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.

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.

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.

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.

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.

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.

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 (δ–), whereas the other atom will have a partial positive charge (δ+). 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.

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.

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.

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.

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.

Chemical Reactions Chemical reactions occur when: One substance is combined with another. Atoms are rearranged in molecules, or small molecules combine to form larger molecules. 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.

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.

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.

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.

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: Cohesive Adhesive Denser as a liquid than as a solid Able to absorb large amounts of energy

A Closer Look at the Properties of Water Cohesion – binding between like molecules Results in high surface tension Adhesion – binding between unlike molecules Water expands as it changes from a liquid to a solid. This is why ice floats! Water has an extraordinarily large capacity for absorbing heat. High specific heat High heat of vaporization

Acid–Base Reactions and pH Proton [hydrogen ion (H+)] concentration is the basis of the pH scale. pH expresses proton concentration in a solution. The pH of pure water is 7. Acids have a pH of less than 7. Bases have a pH of greater than 7. In acid–base reactions, a proton donor (acid) transfers a proton to a proton acceptor (base).

The pH Scale and Buffers The pH scale is logarithmic: pH = −log [H+] Greater H+ concentration – lower pH – more acidic Lower H+ concentration – higher pH – more basic/alkaline Buffers are compounds that minimize changes in pH.

Chemical Evolution Theory Simple molecules present on ancient Earth reacted to create larger, more complex molecules. This may have happened in: The atmosphere Deep-sea vents

How Do Chemical Reactions Happen? Chemical reactions have reactants and products. For example: CO2(g) + H2O(l)  H2CO3(aq) Chemical equilibrium occurs when the forward and reverse reactions proceed at the same rate and the quantities of reactants and products remain constant. Endothermic reactions must absorb heat to proceed, but exothermic reactions release heat.

What Is Energy? Energy is the capacity to do work or supply heat. This capacity exists in one of two ways—as a stored potential or as an active motion.

Potential Energy and Kinetic Energy Stored energy is called potential energy. An object’s position determines its ability to store energy. For example: Electrons in an outer shell (farther from the positively charged nucleus) have more potential energy than do electrons in an inner shell. The energy of movement is called kinetic energy or thermal energy, which is measured as temperature. Low-temperature objects have slower molecules than high-temperature objects.

Heat and the First Law of Thermodynamics Heat is the thermal energy transferred between objects of different temperatures. The first law of thermodynamics states that energy is conserved—it cannot be created or destroyed, but it can be transferred or transformed.

What Makes a Chemical Reaction Spontaneous? Chemical reactions are spontaneous if they proceed on their own, without any continuous external influence such as added energy. The spontaneity of a reaction is determined by two factors: The amount of potential energy Products of spontaneous reactions have less potential energy than the reactants. The degree of order Products of spontaneous reactions are less ordered than the reactants.

The Second Law of Thermodynamics Entropy (S) is the amount of disorder in a group of molecules. The second law of thermodynamics states that entropy always increases. In other words, chemical reactions result in products with less ordered (usable) energy. In general, physical and chemical processes proceed in the direction that results in lower potential energy and increased disorder.

Gibbs Free-Energy Change The Gibbs free-energy change (ΔG) determines whether a reaction is spontaneous or requires energy. ΔG < 0 is an exergonic spontaneous reaction. ΔG > 0 is an endergonic reaction that requires energy input. ΔG = 0 is a reaction that is at equilibrium.

Temperature and Concentration Affect Reactions Breaking and forming bonds depends on collisions between substances. This allows electrons to interact. The rate of a reaction depends upon the number of collisions. The number of collisions is dependent on the temperature and concentration of the reactants: Higher temperature  more collisions  faster reaction Higher concentration  more collisions  faster reaction

The Importance of Carbon Carbon is the most versatile atom on Earth. Because of its four valence electrons, carbon can form many covalent bonds. Carbon-containing molecules can form an almost limitless array of molecular shapes with different combinations of single and double bonds. The formation of carbon–carbon bonds was an important event in chemical evolution.

Functional Groups: Determinants of Chemical Behavior The carbon atoms in an organic molecule furnish the skeleton that gives the molecule its overall shape. Amino and carboxyl groups: Attract or drop a proton, respectively Carbonyl groups: Sites of reactions that link molecules into larger, more-complex compounds Hydroxyl groups: Act as weak acids Phosphate groups: Have two negative charges Sulfhydryl groups: Link together via disulfide bonds