PRESIDENT RAMON MAGSAYSAY STATE UNIVERSITY COLLEGE OF NURSING GENERAL CHEMISTRY chemical symbols & ions IRISH MAE CAÑETE / MARIA ELLAINE LAFUENTE / RODNEY YEPES Introduction IN DEVELOPING OUR IDEAS ABOUT THE COMPOSITION OF ATOMS IN THE PREVIOUS SECTION, WE USED THE FACT THAT ATOMS ARE ELECTRICALLY NEUTRAL TO CONCLUDE THAT THE NUMBERS OF PROTONS AND ELECTRONS MUST BE EQUAL. WHEN THE NUMBER OF PROTONS AND THE NUMBER OF ELECTRONS DO NOT MUCH, THE RESULT IS A SPECIES WITH A NET CHARGE, CALLED AN ION. SUCH SPECIES ARE NO LONGER ATOMS, AND BEHAVIOUR IS MARKEDLY DIFFERENT FROM THAT OF ATOMS. Intended Learning Outcomes AT THE END OF THIS MODULE, IT IS EXPECTED THAT THE LEARNER SHOULD BE ABLE TO: A. DESCRIBE THE BASIC FEATURES OF IONIC AND COVALENT BONDS; B. USE THE PERIODIC TABLE TO DETERMINE WHETHER AN ATOM FORMS A CATION OR AN ANION, AND DETERMINE ITS CHARGE USING THE GROUP NUMBER; AND C. RECOGNIZE THE STRUCTURES OF COMMON POLYATOMIC IONS AND NAME COMPOUNDS THAT CONTAIN THEM discussion Symbols EACH OF THE ELEMENTS HAS BEEN ASSIGNED A CHEMICAL SYMBOL THAT WE CAN THINK OF AS A SHORTHAND WAY OF REPRESENTING THE ELEMENT. THE SYMBOL CONSIST OF ONE OR TWO LETTERS THAT USUALLY BEAR A RESEMBLANCE TO THE ENGLISH NAME OF THE ELEMENT. FOR INSTANCE, CARBON = C, CHROMIUM = CR, CHLORINE = CL, CALCIUM = CA, AND ZINC = ZINC. NOTICE THAT THE FIRST LETTER IS CAPITALIZED, BUT IF THERE IS A SECOND LETTER, IT IS NOT. SOME OF THE ELEMENTS HAVE SYMBOLS HAVE LATIN NAMES. SOME EXAMPLES ARE POTASSIUM (L KALUIM) = K, SODIUM ( NATRIUM) = NA, SILVER (ARGENTIUM) = AG, MERCURY (HYDRARGYRUM) = HG, AND COPPER (CUPRUM) = CU.(CF. THE PERIODIC TABLE OF ELEMENTS). Ions WHEN AN IONS IS DERIVED FROM A SINGLE ATOM IT IS CALLED A MONOATOMIC ION. WHEN GROUPS OF ATOMS CARRY A CHARGE THEY ARE CALLED POLYATOMIC IONS. MONOATOMIC OR POLYATOMIC IONS MAY CARRY EITHER NEGATIVE OR POSITIVE CHARGES. NEGATIVELY CHARGED IONS ARE CALLED ANIONS, AND THEY CONTAIN MORE ELECTRONS THAN PROTONS. SIMILARLY, AN ION WITH MORE PROTONS THAN ELECTRONS HAS A POSITIVE CHARGE AND IS CALLED A CATION. Monatomic ions WE WRITE SYMBOLS FOR IONS ANALOGOUSLY TO THOSE FOR ATOMS, ADDING THE CHARGE ON THE SPECIES AS A SUPERSCRIPT TO THE RIGHT OF THE ATOMIC SYMBOL. FOR MONOATOMIC IONS, THE NUMBER OF PROTONS STILL SPECIFIES THE SYMBOL WE USE. TABLE 6.1 PROVIDES SOME EXAMPLES OF MONOATOMIC IONS. NOTICE THAT THE MONOATOMIC ANIONS HAVE NAMES ENDING IN –IDE, WHEREAS CATIONS SIMPLY HAVE “ION” ADDED TO THE NAME OF THE ELEMENT Table 6.1. 