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Chapter 2 The Chemical Level of Organization Lecture slides prepared by Curtis DeFriez, Weber State University Introduction • Since chemicals compose your body (and all body activities are chemical in nature), it is important to become familiar with the language and fundamental concepts of chemistry. How Matter is Organized • All forms of matter are composed of chemical elements which are substances that cannot be split into simpler substances by ordinary chemical means. – Elements are given letter abbreviations called chemical symbols. – Trace elements are elements in our bodies, present in tiny amounts . Structure of Atoms • Units of matter of all chemical elements are called atoms. An element is a quantity of matter composed of atoms of the same type. Atoms contain: Nucleus: protons (p+) & neutrons (no) Electrons (e-) surround the nucleus as a cloud Structure of Atoms • Electrons are very small and light (mass about 1/2000th that of proton or neutron), often represented as a “planet” orbiting the “sun” (atomic nucleus). In reality, they are found in a “cloud” of probability. This concept, however, made even Einstein’s head hurt – we can use the planets-orbiting-thesun model for this course! Structure of Atoms • Protons and neutrons form the nucleus of an atom; electrons surround the nucleus. • Protons are large, positively-charged particles – The number of protons in the nucleus (called the atomic number) determines the element (gold vs. silver vs. lead). Structure of Atoms • Neutrons are the second large particle that make-up the nucleus of atoms. – Unlike protons, neutrons have no charge. – They do add mass, however, and determine the variety, or “isotope” of a certain element, i.e. carbon-12 vs. carbon-14, which has 2 extra neutrons in nucleus Structure of Atoms • Mass is measured as a dalton (atomic mass unit). – Masses of subatomic particles • Neutron - mass of 1.008 daltons • Proton - mass of 1.007 daltons • Electron has mass of 0.0005 dalton Structure of Atoms • Certain numbers are used to describe different properties of atoms (of elements). – Atomic number is the number of protons in an atom. – Mass number is the sum of protons and neutrons in an atom and indicates how much the atoms “weighs”—this is always a whole number. – Atomic mass (atomic weight) is the average mass of all naturally occurring isotopes—since this is an average, it is not exactly a whole number. Structure of Atoms • The “numbers” of some of the more important elements for the human body are depicted in this graphic: Ions, Molecules, & Compounds • Ions are atoms that have given up or gained an electron in their outer electron shell (also called the valence shell). – Written with its chemical symbol and (+) or (–) Ions, Molecules, & Compounds • Molecules are formed when atoms share electrons. – Written as a molecular formula showing the number of atoms of each element (H2O) – The oxygen gas in the atmosphere we breath is really not oxygen the atom, but a pair of oxygen atoms linked together into an oxygen molecule (O2) . Ions, Molecules, & Compounds • A free radical is an electrically charged atom or group of atoms with an unpaired electron in its outermost shell. • They are unstable and highly reactive. – They can become stable by giving up an electron or taking an electron from another molecule . • Antioxidants are substances that inactivate oxygenderived free radicals. Chemical Bonds • The atoms of a molecule are held together by forces of attraction called chemical bonds. • The likelihood that an atom will form a chemical bond with another atom depends on the number of electrons in its outermost or valence shell. Atomic Structure and Bonds Interactions Animation • Atomic Structure and the Basis of Bonds You must be connected to the internet to run this animation. Chemical Bonds • Ionic bonds form when an atom loses or gains a valence electron. Ions are formed. – Positively and negatively charged ions are attracted to one another. – Cations are positively charged ions that have given up one or more electrons (they are electron donors.) – Anions are negatively charged ions that have picked up one or more electrons that another atom has lost (they are electron acceptors). Chemical Bonds Ionic Bond Formation Chemical Bonds • Covalent bonds are formed by the atoms of molecules sharing one, two, or three pairs of their valence electrons. – Covalent bonds are the strongest chemical bonds. – Single, double, or triple covalent bonds are formed by sharing one, two, or three pairs of electrons, respectively. Chemical Bonds • Covalent bonds may be nonpolar or polar. – In a nonpolar covalent bond, atoms share the electrons equally; nonpolar covalent bonds are the most common types of covalent bonds, as shown in this graphic depicting hydrogen gas, oxygen gas, nitrogen gas, and methane gas: Chemical Bonds • Polar covalent bonds are formed by the unequal sharing of electrons between atoms. • Polar covalent bonds are extremely important because the all-important water molecule makes use of this bond. • In water, oxygen attracts the hydrogen electrons more strongly, making oxygen slightly electronegative as indicated by the negative Greek delta sign.