IONIC COMPOUNDS CHAPTER 6

IONIC COMPOUNDS • IONIC COMPOUNDS ARE MADE UP BY THE CHEMICAL COMBINATION OF METALLIC AND NON-METALLIC ELEMENTS. • MOST ROCKS, MINERALS AND GEMSTONES ARE IONIC COMPOUNDS. • CERAMICS, BRICKS AND KITCHEN CROCKERY ARE MADE FROM CLAYS WHICH CONTAIN IONIC COMPOUNDS. • WHILE MOST OF THE ABOVE ARE MADE UP OF MIXTURES OF DIFFERENT IONIC COMPOUNDS TABLE SALT IS A PURE IONIC COMPOUND MADE UP OF SODIUM CHLORIDE (NACL)

PROPERTIES OF IONIC COMPOUNDS • THINK OF THE PROPERTIES OF ROCKS, BRICKS, CROCKERY AND TABLE SALT. WHAT PROPERTIES DO THEY SHARE? • HAVE HIGH MELTING AND BOILING TEMPERATURES. • ARE HARD BUT BRITTLE

• THEY ALSO: • DO NOT CONDUCT ELECTRICITY IN THE SOLID STATE • THEY WILL ONLY CONDUCT ELECTRICITY IF THEY ARE MELTED OR DISSOLVED IN WATER

STRUCTURE • FROM THE PROPERTIES WE CAN CONCLUDE: • THE FORCES BETWEEN THE PARTICLES ARE STRONG. • THERE ARE NO FREE-MOVING ELECTRONS PRESENT, UNLIKE IN METALS. • THERE ARE CHARGED PARTICLES PRESENT, BUT IN SOLID STATE THEY ARE NOT FREE TO MOVE. • WHEN AN IONIC COMPOUND MELTS, HOWEVER, THE PARTICLES ARE FREE TO MOVE AND THE COMPOUND WILL CONDUCT ELECTRICITY.

THE IONIC BONDING MODEL CHEMISTS BELIEVE THAT WHEN METALLIC AND NON-METALLIC ATOMS REACT TO FORM IONIC COMPOUNDS THE FOLLOWING STEPS OCCUR: • METAL ATOMS LOSE ELECTRONS TO NON-METALLIC ATOMS AND BECOME POSITIVELY CHARGED METAL IONS. • NON-METAL ATOMS GAIN ELECTRONS FROM THE METAL ATOMS AND SO BECOME NEGATIVELY CHARGED NON-METAL IONS. • LARGE NUMBERS OF POSITIVE AND NEGATIVE IONS FORMED IN THIS WAY THEN COMBINE TO FORM A THREE-DIMENSIONAL LATTICE. • THE THREE DIMENSIONAL LATTICE IS HELD TOGETHER STRONGLY BY ELECTROSTATIC FORCES OF ATTRACTION BETWEEN POSITIVE AND NEGATIVE IONS. THIS ELECTROSTATIC FORCE IS CALLED IONIC BONDING.

How many chlorine ions surround each sodium ion and vice versa?

HIGH MELTING TEMPERATURE • EVER NOTICED THAT WHEN YOU EAT FRIES THE FOOD MAY BE HOT BUT THE SALT DOES NOT MELT. • THIS IS BECAUSE TO MELT AND IONIC SOLID ENERGY MUST BE PROVIDED TO ALLOW THE IONS TO BREAK FREE AND MOVE. • NACL HAS A HIGH MELTING TEMP, THIS INDICATES A LARGE AMOUNT OF ENERGY IS NEEDED TO REDUCE THE ELECTROSTATIC ATTRACTION BETWEEN THE OPPOSITELY CHARGED IONS AND ALLOW THEM TO MOVE FREELY.

HARDNESS AND BRITTLENESS • UNLIKE METALS IONIC COMPOUNDS ARE NOT MALLEABLE. THEY BREAK WHEN BEATEN. • A FORCE CAN DISRUPT THE STRONG ELECTROSTATIC FORCES HOLDING THE LATTICE IN PLACE. • A SODIUM CHLORIDE CRYSTAL CANNOT BE SCRATCHED EASILY BUT IF A STRONG FORCE (A HAMMER BLOW) IS APPLIED IT WILL SHATTER. • THIS IS BECAUSE THE LAYERS OF IONS WILL MOVE RELATIVE TO EACH OTHER DUE TO THE FORCE. • DURING THIS MOVEMENT, IONS OF LIKE CHARGE WILL BECOME ADJACENT TO EACH OTHER. RESULTING IN REPULSION

HARDNESS AND BRITTLENESS

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Figure 6.4 The repulsion between like charges causes this sodium chloride crystal to shatter when it is hit sharply.

