Ionic Bonding

Ionic Bonding Occurs when electrons are transferred from one atom to another, forming two ions The ions stay together because of electrostatic attractions Ionic bonds NEVER form molecules Ionic bonds form easily between alkali metals and halogens

The Octet Rule Atoms tend to gain, share, or lose electrons in order to obtain a full set of valence electrons (in most cases this equals 8) An octet of electrons consists of full s and p sublevels on an atom. Exceptions: transition elements and rare earth elements

Example - + Na Cl Na + Cl +

Properties of Ionic Compounds Ionic compounds do not form molecules; they form a crystal lattice The green spheres are Na+ and the red spheres are Cl -

This is a crystal of CaCl2 This is a crystal of CaCl2. Each ion is held rigidly in place by strong electrostatic forces that bond it to several oppositely charged ions

Other Properties Normally form between metals and nonmetals Ionic compounds have ions that form very strong bonds, which makes them hard and brittle They have high melting points and high boiling points When dissolved in water, the solution will conduct electricity

Types of Ionic Compounds Ionic compounds will be a combination of a metal and a nonmetal (if the cation is monatomic) There are two types of ionic compounds Binary Ionic Compounds: contains the ions of only two elements (NaCl, CaCl2) Polyatomic Ionic Compounds: contain at least one polyatomic ion (CaCO3, Mg(OH)2) The names DO NOT indicate the ratio of ions present, but the formulas do

Types of Ions There are two types of ions Monatomic: cation or anion that consists of a single atom. Examples: Na+ and Cl- Polyatomic: two or more atoms that act as a single ion (or particle). Examples: (CO3)2- and (OH)-

Names of Charged Particles When electrons are lost, the ion has an overall positive charge and is called a cation Examples: Na+, Ca2+ When electrons are gained, the ion has an overall negative charge and is called an anion Examples: F-, S2- The negative ions will attract the positive ions and form a bond All ionic compounds are electrically neutral

Ionic Charges Monatomic ions Ions that consist of only one atom Charges often can be determined by using the periodic table Many of the transition metals have more than one charge The charge on the ion is indicated by using a Roman numeral next to the name of the element Cu has a 1+ and a 2+ charge. Cu 1+ is called Copper I, and Cu2+ is called Copper II A few transition metals have only one charge The names of these do not have to include a Roman numeral Zn, Cd

Atoms that commonly form ions Monatomic Ions Group Atoms that commonly form ions Charge on ions 1 H, Li, Na, K, Rb, Cs 1+ 2 Be, Mg, Ca, Sr, Ba 2+ 13 B, Al 3+ 15 N, P, As 3- 16 O, S, Se, Te 2- 17 F, Cl, Br, I 1-

Ionic Compounds Writing formulas for binary ionic compounds Compounds composed of two elements are called binary compounds When the formula is written, the charge of the cation must be balanced by the charge of the anion The overall charge of the ion combination must be zero The cation is always written first in the formula

Writing Formulas for Ionic Compounds To write the formula for an ionic compound: Write the chemical symbol and overall charge of the cation or polyatomic cation: Na+, Ca2+, NH4+ Write the chemical symbol and overall charge of the anion or polyatomic anion: Cl-, O2-, NO3- Add the charges If equal, write the chemical symbols together, e.g. NaCl, CaO, NH4NO3 If not equal, crisscross values of the charges and make them subscripts, e.g. Ca(NO3)2, CaCl2

Crisscross Method (also called “Drop and Swap” Na+ can combine with S2- The value of the charge on Na, which is 1, becomes the subscript for S: S The value of the charge on S, which is 2, becomes the subscript for Na: Na2 The resulting formula is Na2S

Naming Ionic Compounds Naming binary ionic compounds It is important to know the Stock naming system and the charges on cations before naming ionic compounds The process is the reverse of writing formulas

Polyatomic ionic compounds Compounds that contain atoms of three different elements They usually contain a polyatomic ion; must be recognized first to be able to name the compound correctly Tightly bound groups of atoms that behave as a unit and carry a charge Have the suffix –ite and –ate These ions will be given to you on a sheet; you will not have to memorize them Hydroxide and cyanide are the only common polyatomic ions that end in –ide

Common Polyatomic Ions Name NH4+ Ammonium NO2- Nitrite NO3- Nitrate OH- Hydroxide CO32- Carbonate SO42- Sulfate O22- Peroxide C2H3O2- Acetate SO32- Sulfite ClO3- Chlorate You will get these on a chart for tests and quizzes!!

How Do I Write the Formulas for Polyatomic Ions? Put parentheses around any polyatomic ion and add subscripts to the outside of the parentheses. Never change the subscript of a polyatomic ion (it will change the composition of the ion): Ca(OH)2, Ca3(PO4)2 Include the subscript inside the parentheses

Practice Sodium combines with fluorine Chlorine combines with hydrogen The nitrite ion combines with calcium The ammonium ion combines with nitrogen

Percentage Composition The percent by mass of any element in a compound can be found by dividing the mass of the element by the mass of the compound and multiplying by 100% Example: 65g of element X and 45g of element Y in 110 g of compound %X = (65g ÷ 110g) x 100% = 59% %Y = (45g ÷ 110g) x 100% = 41%

Examples: What is the percentage of oxygen in H2O? What is the percentage calcium in Ca(OH)2?

Representing Chemical Compounds The Laws of Definite and Multiple Proportions must be obeyed: Law of Definite Proportions States that in samples of any chemical compound, the masses of the elements are always in the same proportions (e.g. H and O always combine as H2O to form water) Law of Multiple Proportions Whenever element combine to form a compound, they will always combine in small, whole-number ratios with each other (e.g. H2O, CO2, H2CO3, etc)

What is molar mass? Mass of 1 mole of a compound Measured in grams It is the mass in grams of one mole of a given substance It is equal to the average atomic mass of that element, written in grams It depends on the masses of the elements that make up the substance

How to find molar mass Find the molar mass of Na2SO4 Find the molar mass of MgSO4 · 7H2O

How do I convert from moles to mass? You must use molar mass How many moles of water are in 18.01 g of water? What is the mass of 15.2 moles of CO2?

What is empirical formula? Empirical formula: gives the simplest whole number ratio of the atoms of the elements Gives the ratio of the atoms, but doesn’t necessarily give a correct molecular formula

To find empirical formulas To calculate: Convert the mass (g) of each element in the compound to moles Compare the moles in a ratio Put the ratio in lowest terms by dividing by the smallest number in the ratio Round the ratio to whole numbers It is easy to go from % composition to empirical formulas by assuming the percentage is the amount of element in grams (i.e., drop the % and replace with a “g”).

Examples What is the empirical formula of the compounds below? 59.95% O; 40.05% S 48.64% C; 8.16% H; 43.20% O

Molecular vs Empirical Formulas Molecular gives actual number of atoms of each element in a molecule Always a whole # multiple of empirical formula Empirical gives ratios of each element in each compound Subscripts always in lowest possible whole number

To Determine Molecular Mass Divide the molar mass of the unknown compound by the molar mass of the empirical formula This lets you know what multiple of the empirical formula the formula of the unknown is molecular formula = (empirical formula)n

Example A compound was found to contain 65.45% C, 5.45% H, and 29.09% O. The molar mass of the compound is 110.0 g/mol. What is the molecular formula?

Example A colorless liquid composed of 46.68% N and 53.32% O has a molar mass of 60.01 g/mol. What is the molecular formula?