Covalent BondsExamples:Structure of the Atomic NucleusQuick FactsElectron ArrangementDiversity of Carbon-Based MoleculesQuick FactsFunctional GroupsQuick FactsProtein FunctionsProtein StructureHow Enzymes WorkQuick FactsNucleic Acid Building BlockStructure of RNA and DNAQuick Facts:Role of the Nucleus and Ribosomes in Protein SynthesisThe Triplet Nature Of The Genetic CodeHow Genetic Information ChangesQuick FactsPhage Lytic CyclePhage Lysogenic and Lytic CyclesEnergy TransformationsQuick FactsChemical Reactions and ATPQuick FactsRedox ReactionsQuick FactsEnzyme and Substrate ConcentrationsHow Enzymes FunctionEnzyme InhibitionRegulating Enzyme ActionEnzymes & Pathways: Controlling EnzymesQuick FactsTypes of CarbohydratesQuick FactsCellular RespirationNet Input and OutputLocation of Cellular RespirationGlycolysisAcetyl CoA Formation and Citric Acid CycleOxidative PhosphorylationSummaryFermentationQuick FactsEnergy Flow in PlantsPhotosynthesisInputs and OutputsThe Light ReactionsThe Calvin CycleQuick QuestionThe Light ReactionsQuick FactsEnergy Processes: How Photosynthesis Yields SugarHow Photosynthesis Relates to Cellular RespirationPhotosynthesis in Dry ClimatesWater Transport in Plants: The Transpiration-Cohesion-Tension MechanismUnit 1Covalent BondsA 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.Examples: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 eunique 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 neutronsElectron ArrangementA 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 eDiversity of Carbon-Based MoleculesCompounds 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 bondsFunctional GroupsThe 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 eFunctional 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 moleculeas 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