Unit 6 Skill Builder: Writing Ionic FormulasName ______

Date ______Period ____

BACKGROUND: A chemical formula is a way to show the types of elements present in a compound, and the ratio of those elements. Two examples: NaCl and MgBr2. NaCl is the formula for sodium chloride. The elements sodium and chlorine are present in a 1:1 ratio. MgBr2 is the formula for magnesium bromide. The elements magnesium and bromine are present in a 1:2 ratio. For ionic compounds, the ratio of metal to nonmetal ions is determined by what it takes for each element to become stable. There are a couple of

methods that can be used to determine the correct formula for an ionic compound.

PURPOSE: To learn and practice the skill of formula writing.

Method One: Electron Dot diagrams (aka Lewis structures)

The electron dot diagram shows only the electrons in the last level (the valence electrons that are involved in bonding).

Look at the example.The example shown is for sulfur. Since sulfur is in column VIA (“6A”), a sulfur atom has 6 valence e-.So 6 dots are drawn around the symbol for sulfur.

PROCEDURE A. Draw the electron dot diagram for the following elements:

a) sodium
b) chlorine
c) calcium
d) oxygen
e) aluminum
f) barium
g) bromine / Example:

S

The transfer of electrons can be shown with electron dot diagrams. When an ionic compound is formed the metal loses electrons to form a positive ion and the nonmetal gains electrons to form a negative ion. Each loses or gains electrons until the ion formed has a full or stable octet of electrons. An arrow is used to show the transfer of electrons.

Look at the example. The example is for sodium sulfide. Each sodium (Na) atom needs to lose 1e-. But a sulfur atom needs to gain 2e-. So it will take 2 sodium atoms to make 1 sulfur atom stable. So the ratio is 2:1. The formula shows this by placing a subscript of 2 following sodium’s symbol.

PROCEDURE B. Draw the electron dot diagrams to predict the formulas for the following compounds:

a) sodium chloride
b) sodium oxide
c) calcium chloride
d) calcium oxide
e) aluminum chloride
f) aluminum oxide
g) barium bromide
h) barium oxide /

Method Two: Ionic Charge

Charges on the ionscan also be used to predict the formula of the ionic compound. The ions combine in simple whole number ratios to form a neutral compound. The sum of the charges on the ions must add up to

be zero.

PROCEDURE C. Use the charges on the ions to predict the formulas for the following compounds. Write both the name and the formula. Use the charges on the ions to determine the ratio of metal ions to nonmetal ions required toget a sumof zero. Write in subscripts to reflect the ratio.

a) calcium iodided) aluminum oxideg) aluminum chloride

b) magnesium chloridee) lithium sulfideh) sodium chloride

c) barium sulfidef) potassium bromidei) magnesium iodide

The charge method also allows you to predict the formulas for compounds

involvingtransition metals and polyatomic ions.

Transition metalsdo not follow our patterns of the periodic table. Located on the periodic table in the B columns, these metals lose a variable number of electrons. The nomenclature to name the compound must indicate which ion is present. A Roman numeral in parenthesis following the element name shows the charge on the ion.

Example: Iron may lose either two or three electrons. If it loses two electrons, an ion with a charge of positive two is formed and named iron (II) – Fe(II). If it loses three electrons, an ion with a charge of positive three is formed and named iron (III) – Fe(III). So, iron (III) chloride must contain the Fe+3 ion.

PROCEDURE D. Use this nomenclature and the charges on the ions to predict the formula for the following compounds. Write both the name and the formula.

a) copper (II) chloridef) lead (II) nitrate

b) iron (III) oxideg) lead (IV) oxide

c) tin (IV) chlorideh) lead (II) iodide

d) nickel (III) chloridei) iron (III) chloride

e) gold (III) sulfidej) copper (I) sulfate

Polyatomic ionsare a group of atoms held together by a covalent bond with a net charge. The table below shows the most common polyatomic ions.

Nitrate / Sulfate / Carbonate / Phosphate / Hydroxide / Ammonium
NO3- / SO42- / CO32- / PO43- / OH- / NH4+

PROCEDURE D. Use the charges for the polyatomic ions/ions to predict the formulas for the following compounds. Write both the name and the formula.

a) calcium nitratef) ammonium chloride

b) potassium sulfateg) aluminum hydroxide

c) barium sulfateh) lithium phosphate

d) sodium carbonatei) calcium phosphate

e) magnesium hydroxidej) sodium sulfate