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The Periodic Table, Valence Electrons and Bonding How Elements Form Compounds • I CAN describe chemical bonding and determine an ionic chemical formula. Chemical Bonding • Chemical Bonding is the joining of two or more atoms to form a compound. • Bonding occurs when atoms transfer electrons between them, or, in some cases, move close together and share electrons between them. Valence Electrons • During bonding, the only electrons involved are those at the outer edge of the atom’s electron cloud. • These are called VALENCE ELECTRONS. • Atoms can have between 1 and 8 Valence Electrons [Ve-]. • When atoms come into close contact with each other, VALENCE ELECTRONS can be TRANSFERRED from one atom to another. • Typically, atoms on the LEFT SIDE of the periodic table [the METALS] LOSE electrons. • Atoms on the RIGHT SIDE of the periodic table [the NON-METALS] gain electrons. • The number of electrons GAINED or LOST depends on the GROUP NUMBER the atom is in: • LOSE ELECTRONS GAIN ELECTRONS • Group 1 loses 1e• Group 2 loses 2 e• Groups 13 loses 3 e- Group 15 gains 3 eGroup 16 gains 2 eGroups 17 gains 1 e- Atoms become IONS • When an atom gains or loses electrons and there is a change in its overall charge, the atom is transformed into another form called an ION • POSITIVE IONS are called CATIONS and are formed from METALS. • NEGATIVE IONS are called ANIONS and are formed from NON-METALS. Charges of IONS • Family 1 ions = +1 • Family 2 ions = +2 • Family 13 ions = +3 Losing electrons is called OXIDATION and make + charged ions. [Family 14 bonds in a different way – more later] • Family 15 ions = -3 • Family 16 ions = -2 • Family 17 ions = -1 Gaining electrons is called REDUCTION and make - charged ions. • [Family 18 doesn’t form chemical bonds.] IONIC BONDING • IONIC BONDING occurs between two IONS with OPPOSITE CHARGES. • Remember OPPOSITES ATTRACT! Characteristics of IONIC Compounds • Generally consist of a METAL CATION and a NON-METAL ANION. • High melting points • Almost always solid at room temperature • Brittle • Generally not very flammable • Dissolve in water [electrolytes] Forming an IONIC Compound • What would be the formula for a compound resulting from ionic bonding between Potassium and Bromine? • STEPS • 1. Find the elements in the Periodic Table and write down their symbols. • 2. Use the element’s location on the Periodic Table to predict the charge of its ION. • 3. Criss-cross the charges (NO SIGNS) and multiply. • 4. Write the FORMULA. • Write the symbols and charges in a BRACKET above the symbols in this manner: [ +1 ] + [ -1 ] = 0 K Br • BALANCE the charge by criss-crossing the numbers [don’t worry about charge] and multiplying each bracket. 1 [ +1 ] + 1 [ -1 ] = 0 K Br • To write the FORMULA, • Write the symbol of the POSITIVE ION first, the NEGATIVE ION second. – Place the number you MULTIPLIED the bracket by AFTER and slightly BELOW the symbol. – This number is called a SUBSCRIPT. 1[ +1 ] + 1[ -1 ] = 0 K Br K1 Br1 • If the number is ONE, it is understood and you DO NOT have to write it. – Think of X in math….1X = X • So the formula for a compound of Potassium and Bromine is: • KBr • PROBLEM • What is the formula for an ionic compound of Magnesium and Chlorine? • Determine the symbol and charge of both elements and cross the numbers: • 1 [+2 ] + 2 [-1 ] = 0 • Mg Cl • Mg1Cl2 = MgCl2 • Remember ONE is not usually written as a subscript! Problem What is the formula of an ionic compound of Aluminum and Oxygen? •[ +3] + [ -2 ] • Al O •[ +3] + • Al [ -2 ] O • +6 -6 •2 [ +3] + 3 [ -2 ] = 0 • Al O • So the formula becomes: • Al2O3