Formulas for ionic compounds Chemical Names and Formulas for ionic compounds

Significance of a Chemical Formula Review Chemical formulas form the basis of the language of chemistry and reveal much information about the substances they represent. To describe the atomic makeup of compounds chemists use systematic methods for naming compounds and for writing chemical formulas.

Review A chemical formula indicates the relative number of atoms of each kind in a chemical compound. For a molecular compound, the chemical formula gives the number of atoms of each element contained in a single molecule of the compound. Example: octane C8H18 The subscript after the H indicates that there are 18 hydrogen atoms in the molecule. The subscript after the C indicates that there are 8 carbon atoms in the molecule.

Example: aluminum sulfate Al2(SO4)3 The chemical formula for an ionic compound represents one formula unit – the simplest ratio of the compound’s (+) ions and (-) ions. Example: aluminum sulfate Al2(SO4)3 Parentheses surround the polyatomic ion (SO4) to identify it as a unit. The subscript 3 refers to the unit. Al2(SO4)3 3 sulfur atoms and 12 oxygen atoms 2 aluminum atoms Ionic compounds exist as crystalline solids. Aluminum sulfate crystal

Ions A positive ion, called a cation, is produced when one or more electrons are lost from a neutral atom. Metals tend to lose electrons, thus forming positive ions or cations. When a neutral atom gains electrons, an ion with a negative charge is formed, called an anion. Nonmetals tend to gain electrons, thus forming negative ions, or anions.

mono (one) Na+ Monatomic Ions Monatomic ions are ions formed from a single atom. mono (one) Na+ Monatomic cations are identified by the element’s name (Na+ is sodium). When naming monatomic anions, you drop the ending of the element’s name and add –ide. F-, Fluorine → Fluoride

The periodic table is very useful when we want to know what type of ion is formed by a given atom. Note that the Group 1 atoms all form 1+ ions and the Group 2 metals all form 2+ ions.

Many of the transition metals lose different numbers of electrons and follow the Stock System of naming ions and elements. Example: copper(I) → Cu+ or copper(II) → Cu2+ copper(I) ion – lost one electron copper(II) ion – lost two electrons

Binary Ionic Compounds Binary ionic compounds are compounds composed of two different elements. The total number of positive and negative charges must be equal. For any ionic compound, Total charge of cations + Total charge of anions = Zero net charge Use your ion handout to help you with the charges on ions.

Polyatomic Ions In a polyatomic ion, two or more atoms are covalently bonded together. N O

1– N O Together, they carry a charge.

When balancing charges in an ionic compound, the polyatomic ions acts as a single unit of charge. 1- 1- 2+ N O N O Mg Mg(NO3)2

1- 1- 2+ N O N O Mg Mg(NO3)2 2+

Mg(NO3)2 2+ 2- 1- 1- 2+ the charges are balanced Remember a parentheses is placed around the polyatomic ion to identify it as a unit.

Naming Binary Ionic Compounds The nomenclature, or naming system, or binary ionic compounds involves combining the names of the compound’s positive and negative ions. The name of the cation is given first, followed by the name of the anion (-ide). Example: Al2O3 aluminum oxide

Some elements such as iron, form two or more cations with different charges. The Stock System of nomenclature uses a Roman numeral to indicate an ion’s charge. The numeral is enclosed in parentheses and placed immediately after the metal’s name. Example: Fe2+ - iron(II), Fe3+ - iron(III).

Example: write the name for the following compound. SnI4 First determine the charge on the Sn ion. The iodine ion has a charge of 1- and the formula contains 4 I- ions therefore, the overall negative charge is 4-. In order for the charges to equal the Sn ion must have a charge of 4+. The name would be tin(IV) iodide.

Compounds containing Polyatomic Ions Most polyatomic ions are negatively charged and are oxyanions – contain oxygen. Compounds containing polyatomic ions are named in the same way as binary ionic compounds. Name the cation first, then the anion. Example: Ca(OH)2 Name cation first, then the anion. calcium hydroxide

If more than one oxyanion is formed by the same two elements (Ex If more than one oxyanion is formed by the same two elements (Ex. N and O) the most common ion is given the ending – ate. The one with one less oxygen ends in –ite. Example: NO2- - nitrite, NO3-, nitrate. Some elements can form more than two types of oxyanions. Example: chlorine can form ClO- (hypochlorite) ClO2- (chlorite) ClO3- (chlorate) ClO4- (perchlorate)