The Chemical Context of Life Chapter 2 Matter

The Chemical Context of Life Chapter 2

Matter § Matter consists of chemical elements in pure form and in combinations called compounds; living organisms are made of matter. § Matter -- Anything that takes up space and has mass. § Element -- A substance that cannot be broken down into other substances by chemical reactions; all matter made of elements. § Life requires about 25 chemical elements § 96% of living matter is composed of C, O, H, N. § Most of remaining 4% is P, S, Ca, K. § Trace element -- required by organisms in extremely small quantities: Cu, Fe, I, etc.

Matter cont. § Compound -- A pure substances made of two or more elements combined in a fixed ratio. § Have characterisitics different than the elements that make them up (emergent property). § Na and Cl have very different properties from Na. Cl. § Difference between mass and weight: § Mass -- measure of the amount of matter an object contains; constant. § Weight -- measure of how strongly an object is pulled by earth's gravity; varies.

Nutrient Deficiencies

Atomic structure determines the behavior of an element § Atom -- Smallest possible unit of matter that retains the physical and chemical properties of its element. § Subatomic Particles § 1. Neutrons (no charge/neutral; found in nucleus; ~ 1 amu). § 2. Protons (+1 charge; found in nucleus; ~ 1 amu). § 3. Electrons (-1 charge; electron cloud; 1/2000 amu). § One amu approx equal to 1. 7 x 10 -24 g.

Atomic Number and Atomic Weight § Atomic number = Number of protons in an atom of a particular element. § All atoms of an element have the same atomic number. § In a neutral atom, # protons = # electrons. Mass number -- Number of protons and neutrons in an atom; not the same as an element's atomic weight.

§ § Examples 23 Mg 12 Mass number ? ? § 23 § # of protons ? ? § 12 § 14 C § 6 12 # of electrons ? ? 12 Mass number ? ? § 14 § # of protons ? ? § 6 Atomic number ? ? # of neutrons ? ? 11 Atomic number ? ? 6 # of electrons ? ? 6 # of neutrons ? ? 8

Isotopes § Isotopes -- Atoms of an element that have the same atomic number but different mass number; different number of neutrons. § Half-life -- Time for 50% of radioactive atoms in a sample to decay. § Biological applications of radioactive isotopes include: § 1. Dating geological strata and fossils. § Radioactive decay is at a fixed rate; by comparing the ratio of radioactive and stable isotope, age can be estimated. in a fossil with the § Ratio of Carbon-14 to Carbon-12 is used to date fossils less than 50, 000 years old.

Isotopes cont. § 2. Radioactive tracers § Chemicals labelled with radioactive isotopes are used to trace the steps of a biochemical reaction or to determine the location of a particular substance within an organism. § Isotopes of P, N and H were used to determine DNA structure. § Used to diagnose disease. § 3. Treatment of cancer § Can be hazardous to cells.

Energy Levels § Electrons are directly involved in chemical reactions. § They have potential energy because of their position relative to the positively charged nucleus. § There is a natural tendency for matter to move to the lowest state of potential energy. § Different fixed potential energy states for electrons are called energy levels or electron shells. § Electrons with lowest potential energy are in energy levels closest to the nucleus. § Electrons with greater energy are in energy levels further from nucleus. § Electrons may move from one energy level to another.

Electron Configuration and Chemical Properties § Electron configuration -- Distribution of electrons in an atom's electron shells; determines its chemical behavior. § Chemical properties of an atom depend upon the number of valence electrons (electrons in the outermost energy level. § Octet rule -- A valence shell is complete when it contains 8 electrons (except H and He). § An atom with an incomplete valence shell is chemically reactive (tends to form chemical bonds until it has 8 electrons to fill the valence shell). § Atoms with the same number of valence electrons show similar chemical behavior.

Bonding in Molecules § Chemical bonds -- Attractions that hold molecules together. § Molecules --Two or more atoms held together by chemical bonds. § Covalent bond -- formed between atoms by sharing a pair of valence electrons; common in organic compounds. § Single covalent bond -- Bond between atoms formed by sharing a single pair of valence electrons. § Double bond -- share two pairs of valence electrons. § Triple bond -- share three pairs of valence electrons. § Compound = A pure substance composed of two or more elements combined in a fixed ratio. § For example: water (H 2 O), methane (CH 4).

Nonpolar Covalent Bonds § Electronegativity -- Atom's ability to attract and hold electrons. § • The more electronegative an atom, the more strongly it attracts shared electrons. § • Scale determined by Linus Pauling: § O = 3. 5; N = 3. 0; S and C = 2. 5; P and H = 2. 1. § § Nonpolar bond -- Covalent bond formed by an equal sharing of electrons between atoms. § • Occurs when electronegativity of both atoms is about the same. § • Molecules made of one element usually have nonpolar covalent bonds (H 2 and O 2).

Polar Covalent Bonds § Polar bond -- Covalent bond formed by an unequal sharing of electrons between atoms. § • Occurs when the atoms involved have different electronegativities. § • In water, electrons spend more time around the oxygen than the hydrogens. This causes the oxygen atom to have a slight negative charge and the hydrogens to have a slight positive charge.

Ionic Bonds § Ion -- Charged atom or molecule. § Anion -- An atom that has gained one or more electrons from another atom; negatively charged. § Cation -- An atom that has lost one or more electrons; positively charged. § Ionic bond -- Bond formed by the electrostatic attraction after the complete transfer of an electron from a donor atom to an acceptor. § Strong bonds in crystals, but fragile bonds in water. § Ionic compounds are called salts (e. g. Na. Cl or table salt).

Biologically important weak bonds § Include: Hydrogen bonds; Ionic bonds in aqueous solutions; Van der Waals forces. § Hydrogen bond -- Bond formed by the charge attraction when a hydrogen atom covalently bonded to one electronegative atom is attracted to another electronegative atom. § Van der Waals -- charge attraction between oppositely charged portions of polar molecules.