Unit 1 Examples Covalent Bonds A covalent bond is the sharing of a pair of outer shell electrons by two atoms For example each of these hydrogen atoms has one electron in its outer shell but needs two electrons to complete its outer shell If the two hydrogen atoms share electrons they can both complete their outer shells The shared pair of electrons constitutes a covalent bond shown in shorthand as a line The covalently bonded hydrogen atoms form a molecule of hydrogen gas A molecule is defined as two or more atoms held together by covalent bonds a An oxygen atom needs two electrons to complete its outer shell Two oxygen atoms can share two pairs of electrons A molecule of oxygen gas is held together by a double covalent bond two shared pairs of electrons b A carbon atom needs four electrons to complete its outer shell It can share electrons with four hydrogen atoms forming a methane molecule containing four single covalent bonds Methane is a compound a substance formed by the combination of two or more elements We call methane natural gas it is the fuel burned in gas stoves and furnaces c An oxygen atom needs two additional electrons to fill its outer shell Thus it can form two single covalent bonds An oxygen atom can share electrons with two hydrogen atoms forming a molecule of water containing two single covalent bonds Structure of the Atomic Nucleus These are five atoms that are important in living things The nucleus of each atom has a characteristic number of protons which are positively charged and neutrons which are electrically neutral Each atom of a particular element has the same 1 P a g e unique number of protons This is the element s atomic number The number of particles in the nucleus protons plus neutrons is called the mass number Most elements have two or more isotopes whose atoms vary a bit in neutron number and mass number Quick Facts Atomic number is the number of protons in an atom Atoms with the same atomic number and different atomic masses are referred to as isotopes Atomic mass is equal to the number of protons plus the number of neutrons Electron Arrangement A cloud of negatively charged electrons surrounds the nucleus In an uncharged atom the number of electrons is equal to the number of protons Electrons occupy specific energy levels called electron shells shown here as concentric circles The innermost shell can hold up to two electrons the next few shells can hold up to eight electrons apiece The chemical properties of an atom depend mostly on the number of electrons in its outermost electron shell An atom is most stable when it has a full outermost shell two electrons for small atoms such as hydrogen and eight electrons for larger atoms such as carbon or chlorine By transferring or sharing electrons atoms complete their outer shells These interactions between atoms create chemical bonds 2 P a g e Diversity of Carbon Based Molecules Compounds composed of only carbon atoms and hydrogen atoms are called hydrocarbons The carbon skeletons of hydrocarbons can vary in many ways as demonstrated by these ball and stick models For each pair of molecules we have shown how the molecule on the right differs from the molecule on the left Quick Facts A carbon atom can form up to four covalent bonds Functional Groups The characteristics of an organic molecule depend not only on its carbon skeleton but also on groups of atoms with distinctive properties called functional groups attached to the skeleton This is an organic molecule called a tripeptide 3 P a g e Functional Group A specific configuration of atoms commonly attached to the carbon skeletons of organic molecules and usually involved in chemical reactions Hydroxyl Group O H This functional group is polar because the electronegative oxygen has a strong attraction for electrons Water is attracted to this group and therefore molecules that have this group such as sugars tend to dissolve in water Alcohols are characterized by this functional group Carbonyl Group C O This group is polar because oxygen has a strong attraction for electrons If this group is at the end of a carbon skeleton the molecule is called an aldehyde otherwise it is called a ketone Carboxyl Group O C O H The two electronegative oxygens of this group pull electrons away from the hydrogen atom This weakens the bond between oxygen and hydrogen and the hydrogen atom tends to dissociate from the molecule as a hydrogen H ion Because it donates hydrogen ions this group is acidic and molecules that contain these groups are known as carboxylic acids Amino Group H N H The slightly electronegative nitrogen atom in this group tends to pick up hydrogen ions from the surrounding solution and thus this functional group acts as a base Because of the added hydrogen ion this group usually has a 1 charge in the cell This group is characteristic of organic molecules called amines Sulfhydryl Group S H Molecules termed thiols are characterized by this group which resembles a hydroxyl group These groups interact to help stabilize the structure of many proteins Phosphate Group PO4 The electronegative oxygens of this functional group draw electrons and as a result the group acts as an acid losing hydrogen ions to the surrounding solution This dissociation leaves the group with a negative charge This group is important in ATP and the transfer of energy between organic molecules 4 P a g e Quick Facts A carbonyl group in the middle of a carbon skeleton characterizes ketones By donating hydrogen ions carboxyl groups act as an acid Hydroxyl groups are characteristic of alcohol Some biologically important functional groups include the hydroxyl group the carbonyl group the carboxyl group the amino group the sulfhydryl group and the phosphate group Phosphate groups for example are an important part of the structure of ATP adenosine triphosphate an energy storage molecule in cells Protein Functions Proteins the purple blobs in this close up of an animal cell are the most complicated molecules known A cell contains thousands of kinds of proteins which carry out a variety of functions In most cases a protein s function depends on its complex three dimensional structure Note how function depends on protein shape and changes in shape Hormone Receptor Signal proteins include hormonal proteins that help coordinate an organism s activities by acting as signals between cells For example insulin a hormonal protein secreted by the pancreas signals an animal s cells to take in and use