOMS AND COMPOUNDS 2 1 Organisms are composed of elements in combinations called compounds o Matter anything that occupies space and has a mass Living organisms are composed of matter which is anything that occupies space and has mass weight o Matter is found in 3 physical states solid liquid and gas o Matter is composed of chemical elements An element is a substance that cannot be broken down to other substances by ordinary chemical means There are 92 elements in nature only a few exist in a pure state Elements o Periodic table displays elements according to valence electrons o Valence electrons number of electrons in outermost energy level o Inert nonreactive elements have all eight electrons o Octet rule atoms tend to establish completely full outer energy levels 2 o A compound is a substance consisting of two or more different elements in a fixed o Compounds are more common than pure elements o Sodium chloride table salt is a common compound of equal parts of sodium Na o About 25 elements are essential for human life o Four elements make up about 96 of the weight of most living organisms These are ratio and chlorine Cl oxygen carbon hydrogen and nitrogen o Trace elements are essential but are only needed in minute quantities Question Explain how table salt illustrates the theme of emergent properties Answer the element that make up the edible crystal of table salt sodium and chlorine are in pure form a metal and a poisonous gas 2 2 CONNECTION Trace elements are common additives to food and water o Some trace elements are required to prevent disease Without iron your body cannot transport oxygen An iodine deficiency prevents production of thyroid hormones resulting in goiter Fluoride is usually added to municipal water and dental products to help reduce tooth decay o Several chemicals are added to food to help preserve it make it more nutritious and or make it look better 3 o Check out the nutrition facts label on foods and drinks you purchase Question In addition to iron what other trace elements are found in the cereal in figure 2 2C Does one serving provide the total daily amount needed of these elements Answer Zing and copper one serving provides 100 of the zinc but only 4 of the the copper needed in a day 2 3 Atoms consist of protons neutrons and electrons o Each element consists of one kind of atom o An atom is the smallest unit of matter that still retains the properties of an element o Atoms are composed of Protons Positively Located in the nucleus charged particles Neutrons Neutral Located in the nucleus particles Electrons charged particles Found in orbitals surrounding the nucleus Negatively The negative charge of electrons and the positive charge of protons keep electrons near the nucleus o Atomic number The number of protons in the atom 4 o Mass number The SUM of the number of protons and neutrons in the nucleus o The atomic mass is approximately equal to its mass number o Although all atoms of an element have the same atomic number some differ in mass number o Different isotopes of an element have the same number of protons but different numbers of neutrons o Different isotopes of an element behave identically in chemical reactions o In radioactive isotopes the nucleus decays spontaneously giving off particles and energy Half life time it takes for one half of the atoms in a sample to decay Question A nitrogen atom has 7 protons and its most common isotope has 7 neutrons A radioactive isotope of nitrogen has 9 neutrons What is the atomic number and mass number of this radioactive nitrogen Answer Atomic Number 7 Atomic Mass 16 2 4 CONNECTION Radioactive isotopes can help or harm us o Living cells cannot distinguish between isotopes of the same element Therefore radioactive compounds in metabolic processes can act as tracers This radioactivity can be detected by instruments Geiger counter By using these instruments the fate of radioactive tracers can be monitored in living organisms o Radioactive tracers are frequently used in medical diagnosis o Sophisticated imaging instruments are used to detect them 5 An imaging instrument that uses positron emission tomography PET detects the location of injected radioactive materials PET is useful for diagnosing heart disorders and cancer and in brain o In addition to benefits there are also dangers associated with using radioactive Uncontrolled exposure can cause damage to some molecules in a living cell Chemical bonds are broken by the emitted energy which causes abnormal research substances especially DNA bonds to form QUESTION Why are radioactive isotopes useful as tracers in research on the chemistry of life ANSWER Organisms incorporate radioactive isotopes of an element into their molecules and researchers can use special scanning device to detect the presence of these isotopes in biological pathways or location in the body II CHEMICAL BONDS 2 5 The distribution of electrons determines an atom s chemical properties o Of the three subatomic particles protons neutrons and electrons only electrons are directly involved in the chemical activity of an atom o Electrons can be located in different electron shells each with a characteristic distance from the nucleus An atom may have one two or more electron shells Information about the distribution of electrons is found in the periodic table of the elements o The number of electrons in the outermost shell called the valence shell determines the chemical properties of the atom o Atoms whose outer shells are not full tend to interact with other atoms in ways that enable them to complete or fill their valence shells o When two atoms with incomplete outer shells react each atom will share donate or receive electrons so that both partners end up with completed outer shells o These interactions usually result in atoms staying close together held by attractions called chemical bonds o Ions are charged particles unbalanced Cation more protons than electrons net positive charge Anion fewer protons than electrons net negative charge QUESTION how many electrons and electron shells does