Campbell Biology 10th Edition Fall 2015 UCONN BIO 1107 Unit 1 Chapter 2 The Chemical Context of Life Key Concepts 2 1 Matter consists of chemical elements in pure form and in combinations called compounds 2 2 An element s properties depend on the structure of its atoms The formation and function of molecules depend on chemical 2 3 bonding between atoms 2 4 Chemical reactions make and break chemical bonds Concept 2 1 Matter consists of chemical elements in pure form and in combinations called compounds 1 Matter a Exists in many forms and has an endless assortment of variation anything that takes up space and has mass 2 Elements and Compounds a An element is a substance that cannot be broken down to other substances by chemical reactions i Scientists recognize 92 elements that are naturally occurring in nature b A compound is a substance consisting of two or more different elements combined in a fixed ratio c Elements of Life i Of the 92 elements about 20 25 are essential elements that an organism needs to live a healthy life and reproduce 1 Oxygen O 2 Carbon C 3 Hydrogen H 4 Nitrogen N a Those make up 96 of living matter ii Calcium Ca phosphorous P potassium K sulfur S and iii Trace elements a few others account for the last 4 of an organism s mass are required by an organism in only minute quantities Concept 2 2 An element s properties depend on the structure of its atoms 1 An atom is the smallest unit of matter that still retains the properties of an element a So small it would take a million of them to stretch across the period 2 Subatomic Particles a Atoms are composed of even smaller particles i Protons ii Neutrons iii Electrons electron cloud b Protons and neutrons are packed tightly together to form the atomic nucleus the center of the atom i Protons give the atom a positive charge ii Rapidly moving electrons form an electron cloud with a c The neutron and the proton are almost identical in mass each negative charge about 1 7 x 10 24 gram g d For atoms and subatomic particles including molecules the Dalton is used as a form of measurement i Named after John Dalton British scientist who helped developed atomic theory around 1800 ii The Dalton is the same as the atomic mass unit or amu iii Neutrons and protons have masses that are close to 1 dalton e Electrons are about 1 2000th of a neutron or proton so they can be ignored when measuring mass 3 Atomic Number and Atomic Mass a The number of protons in an element which is unique to it is the atomic number and is written as a subscript to the left of the symbol for the element i Mass number number of protons number of neutrons ii Atomic number number of protons number of electrons in a neutral atom b The mass number is an approximation of the total mass of an atom called it s atomic mass 4 Isotopes a Some atoms have more neutrons than other atoms of the same element therefore having a different mass these are called the isotopes of the element i In nature an element occurs as a mixture of its isotopes b The most common isotope is Carbon 12 or 12 6 C which accounts c Both 12C and 6C are stable isotopes meaning that their nuclei do for about 99 of the carbon in nature not have a tendency to lose subatomic particles d A radioactive isotope like 14C is one in which the nucleus decays spontaneously giving off particles and energy e Radioactive Tracers i Radioactive isotopes are used as diagnostic tools in medicine 1 Cells can use radioactive atoms just as they would use nonradioactive isotopes of the same element 2 The isotopes are incorporated into biologically active molecules which are then used as tracers to track atoms during metabolism the chemical process of an organism 3 Tracers are also used in combination with complex imaging machines such as PET scan ii Radiometric Dating 1 A parent isotope decays into its daughter isotope at a fixed rate expressed as the half life of the isotope a The time it takes for 50 of the parent isotope to decay b Not affected by temperature pressure or any other environmental variable 2 Radiometric dating helps scientists measure the ratio of different isotopes and calculate how many half lives in years have passed since an organism was fossilized or a rock was formed 5 The Energy Levels of Electrons i Potential energy a Only electrons are directly involved in chemical reactions b Energy is defined as the capacity to cause change is the energy one possesses because of its location or structure 1 It takes work to move a given electron farther away from the nucleus so the more distant an electron is the greater its potential energy c Electrons are found in different electron shells each having its own carrying capacity 6 Electron Distribution and Chemical Properties a Electrons like all matter tend to exist in the lowest available state of potential energy b The chemical behavior of an atom depends on the number of electrons in its outermost shell or valence electrons c An atom with a completed valence shell is unreactive it will not interact readily with other atoms 7 Electron Orbitals a Each concentric circle in electron orbitals only represents the average distance between an electron in that shell and the nucleus b Three dimensional space where an electron is found 90 of the time is called an orbital c The first electron cell has only one spherical s orbital called 1s i The second shell has four orbitals one large spherical s orbital called 2s and three dumbbell shaped p orbitals called 2p orbitals ii The third shell and other higher electron shells also have s and p orbitals as well as orbitals of more complex shapes 1 No more than two electrons can occupy a single orbital Concept 2 3 The formation and function of molecules depend on chemical bonding between atoms The strongest types of bonds are covalent and ionic when in dry ionic compounds 1 Covalent Bonds a A covalent bond is the sharing of a pair of valence electrons i When the two hydrogen atoms come close enough for their 1s orbitals to overlap they can share their electrons ii Electron sharing can be depicted by an electron distribution diagram or a Lewis dot structure in which element symbols are surrounded by dots that represent the valence electrons 1 You can also use a structural formula H H where the line represents a single bond b The attraction of a particular atom for the electrons of a covalent bond is called its electronegativity c In a covalent bond between two atoms of the same element the electrons are shared equally