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Chemical Formulas ( Molecular Formulas - Empirical Formulas)

** Chemists use Chemical formulas to express the composition of molecules and ionic compounds in terms of chemical symbols.

** By composition we mean not only the elements present but also the ratios in which the atoms are combined.

** Here we are mainly concerned with two types of formulas: molecular formulas and empirical formulas.

Molecular Formulas

** A molecular formula shows the exact number of atoms of each element in the smallest unit of a substance.

** In our discussion of molecules, each example was given with its molecular formula in parentheses. Thus, H2 is the molecular formula for hydrogen, O2 is oxygen, O3 is ozone, and H2O is water.

** The subscript numeral indicates the number of atoms of an element present. There is no subscript for O in H2O because there is only one atom of oxygen in a molecule of water, and so the number “one” is omitted from the formula.

** Note that oxygen (O2) and ozone (O3) are allotropes of oxygen.Allotrope** An allotrope is one of two or more distinct forms of an element.

** Two allotropic forms of the element carbon—diamond and graphite—are dramatically different not only in properties but also in their relative cost.

Molecular Models   

** Molecules are too small for us to observe directly. An effective means of visualizing them is by the use of molecular models.

** Two standard types of molecular models are currently in use: ball-and-stick models and space-filling models (Figure).

(1) ball and- stick models

** In ball and- stick model kits, the atoms are wooden or plastic balls with holes in them.

** Sticks or springs are used to represent chemical bonds.

** The angles they form between atoms approximate the bond angles in actual molecules.

** With the exception of the H atom, the balls are all the same size and each type of atom is represented by a specific color.

(2) space-filling models

** In space-filling models, atoms are represented by truncated balls held together by snap fasteners, so that the bonds are not visible.** The balls are proportional in size to atoms.

** The first step toward building a molecular model is writing the structural formula, which shows how atoms are bonded to one another in a molecule. For example, it is known that each of the two H atoms is bonded to an O atom in the water molecule. Therefore, the structural formula of water is H-O-H.

** A line connecting the two atomic symbols represents a chemical bond.

Difference between ball and- stick models and space-filling models

** Ball-and-stick models show the three-dimensional arrangement of atoms clearly, and they are fairly easy to construct. However, the balls are not proportional to the size of atoms. Furthermore, the sticks greatly exaggerate the space between atoms in a molecule.

** Space-filling models are more accurate because they show the variation in atomic size. Their drawbacks are that they are time-consuming to put together and they do not show the three-dimensional positions of atoms very well.

Empirical Formulas

** The empirical formula tells us which elements are present and the simplest whole-number ratio of their atoms, but not necessarily the actual number of atoms in a given molecule.

** The empirical formula are the simplest chemical formulas;

** they are written by reducing the subscripts in the molecular formulas to the smallest possible whole numbers.

** The word “empirical” means “derived from experiment.” As we will see later, empirical formulas are determined experimentally.

Example(1): H2O2

** The molecular formula of hydrogen peroxide, a substance used as an antiseptic and as a bleaching agent for textiles and hair, is H2O2.

** This formula indicates that each hydrogen peroxide molecule consists of two hydrogen atoms and two oxygen atoms.

** The ratio of hydrogen to oxygen atoms in this molecule is 2:2 or 1:1.

** The empirical formula of hydrogen peroxide is HO.

Example(2): N2H

** The compound hydrazine (N2H4), which is used as a rocket fuel.

** The empirical formula of hydrazine is NH2.Although the ratio of nitrogen to hydrogen is 1:2 in both the molecular formula (N2H4) and the empirical formula (NH2), only the molecular formula tells us the actual number of N atoms (two) and H atoms (four) present in a hydrazine molecule.

Molecular Formulas and Empirical Formulas

** Molecular formulas are the true formulas of molecules.

** If we know the molecular formula, we also know the empirical formula, but the reverse is not true.

** Why, then, do chemists bother with empirical formulas? , when chemists analyze an unknown compound, the first step is usually the determination of the compound’s empirical formula. With additional information, it is possible to deduce the molecular formula.

** For many molecules, the molecular formula and empirical formula are one and the same. Some examples are water (H2O), ammonia (NH3), carbon dioxide (CO2), and methane (CH4).

Formula of Ionic Compounds

** The formulas of ionic compounds are usually the same as their empirical formulas because ionic compounds do not consist of discrete molecular units.

** For example, a solid sample of sodium chloride (NaCl) consists of equal numbers of Na+ and Cl- ions arranged in a three-dimensional network (Figure).

** In such a compound, there is a 1:1 ratio of cations to anions so that the compound is electrically neutral.

** As you can see in Figure above, no Na+ ion in NaCl is associated with just one particular Cl- ion. In fact, each Na+ ion is equally held by six surrounding Cl- ions and vice versa.

** Thus, NaCl is the empirical formula for sodium chloride.

** In other ionic compounds, the actual structure may be different, but the arrangement of cations and anions is such that the compounds are all electrically neutral.

** Note that the charges on the cation and anion are not shown in the formula for an ionic compound.

** In order for ionic compounds to be electrically neutral, the sum of the charges on the cation and anion in each formula unit must be zero.

** If the charges on the cation and anion are numerically different, we apply the following rule to make the formula electrically neutral: The subscript of the cation is numerically equal to the charge on the anion, and the subscript of the anion is numerically equal to the charge on the cation.

** If the charges are numerically equal, then no subscripts are necessary. This rule follows from the fact that because the formulas of most ionic compounds are empirical formulas, the subscripts must always be reduced to the smallest ratios.

** Let us consider some examples:

(1) Potassium Bromide

** The potassium cation K+ and the bromine anion Br- combine to form the ionic compound potassium bromide.

** The sum of the charges is +1 + (-1) = 0, so no subscripts are necessary.

** The formula is KBr. 

(2) Zinc Iodide

** The zinc cation Zn2+ and the iodine anion I- combine to form zinc iodide.The sum of the charges of one Zn2+ ion and one I- ion is +2 + (-1) =  +1.

** To make the charges add up to zero we multiply the -1 charge of the anion by 2 and add the subscript “2” to the symbol for iodine.

** Therefore, the formula for zinc iodide is ZnI2.

(3) Aluminum Oxide

** The cation is Al3+ and the oxygen anion is O2-.The following diagram helps us determine the subscripts for the compound formed by the cation and the anion.

** The sum of the charges is 2(+3) + 3(-2) = 0.

** Thus, the formula for aluminum oxide is Al2O3.

Reference: General Chemistry: The Essential Concepts / Raymond Chang , Jason Overby . ( sixth edition)

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