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Atoms, Molecules, and Life Chapter 2 2.1 What Are Atoms? • Atoms Are Basic Structural Units Composed of Still Smaller Particles • Electrons Orbit the Nucleus at Fixed Distances • Life Depends on the Ability of Electrons to Capture and Release Energy Atoms Atoms are the fundamental structural units of matter and are composed of three types of particles In the central nucleus there are positively charged protons and uncharged neutrons In orbit around the nucleus are negatively charged particles called electrons Atoms are electrically neutral because they have an equal number of positive protons as negative electrons Atomic Number & Elements The number of protons in the nucleus of an atom is known as the atomic number An element is a substance that cannot be broken down by ordinary chemical reactions All atoms belong to one of 96 types of naturally occurring elements Elements & Isotopes The atomic number (number of protons) is the defining value for an element • All atoms of an element have the same atomic number • e.g. Carbon has 6 protons, nitrogen has 7 Atoms of an element may vary in the number of neutrons they have in the nucleus • Variant atomic forms of an element are called isotopes • Some isotopes are radioactive and are used in research Elements may occur as solids, liquids, or gases at room temperature Electrons & Electron Shells Larger atoms can accommodate more electrons than smaller ones can Electrons are distributed about the nucleus of an atom in electron shells • The first shell or energy level holds 2 electrons • The second shell holds up to 8 Roles of the nucleus and the electrons • The nucleus provides stability • The electrons interact with other atoms (e.g. form bonds) Atoms Interact Molecules are made of two or more atoms bonded together through electron shell interactions A substance made of atoms of different elements is a compound Reactions between atoms depend upon the configuration of electrons in the outermost electron shell Atoms Interact Atoms will not react with other atoms if the outermost shell is completely empty or full (such atoms considered inert) • Example: Neon, with 8 electrons in outermost shell (full) Atoms will react with other atoms if the outermost shell is partially full (such atoms considered reactive) • Example: Oxygen, with 6 electrons in outermost shell (can hold 2 more electrons) Atoms Interact Reactive atoms gain stability by electron interactions (chemical reactions) • Electrons can be lost to empty the outermost shell • Electrons can be gained to fill the outermost shell • Electrons can be shared with another atom where both atoms have full outermost shells Hydrogen and oxygen atoms gain stability by interacting with each other Single electrons from each of two hydrogen molecules fill the outer shell of an oxygen atom Attractive forces (chemical bonds) hold atoms together in molecules Ions and Ionic Bonds Atoms that have lost electrons become positively charged ions (e.g. sodium: Na+) Atoms that have gained electrons become negatively charged ions (e.g. chlorine: Cl-) Oppositely charged ions are attracted to each other are bound into a molecule by ionic bonds Covalent Bonds Atoms with partially full outer electron shells can share electrons Two electrons (one from each atom) are shared in a covalent bond Covalent bonds are found in H 2 (single bond), O2 (double bond), N2 (triple bond) and H2O Covalent bonds are stronger than ionic bonds but vary in their stability Polar Covalent Bonds In diatomic molecules like H2, both atoms exert the same pulling force on bond electrons: the covalent bond is nonpolar Atoms within a molecule may have different nuclear charges Those atoms with greater positive nuclear charge pull more strongly on electrons in a covalent bond Polar Covalent Bonds In molecules where atoms of different elements are involved (H2O), the electrons are not always equally shared: these covalent bonds are polar A molecule with polar bonds may be polar overall H2O is a polar molecule • The (slightly) positively charged pole is around each hydrogen • The (slightly) negatively charged pole is around the oxygen Hydrogen Bonds Polar molecules like water have partially charged atoms at their ends Hydrogen bonds form when partial opposite charges in different molecules attract each other • The partially positive hydrogens of one water molecule are attracted to the partially negative oxygen on another Hydrogen Bonds Polar biological molecules can form hydrogen bonds with water, each other, or even within the same molecule Hydrogen bonds are rather weak but can collectively be quite strong Water Interacts with Many Molecules Water is an excellent solvent • A wide range of substances dissolve in water to form solutions Water-soluble molecules are hydrophilic • Water molecules are attracted to and can surround ions or polar molecules (dissolving them), such as sugars and amino acids Water Interacts with Many Molecules Water-insoluble molecules are hydrophobic • Water molecules repel and drive together uncharged and nonpolar molecules like fats and oils • The “clumping” of nonpolar molecules is called hydrophobic interaction Water Molecules Tend to Stick Hydrogen bonding between water molecules produces high cohesion • Water cohesion explains how water molecules can form a chain in delivering moisture to the top of a tree Cohesion of water molecules along a surface produces surface tension • Fishing spiders and water striders rely on surface tension to move across the surface of ponds Water Molecules Tend to Stick Water molecules stick to polar or charged surfaces in the property called adhesion • Adhesion helps water climb up the thin tubes of plants to the leaves Acid, Basic, and Neutral Solutions A small fraction of water molecules break apart into ions: H2O OH- + H+ Solutions where H+ > OH- are acidic • e.g. Hydrochloric acid ionizes in water: HCl H+ + Cl• Lemon juice and vinegar are naturally produced acids Acid, Basic, and Neutral Solutions Solutions where OH- > H+ are basic • e.g. Sodium hydroxide ionizes in water: NaOH Na+ + OH• Baking soda, chlorine bleach, and ammonia are basic The degree of acidity of a solution is measured using the pH scale • • • pHs 0-6 are acidic (H+ > OH-) pH 7 is neutral (H+ = OH-) pH 8-14 is basic (OH- > H+) Buffers Maintain Constant pH A buffer is a compound that accepts or releases H+ in response to pH change The bicarbonate buffer found in our bloodstream prevents pH change If the blood becomes too acidic, bicarbonate accepts (and absorbs) H+ to make carbonic acid HCO3H2CO3 bicarbonate carbonic acid + H+ hydrogen ion Buffers Maintain Constant pH If the blood becomes too basic, carbonic acid liberates hydrogen ions to combine with OH- to form water H2CO3 + OH HCO3+ H 2O carbonic acid water hydroxide ion bicarbonate Water Stabilizes Temperature Very low or very high temperatures may damage enzymes or slow down important chemical reactions Water moderates the effect of temperature change • Temperature reflects the speed of molecular motion • It requires 1 calorie of energy to raise the temperature of 1g of water 1oC (the specific heat of water), so it heats up very slowly Water Stabilizes Temperature Water requires a lot of energy to turn from liquid into a gas (heat of vaporization) • Evaporating water uses up heat from its surroundings, cooling the nearby environment (as occurs during sweating) • Because the human body is mostly water, a sunbather can absorb a lot of heat energy without sending her/his body temperature soaring Water Stabilizes Temperature Water requires a lot of energy to be withdrawn in order to freeze (heat of fusion) Water freezes more slowly than other liquids Most substances become denser when they solidify from a liquid Ice is unusual because it is less dense than liquid water Water molecules spread apart slightly during the freezing process