1 Examples of Monoatomic Ions MANY MONOATOMIC CATIONS AND ANIONS EXIST. THESE IONS CAN EXIST IN THE GAS PHASE, AND MANY ARE IMPORTANT IN ATMOSPHERIC CHEMISTRY. BUT WE ENCOUNTER IONS MOST FREQUENTLY WHEN DEALING WITH THE CHEMISTRY OF SUBSTANCES DISSOLVED IN WATER. FOR EXAMPLE, SODIUM ATOMS LOSE AN ELECTRON RELATIVELY EASILY TO FORM THE SODIUM CATION, NA+. BECAUSE IT STILL HAS 11 PROTONS, THIS ION RETAINS THE SYMBOL OF SODIUM, YET IT DOES NOT BEHAVE AT ALL LIKE AN ATOM OF SODIUM. POLYATOMIC IONS SOMETIMES IONS ARE COMPOSED OF MORE THAN ONE ELEMENT. THE ION BEARS A CHARGE BECAUSE THE TOTAL NUMBER OF ELECTRONS IT CONTAINS IS DIFFERENT FROM THE TOTAL NUMBER OF PROTONS IN THE NUCLEI OF ALL OF THE ATOMS. • A POLYATOMIC ION IS A CATION OR ANION THAT CONTAINS MORE THAN ONE ATOM. DISSOLVING NACL IN WATER THE ATOMS IN THE POLYATOMIC ION ARE HELD TOGETHER BY COVALENT BONDS, BUT SINCE THE ION BEARS A CHARGE, IT BONDS TO OTHER IONS BY IONIC BONDING. FOR EXAMPLE, CALCIUM SULFATE, CASO4, IS COMPOSED OF A CALCIUM CATION, CA2+, AND THE POLYATOMIC ANION SULFATE, SO42–. CASO4 IS USED TO MAKE PLASTER CASTS FOR BROKEN BONES. WE WILL ENCOUNTER ONLY TWO POLYATOMIC CATIONS: H3O+, THE HYDRONIUM ION, WHICH WILL PLAY A KEY ROLE IN THE ACID–BASE CHEMISTRY, AND NH4+, THE AMMONIUM ION. IN CONTRAST, THERE ARE SEVERAL COMMON POLYATOMIC ANIONS, MOST OF WHICH CONTAIN A NONMETAL LIKE CARBON, SULFUR, OR PHOSPHORUS, USUALLY BONDED TO ONE OR MORE OXYGEN ATOMS. COMMON EXAMPLES INCLUDE CARBONATE (CO32), SULFATE (SO42–), AND PHOSPHATE (PO43–). TABLE 6.2 LISTS THE MOST COMMON POLYATOMIC ANIONS. THE NAMES OF MOST POLYATOMIC ANIONS END IN THE SUFFIX -ATE. EXCEPTIONS TO THIS GENERALIZATION INCLUDE HYDROXIDE (–OH) AND CYANIDE (–CN). TWO OTHER ASPECTS OF NOMENCLATURE ARE WORTHY OF NOTE. • THE SUFFIX -ITE IS USED FOR AN ANION THAT HAS ONE FEWER OXYGEN ATOM THAN A SIMILAR ANION NAMED WITH THE -ATE ENDING. THUS, SO42– IS SULFATE, BUT SO32– IS SULFITE. • WHEN TWO ANIONS DIFFER IN THE PRESENCE OF A HYDROGEN, THE WORD HYDROGEN OR THE PREFIX BI- IS ADDED TO THE NAME OF THE ANION. THUS, SO42– IS SULFATE, BUT HSO4– IS HYDROGEN SULFATE OR BISULFATE. TABLE 6.1.2 NAMES OF COMMON POLYATOMIC ANIONS mathematical DESCRIPTION THE BEHAVIOR AND INTERACTION OF ELECTRICAL CHARGES ARE IMPORTANT TOPICS IN PHYSICS, BUT THEY ALSO PROVIDE A BASIS FOR THINKING ABOUT MANY ASPECTS OF CHEMISTRY. FOR OUR CURRENT INTERESTS, WE WILL POINT OUT JUST TWO FUNDAMENTAL IDEAS ABOUT ELECTRICAL CHARGE. FIRST, OPPOSITE CHARGES ATTRACT EACH OTHER AND LIKE CHARGES REPEL ONE ANOTHER. AND SECOND, ELECTRICAL CHARGE IS CONSERVED. THESE TWO IDEAS HAVE IMPORTANT IMPLICATIONS FOR THE FORMATION IF IONS IN CHEMICAL PROCESS. FIRST OF ALL, BECAUSE CHARGED IS CONSERVED, WE CAN SAY THAT IF A NEUTRAL ATOM OR MOLECULE IS TO BE CONVERTED INTO AN ION, SOME OPPOSITELY CHARGED PARTICLE – MOST LIKELY AN ELECTRON OR ANOTHER ION – MUST BE PRODUCED AT THE SAME TIME. MOREOVER, BECAUSE OPPOSITE CHARGES ATTRACT ONE ANOTHER, SOME ENERGY INPUT IS ALWAYS NEEDED TO CONVERT A NEUTRAL ATOM OR MOLECULE INTO A PAIR OF OPPOSITELY CHARGED PARTICLES. THE STATEMENT THAT “OPPOSITES ATTRACT AND LIKES REPEL” CAN BE QUANTIFIED