ELECTRICAL CONDUCTIVITY • IN THE SOLID FORM, IONS IN SODIUM CHLORIDE ARE HELD IN THE CRYSTAL LATTICE AND ARE NOT FREE TO MOVE SO CANNOT CONDUCT ELECTRICITY. • WHEN THE SOLID MELTS THE IONS ARE FREE TO MOVE. • THE MOVEMENT OF THESE CHARGED PARTICLES TO AN ELECTRODE COMPLETES AN ELECTRICAL CIRCUIT. • IN A SIMILAR WAY, WHEN SODIUM CHLORIDE DISSOLVES IN WATER, THE IONS SEPARATE AND ARE FREE TO MOVE TOWARDS THE OPPOSITE CHARGE.

CONDUCTING ELECTRICITY

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IONIC COMPOUNDS • SO THE METAL ATOMS LOSE AN ELECTRON TO THE NON-METAL ATOMS. • IN DOING SO, BOTH ATOMS WILL OFTEN ACHIEVE THE ELECTRONIC CONFIGURATION OF THE NEAREST NOBLEST GAS, WHICH IS PARTICULARLY STABLE.

ELECTRON TRANSFER DIAGRAMS • WHEN SODIUM AND CHLORIDE REACT TOGETHER SODIUM LOSES AN ELECTRON AND CHLORINE GAINS AN ELECTRON.

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NOTES: • WHEN A NON-METAL ATOM GAINS ONE OR MORE ELECTRONS, THE NAME OF THE NEGATIVE ION ENDS IN –IDE. • WHEN A METAL ATOM LOSES ONE OR MORE ELECTRONS THE NAME OF THE POSITIVE ION IS THE SAME AS THE METAL AND IS ALWAYS NAMED FIRST. • FOR EXAMPLE: SODIUM CHLORIDE

POLYATOMIC IONS • SOME IONS CONTAIN MORE THAN ONE ATOM. • THESE ARE CALLED POLYATOMIC IONS. • THEY INCLUDE NITRATE (NO3-) AND HYDROXIDE (OH-). WHAT ELSE? • IF MORE THAN ONE OF THESE IONS IS USED TO BALANCE THE CHARGE OF A COMPOUND, THEN IT IS PLACED IN BRACKETS WITH THE REQUIRED NUMBER WRITTEN AS A SUBSCRIPT AFTER THE BRACKETS. FOR EXAMPLE MG(NO3)2 AND AL(OH)3 • BRACKETS ARE NOT REQUIRED FOR THE FORMULA OF SODIUM NITRATE NANO3, WHERE THERE IS ONLY ONE NITRATE ION PRESENT FOR EACH SODIUM ION.

METALLIC BONDS

METALS • METALS CONSIST OF CLOSELY PACKED CATIONS FLOATING IN A “SEA OF ELECTRONS”. • ALL OF THE ATOMS ARE ABLE TO SHARE THE ELECTRONS. • THE ELECTRONS ARE NOT BOUND TO INDIVIDUAL ATOMS.

CATIONS PACKED IN “A SEA OF ELECTRONS”

PROPERTIES OF METALS • GOOD CONDUCTORS

• DUCTILE • MALLEABLE • ELECTRONS ACT AS A LUBRICANT, ALLOWING CATIONS TO MOVE PAST EACH OTHER

• METALS HAVE A CRYSTALLINE STRUCTURE

WHY IS A METAL DUCTILE BUT AN IONIC COMPOUND IS NOT?

ALLOYS • MIXTURES OF TWO OR MORE ELEMENTS, AT LEAST ONE OF WHICH IS A METAL. • MADE BY MELTING, MIXING, THEN COOLING THE METALS. • MAY CONTAIN NON-METALS LIKE CARBON.

ALLOYS • PROPERTIES ARE SUPERIOR TO THEIR COMPONENTS. • STERLING SILVER • 92.5% AG, 7.5% CU • HARDER THAN SILVER

• BRONZE • 7:1 CU TO SN (TIN)

• STAINLESS STEEL

• FE 80.6%, CR 18%, C 0.4%, NI 1% • CAST IRON -FE 96%, C 4%