1 Chapter 3 Chemical Compounds. 2 Chemical Formulas; Molecular and Ionic Substances The chemical formula of a substance is a notation using atomic symbols

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Chapter 3

Chemical Compounds

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Chemical Formulas; Molecular and Ionic Substances

The chemical formula of a substance is a notation using atomic symbols with subscripts to show the relative proportions of atoms of the different elements in a substance.

Consider the formula of aluminum oxide, Al2O3. This formula implies that the compound is composed of aluminum atoms and oxygen atoms in the ratio 2:3.

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A molecule is a definite group of atoms (two or more nonmetals) that are chemically bonded together – that is, tightly connected by attractive forces.

Chemical Formulas; Molecular Substances

Molecular substances

A molecular substance is a substance that is composed of molecules, all of which are alike.

A molecular formula gives the exact number of atoms of elements in a molecule.

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Molecular Compounds

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Naming Binary Molecular Compounds

• Binary molecular compounds consist of molecules that contain atoms of only two elements (nonmetals).

• In binary molecular compounds the more metallic element is listed first

Increasing metallic character

Increasin

g metallic ch

aracter

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Naming Binary Molecular Compounds

Nonmetal + Nonmetal

Prefixes

mono =1 hexa = 6

di = 2 hepta = 7

tri = 3 octa = 8

tetra = 4 nona = 9

penta =5 deca = 10

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Binary Molecular Compounds

(prefix)name of the first nonmetal +

(prefix)modified name of the second nonmetal

Oxygen - ox + ide = oxideChlorine - chlor + ide = chlorideFluorine – fluor + ide = fluorideHydrogen – hydr + ide = hydrideNitrogen – nitr + ide = nitrideSulfur – sulf + ide = sulfide

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Binary Molecular Compounds

CO - carbon monoxide

SO2 – sulfur dioxide

P2O5 – diphosphorus pentoxide

S2Cl2 – disulfur dichloride

N2O – dinitrogen monoxide

use prefixes in front of both elements; exception mono is never used in front of the first element

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Compounds that consist of ions

Chemical Formulas; Ionic Substances

Ionic substances

An ion is an electrically charged particle obtained from an atom or chemically bonded group of atoms by adding or removing electrons.

When an atom loses an electron it becomes a positively charged - cationWhen an atom gains an electron it becomes a negatively charged - anion

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Formation of Ionic Compound, NaCl

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NaCl “table salt”

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The formula of an ionic compound is written by giving the smallest possible whole-number ratio of different ions in the substance.

Chemical Formulas; Ionic Substances

The formula unit of the substance is the group of atoms or ions explicitly symbolized by its formula.

Sodium chloride NaCl , not Na6Cl6

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Naming Binary Ionic CompoundsMetal + Nonmetal

Name of the metal + modified name of a nonmetal

Oxygen - ox + ide = oxideChlorine - chlor + ide = chlorideFluorine – fluor + ide = fluorideHydrogen – hydr + ide = hydrideNitrogen – nitr + ide = nitrideSulfur – sulf + ide = sulfide

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Naming Binary Ionic Compounds

Charges Group 1A – 1+Group 2A – 2+Group 3A – 3+Group 5A – 3-Group 6A – 2-Group 7A – 1-

Main group metallic elements have the charge equal to the group number.

Most nonmetallic elements have the charge equal to the group number – 8

( for example oxygen 6 - 8 = - 2)

We do not use prefixes for binary ionic compounds!!!

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Charges on Some Common Monatomic Cations and Anions

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Binary CompoundsMetal + Nonmetal

NaCl - sodium chloride

KBr – potassium bromide

LiI – lithium iodide

MgBr2 – magnesium bromide

Na2O– sodium oxide

Al2O3 – aluminum oxide

The total of charges = 0 Al2O3 2 x (+3) + 3 x (-2) = 0

3+ 2-Al O2 3

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Binary CompoundsMetal + Nonmetal

Transition Metals

FeO - Fe2O3 – CuO – Cu2O -

iron (II) oxideiron (III) oxidecopper (II) oxidecopper (I) oxide

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Mass and Moles of a Substance

The molar mass of a substance is the mass of one mole of a substance.

For all substances, molar mass, in grams per mole, is numerically equal to the formula weight in atomic mass units.

That is, one mole of any element weighs its atomic mass in grams.

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Molar Mass of Compounds

Calculate the molar mass of butane C4H10

C = 12.01 g/mol × 4 = 48.04 g/mol

H = 1.01 g/mol × 10 = 10.10 g/mol

C4H10 = 58.14 g/mol

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Determining Chemical Formulas

The percent composition of a compound is the mass percentage of each element in the compound.

We define the mass percentage of “A” as the parts of “A” per hundred parts of the total, by mass. That is,

%100whole the of mass

whole in A"" of mass A"" % mass

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Mass Percentages from Formulas

Let’s calculate the percent composition of butane, C4H10.

C % 82.63% 100g 58.14g 48.04

C %

4 moles of carbon × 12.01g/mol = 48.04 g

10 moles of hydrogen × 1.01 g/mol = 10.10 g

Total mass of 1 mol of C4H10 = 58.14 g

H % 17.37% 100g 58.14

g 10.10H %

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C

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We can calculate the percent composition by mass of a compound if we know its formula.

If we know the percent composition of a compound we can determine its empirical formula.

An empirical formula (or simplest formula) for a compound is the formula of the substance in in which the ratio of atoms or ions gives the smallest possible integer value

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Benzoic acid is a white, crystalline powder used as a food preservative. The compound contains 68.8% C, 5.0% H, and 26.2% O by mass. What is its empirical formula?

Cx HyOz

In other words, give the smallest whole-number

ratio of the subscripts (moles) in the formula

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For the purposes of this calculation, we will assume we have 100.0 grams of benzoic acid.

Since x, y, and z in our formula represent mole-mole ratios, we must first convert mass to moles.

Cx HyOz

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3.501.63(8)C 728.5 mol

3.021.63(8)H mol95.4

1.001.63(8)O )8(63.1 mol

The empirical formula is C7H6O2 .

To calculate the ratio of moles:

Multiply each number by 2 to get integers.

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Determining the molecular formula from the empirical formula.

An empirical formula gives only the smallest whole-number ratio of atoms in a formula.

The molecular formula could be a multiple of the empirical formula

To determine the “true” molecular formula,

we must know the molar mass of the compound.

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C6H6

C2H6

C3H7O2

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A compound is 39.9% C, 6.70% H and 53.4% oxygen. The molecular weight of this compound is 60.0 g/mol. Determine the molecular formula of this compound.

CxHyOz

Documents

1 Chapter 3 Chemical Compounds. 2 Chemical Formulas; Molecular and Ionic Substances The chemical formula of a substance is a notation using atomic symbols