MATHEMATICALLY. COULOMB’S LAW DESCRIBES THE INTERACTION OF CHARGED PARTICLES. THE ATTRACTION OF OPPOSITE CHARGES AND THE REPULSION OF LIKE CHARGES ARE BOTH DESCRIBED MATHEMATICALLY BY ONE SIMPLE EQUATION: HERE AND ARE THE CHARGES, IS A CONSTANT CALLED THE PERMITTIVITY OF A VACUUM, AND R IS THE DISTANCE BETWEEN THE CHARGES. F IS THE FORCE THE OBJECTS EXERT ON ONE ANOTHER AS A RESULT OF THEIR CHARGES. CATIONS AND ANIONS · THERE ARE TWO TYPES OF IONS CALLED CATIONS AND ANIONS. CATIONS ARE POSITIVELY CHARGED IONS. A CATION HAS FEWER ELECTRONS THAN PROTONS. ·ANIONS ARE NEGATIVELY CHARGED IONS. AN ANION HAS MORE ELECTRONS THAN PROTONS. THE NATURE AND MAGNITUDE OF THE CHARGE ON AN ION DEPEND ON THE POSITION OF AN ELEMENT IN THE PERIODIC TABLE. IN FORMING AN ION, AN ATOM OF A MAIN GROUP ELEMENT LOSES OR GAINS ELECTRONS TO OBTAIN THE ELECTRONIC CONFIGURATION OF THE NOBLE GAS CLOSEST TO IT IN THE PERIODIC TABLE. THIS GIVES THE ION AN ESPECIALLY STABLE ELECTRONIC ARRANGEMENT WITH A COMPLETELY FILLED SHELL OF ELECTRONS; THAT IS, THE ELECTRONS COMPLETELY FILL THE SHELL FARTHEST FROM THE NUCLEUS. FOR EXAMPLE, SODIUM (GROUP 1A) HAS AN ATOMIC NUMBER OF 11, GIVING IT 11 PROTONS AND 11 ELECTRONS IN THE NEUTRAL ATOM. THIS GIVES SODIUM ONE MORE ELECTRON THAN NEON, THE NOBLE GAS CLOSEST TO IT IN THE PERIODIC TABLE. IN LOSING ONE ELECTRON, SODIUM FORMS A CATION WITH A +1 CHARGE, WHICH STILL HAS 11 PROTONS, BUT NOW HAS ONLY 10 ELECTRONS IN ITS ELECTRON CLOUD. WHAT DOES THIS MEAN IN TERMS OF VALENCE ELECTRONS? A NEUTRAL SODIUM ATOM, WITH AN ELECTRONIC CONFIGURATION OF 1S22S22P63S1, HAS A SINGLE VALENCE ELECTRON. LOSS OF THIS VALENCE ELECTRON FORMS A SODIUM CATION, SYMBOLIZED AS NA+, WHICH HAS THE ESPECIALLY STABLE ELECTRONIC CONFIGURATION OF THE NOBLE GAS NEON, 1S22S22P6. THE SODIUM CATION NOW HAS EIGHT ELECTRONS THAT FILL THE 2S AND THREE 2P ORBITALS. MAGNESIUM (GROUP 2A) HAS 12 PROTONS AND 12 ELECTRONS IN THE NEUTRAL ATOM. THIS GIVES MAGNESIUM TWO MORE ELECTRONS THAN NEON, THE NOBLE GAS CLOSEST TO IT IN THE PERIODIC TABLE. IN LOSING TWO ELECTRONS, MAGNESIUM FORMS A CATION WITH A +2 CHARGE, WHICH STILL HAS 12 PROTONS, BUT NOW HAS ONLY 10 ELECTRONS IN ITS ELECTRON CLOUD. IN TERMS OF VALENCE ELECTRONS, A NEUTRAL MAGNESIUM ATOM, WITH AN ELECTRONIC CONFIGURATION OF 1S22S22P63S2, HAS TWO VALENCE ELECTRONS. LOSS OF THESE VALENCE ELECTRONS FORMS A MAGNESIUM CATION, SYMBOLIZED AS MG2+, WHICH HAS THE ESPECIALLY STABLE ELECTRONIC CONFIGURATION OF THE NOBLE GAS NEON, 1S22S22P6. THE MAGNESIUM CATION NOW HAS EIGHT ELECTRONS THAT FILL THE 2S AND THREE 2P ORBITALS. + + +SODIUM AND MAGNESIUM ARE EXAMPLES OF METALS. • METALS ARE FOUND ON THE LEFT SIDE OF THE PERIODIC TABLE. • METALS FORM CATIONS. • BY LOSING ONE, TWO, OR THREE ELECTRONS, AN ATOM FORMS A CATION WITH A COMPLETELY FILLED OUTER SHELL OF ELECTRONS.