Concept 2 1 Matter consists of chemical elements in pure form and in combinations called compounds Organisms are composed of matter Matter is anything that takes up space and has mass Matter is made up of elements An element is a substance that cannot be broken down to other substances by chemical reactions A compound is a substance consisting of two or more elements in a fixed ratio A compound has characteristics different from those of its elements Copyright 2008 Pearson Education Inc publishing as Benjamin Cummings Elements and Compounds Sodium Chlorine Copyright 2008 Pearson Education Inc publishing as Benjamin Cummings Sodium chloride Essential Elements of Life About 25 of the 92 elements are essential to life Carbon hydrogen oxygen and nitrogen make up 96 of living matter Most of the remaining 4 consists of calcium phosphorus potassium and sulfur Trace elements are those required by an organism in minute quantities e g Iodine I Iron Fe Copyright 2008 Pearson Education Inc publishing as Benjamin Cummings Concept 2 2 An element s properties depend on the structure of its atoms Each element consists of unique atoms An atom is the smallest unit of matter that still retains the properties of an element Atoms are composed of subatomic particles Relevant subatomic particles include Neutrons no electrical charge Protons positive charge Electrons negative charge Copyright 2008 Pearson Education Inc publishing as Benjamin Cummings Neutrons and protons form the atomic nucleus Electrons form a cloud around the nucleus Neutron mass and proton mass are almost identical and are measured in daltons An element s atomic number is the number of protons in its nucleus An element s mass number is the sum of protons plus neutrons in the nucleus Atomic mass the atom s total mass can be approximated by the mass number Copyright 2008 Pearson Education Inc publishing as Benjamin Cummings Isotopes All atoms of an element have the same number of protons but may differ in number of neutrons Isotopes are two atoms of an element that differ in number of neutrons Radioactive isotopes decay spontaneously giving off particles and energy Some applications of radioactive isotopes in biological research are Dating fossils Tracing atoms through metabolic processes Diagnosing medical disorders Copyright 2008 Pearson Education Inc publishing as Benjamin Cummings TECHNIQUE Incubators Compounds including radioactive tracer 3 1 2 bright blue 10 C 15 C 20 C Human cells 1 Human cells are incubated with compounds used to make DNA One compound is labeled with 3H 2 The cells are placed in test tubes their DNA is isolated and unused labeled compounds are removed 4 25 C 5 30 C 6 35 C 7 40 C 8 45 C 9 50 C DNA old and new 3 The test tubes are placed in a scintillation counter RESULTS Counts per minute 1 000 Fig 2 6 30 20 Optimum temperature for DNA synthesis 10 0 10 20 30 40 50 Temperature C The Energy Levels of Electrons Energy is the capacity to cause change Potential energy is the energy that matter has because of its location or structure The electrons of an atom differ in their amounts of potential energy An electron s state of potential energy is called its energy level or electron shell The chemical behavior of an atom is determined by the distribution of electrons in electron shells Copyright 2008 Pearson Education Inc publishing as Benjamin Cummings Fig 2 8 a A ball bouncing down a flight of stairs provides an analogy for energy levels of electrons Third shell highest energy level Second shell higher energy level First shell lowest energy level b Atomic nucleus Energy absorbed Energy lost Valence electrons are those in the outermost shell or valence shell The chemical behavior of an atom is mostly determined by the valence electrons Elements with a full valence shell are chemically inert An orbital is the three dimensional space where an electron is found 90 of the time Each electron shell consists of a specific number of orbitals Copyright 2008 Pearson Education Inc publishing as Benjamin Cummings Fig 2 10 4 Neon with two filled shells 10 electrons a Electron distribution diagram b Separate electron orbitals First shell Second shell y x z 1s orbital 2s orbital Three 2p orbitals c Superimposed electron orbitals 1s 2s and 2p orbitals Concept 2 3 The formation and function of molecules depend on chemical bonding between atoms Atoms with incomplete valence shells can share or transfer valence electrons with certain other atoms These interactions usually result in atoms staying close together held by attractions called chemical bonds Types of Chemical bonds 1 Covalent Bonds results from Sharing of electrons 2 Ionic or electrovalent bonds results from transfer of electrons 3 Hydrogen Bonds results between Hydrogen atom of one molecule and an electronegative atom of another molecule Copyright 2008 Pearson Education Inc publishing as Benjamin Cummings 1 Covalent Bonds A covalent bond is the sharing of a pair of valence electrons by two atoms In a covalent bond the shared electrons count as part of each atom s valence shell A molecule consists of two or more atoms held together by covalent bonds A single covalent bond or single bond is the sharing of one pair of valence electrons A double covalent bond or double bond is the sharing of two pairs of valence electrons Copyright 2008 Pearson Education Inc publishing as Benjamin Cummings Fig 2 12 Name and Molecular Formula a Hydrogen H2 b Oxygen O2 c Water H2O d Methane CH4 ElectronLewis Dot Spacedistribution Structure and filling Model Diagram Structural Formula Covalent bonds can form between atoms of the same element or atoms of different elements A compound is a combination of two or more different elements Bonding capacity is called the atom s valence Electronegativity is an atom s attraction for the electrons in a covalent bond The more electronegative an atom the more strongly it pulls shared electrons toward itself Copyright 2008 Pearson Education Inc publishing as Benjamin Cummings In a nonpolar covalent bond the atoms share the electron equally In a polar covalent bond one atom is more electronegative and the atoms do not share the electron equally Unequal sharing of electrons causes a partial positive or negative charge for each atom e g Water molecule O H H H2O Copyright 2008 Pearson Education Inc publishing as Benjamin Cummings 2 Ionic Bonds Atoms sometimes strip electrons from their bonding partners An example is the transfer of an electron from sodium to