Key Concepts Molecules form when atoms bond to each other Chemical bonds are based on electron sharing The degree of electron sharing varies depending on the type of bond formed Of all small molecules water is the most important for life Water is highly polar and readily forms hydrogen bonds making it an extremely efficient solvent 2011 Pearson Education Inc Key Concepts Energy is the capacity to do work or supply heat and can be 1 a stored potential or 2 an active motion Chemical energy is a form of potential energy stored in chemical bonds Chemical reactions tend to be spontaneous if they lead to lower potential energy and higher entropy and nonspontaneous if they require an input of energy Most of the important compounds in organisms contain carbon 2011 Pearson Education Inc Basic Atomic Structure Atoms are composed of Protons positively charged particles Neutrons neutral particles Electrons negatively charged particles Protons and neutrons are located in the nucleus Electrons are found in orbitals surrounding the nucleus 2011 Pearson Education Inc 2011 Pearson Education Inc Elements The Building Blocks of Chemical Evolution Every different atom has a characteristic number of protons in the nucleus called the atomic number Atoms with the same atomic number have the same chemical properties and belong to the same element Forms of an element with different numbers of neutrons are isotopes The mass number is the number of protons neutrons of the most common isotope 2011 Pearson Education Inc Electron Arrangement around the Nucleus Electrons move around atomic nuclei in specific regions called orbitals Each orbital can hold up to two electrons Orbitals are grouped into levels called electron shells Electron shells are numbered with smaller numbers closer to the nucleus The electrons in the outermost shell are called valence electrons Elements commonly found in organisms have at least one unpaired valence electron The number of unpaired electrons in an atom is its valence 2011 Pearson Education Inc 2011 Pearson Education Inc Chemical Bonding Unfilled electron orbitals allow formation of chemical bonds and atoms are most stable when each electron orbital is filled Covalent bond Each atom s unpaired valence electrons are shared by both nuclei to fill their orbitals Substances held together by covalent bonds are called molecules Ionic bond Electrons are transferred from one atom to another 2011 Pearson Education Inc Covalent Bonds Electrons are not always shared equally An atom in a molecule with a high electronegativity will hold the electrons more tightly and have a partial negative charge whereas the other atom will have a partial positive charge Differences in electronegativity dictate how electrons are distributed in covalent bonds Nonpolar covalent bond Electrons are evenly shared between two atoms and the bond is symmetrical Polar covalent bond Electrons are asymmetrically shared 2011 Pearson Education Inc 2011 Pearson Education Inc 2011 Pearson Education Inc Ions and Ionic Bonds An atom or molecule that carries a charge is called an ion Cation An atom that loses an electron and becomes positively charged Anion An atom that gains an electron and becomes negatively charged The resulting attraction between oppositely charged ions is an ionic bond 2011 Pearson Education Inc 2011 Pearson Education Inc The Electron Sharing Continuum The degree to which electrons are shared in chemical bonds forms a continuum from equal sharing in nonpolar covalent bonds to unequal sharing in polar covalent bonds to the transfer of electrons in ionic bonds 2011 Pearson Education Inc 2011 Pearson Education Inc How Many Bonds Can an Atom Have The number of unpaired electrons determines the number of bonds an atom can make Atoms with more than one unpaired electron can form multiple single bonds or double or triple bonds 2011 Pearson Education Inc 2011 Pearson Education Inc Representing Molecules The shape of a simple molecule is governed by the geometry of its bonds Molecular formulas indicate the numbers and types of atoms in a molecule e g H2O CH4 Structural formulas indicate which atoms are bonded together and whether the bonds are single double or triple bonds Ball and stick models and space filling models show 3D geometry 2011 Pearson Education Inc 2011 Pearson Education Inc Chemical Reactions Chemical reactions occur when 1 One substance is combined with another Atoms are rearranged in molecules or small molecules combine to form larger molecules 2 One substance is broken down into another substance Molecules are split into atoms or smaller molecules In most cases chemical bonds are broken and new bonds form 2011 Pearson Education Inc Quantifying Molecules The molecular weight of a molecule is the sum of the mass numbers of all the atoms in the molecule One mole or 6 022 1023 molecules has a mass equal to the molecular weight expressed in grams The concentration of a substance in a solution is typically expressed as molarity M which is the number of moles per liter 2011 Pearson Education Inc Why Is Water Such an Efficient Solvent Life is based on water because water is a great solvent The covalent bonds in water are polar because oxygen has a greater electronegativity than hydrogen Oxygen has a partial negative charge Hydrogen has a partial positive charge Hydrogen bonds are the weak electrical attractions between the partially negative oxygen of one water molecule and the partially positive hydrogen of a different water molecule Can also form between a water molecule and another polar molecule 2011 Pearson Education Inc 2011 Pearson Education Inc Water and Hydrogen Bonds Ions and polar molecules stay in solution because of their interactions with water s partial charges These atoms and molecules are said to be hydrophilic Uncharged and nonpolar compounds do not dissolve in water and are said to be hydrophobic Hydrogen bonding makes it possible for almost any charged or polar molecule to dissolve in water 2011 Pearson Education Inc 2011 Pearson Education Inc Correlation of Water s Structure and Properties Water is unique due to its small size bent shape highly polar covalent bonds and overall polarity Water also has several remarkable properties largely due to its ability to form hydrogen bonds Water is 1 Cohesive 2 Adhesive 3 Denser as a liquid than as a solid 4 Able to absorb large amounts of energy 2011 Pearson Education Inc A Closer Look at the Properties of Water Cohesion binding between