Life’s Chemical Basis Chapter 2

Building Block of Matter Atoms: Tiny, indivisible particles, the building blocks of matter Made up of neutrons (n0) in the nucleus protons (p+) in the nucleus electrons (e-) orbiting around the nucleus A typical atom has the same number of protons and electrons Atomic Number: number of protons found in nucleus of the atom – IT NEVER CHANGES

Building Blocks of Matter Element: pure substance that consists only of atoms with the same number of protons Periodic Table: tabular arrangement of all known elements by their atomic number

Building Blocks of Matter Although all atoms of an element have the same number of protons, they can differ in the number of other subatomic particles Isotopes: forms of an element that differ in the number of neutrons their atoms carry Mass Number: total number of protons and neutrons in the atomic nucleus Radioisotope: isotope with an unstable nucleus

Building Blocks of Matter Ion: an atom that has gained/lost electrons in order to achieve a full valence shell, ions carry a charge If it gives up an electron, it becomes positively charged If it gains an electron, it becomes more negative

Building Blocks of Matter Chemical Bond: attractive force that arises between two atoms when their electrons interact Compound: a pure substance formed when two or more different elements combine Each has a chemical formula (H2O) and formed by a specific combination of elements in a fixed ratio

Building Blocks of Matter Ionic bond: an electrical attraction between two oppositely charged atoms or groups of atoms Ions retain their respective charges when in an ionic bond, therefore, one end of the ionic bond has a positive charge, and the other has a negative charge

Building Blocks of Matter Polarity: separation of charge into positive and negative regions Electronegativity: an atom’s ability to pull electrons away from other atoms

Building Blocks of Matter Covalent Bond: forms when a pair of electrons is shared Formed between atoms with a small or no difference in electronegativity Covalent bonds are often stronger than ionic bonds, but not always

Building Blocks of Matter Covalent bonds in compounds are usually polar When atoms share electrons unequally in covalent bond, they are called polar covalent Example: water molecules Atoms that share electrons equally are nonpolar covalent

Water H2O has unique properties that arise from the two polar covalent bonds in each molecule The oxygen atom carries a slight negative charge; the hydrogen atoms carry a slight positive charge The polarity of individual H2O molecules attracts them to one another in a hydrogen bond Hydrogen bond: attraction between a covalently bonded hydrogen atom and another atom taking part in a separate polar covalent bond

Water Hydrogen bonds form and break more easily than covalent or ionic bonds, BUT collectively are quite strong Hydrogen bonds stabilize DNA and protein structures Extensive hydrogen bonding among H2O molecules gives liquid H2O several special properties that make life possible

Water 1. Water is a Universal Solvent Solvent: substance in which another is dissolved Solute: the substance that is dissolved Substances that dissolve easily in water are hydrophilic (water loving) Ionic solids, such as sodium chloride (NaCl), and nonionic solids (e.g., sugars) dissolve in water Substances that resist dissolving in water are hydrophobic (e.g., oils)

Water 2. Water has Cohesion Cohesion: tendency of molecules to resist separating from one another Water has cohesion because hydrogen bonds collectively exert a continuous pull on its individual molecules REQUIRES ENERGY The strong attraction to other water creates high surface tension. This causes droplets to form.

Water 3. Water has Adhesion Adhesion: force of attraction between unlike molecules It forms hydrogen bonds with other molecules on other surfaces (think of tape) This causes capillary action (water travelling up the stems of a plant)

Water 4. Water Stabilizes Temperature Because of hydrogen bonding, it takes more heat to raise the temperature of water compared with other liquids Below 0°C (32°F), water molecules become locked in the bonding pattern of ice Sheets of ice that form on the surface of ponds, lakes, and streams insulate the water Protects aquatic organisms during cold winters

Acids & Bases Remember, water is a universal solvent because of its polarity Your body is about 70% water When water is liquid, some of its molecules spontaneously separate into hydrogen ions (H+) and hydroxide ions (OH–)

Acids & Bases Acid: Substance that releases hydrogen ions (H+) when dissolved in water The more hydrogen released, the more acidic the solution becomes Base: Substance that releases hydroxide ions (OH-) when dissolved in water The more hydroxide released, the more basic the solution becomes

pH Scale pH: measure of hydrogen ions in a solution Bases accept hydrogen ions in water, pH > 7 Acids release hydrogen ions in water, pH < 7 The closer a substance is to 7, the weaker it is

Acids & Bases Acids and bases are key substances in biology. Most biological molecules can function properly only within a narrow range of pH – HOMEOSTASIS! Buffers: mixtures that can react with acids or bases to keep the pH within a particular range. Antacids, blood

Acids & Bases The fluids inside cells stay within a consistent range of pH because they are buffered Buffer failure can be catastrophic in a biological system Burning fossil fuels such as coal releases sulfur and nitrogen compounds that affect the pH of rain

Mercury Rising Mercury is a naturally occurring element, mostly safely locked away in rocks Volcanic and human activity release it into the atmosphere and it combines with carbon to form methylmercury This easily crosses skin and mucous membranes, causing damage to the nervous system, brain, kidneys, and other organs

Mercury Rising The toxin can build up to high levels if even small amounts are ingested on a regular basis, and can take months or years to be cleared from the body Large predatory fish have a lot of mercury U.S. Environmental Protection Agency recommends that adult humans ingest less than 0.1 microgram of mercury per kilogram of body weight per day