The Number of Atoms in a Formula

1 Given the formula of a chemical compound (or a name from which the formula may be written), state the number of atoms of each element in the formula unit. 

In writing the formula of a substance, subscript numbers are used to indicate the number of atoms or groups of atoms of each element in the formula unit of the substance. There is one exception If that number is one, it is omitted. The formula of sodium nitrate is NaN03. The formula unit contains one sodium atom, one nitrogen atom, and three oxygen atoms. 

The formula of calcium nitrate is Ca(N03)2- How many atoms of each element are in this formula Applying the information from the preceding paragraph, you 

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As the number of atoms in a formula increases, the number of possible constitutional isomers increases dramatically. As an illustration, consider the series of hydrocarbons (compounds made up of only carbon and hydrogen) shown in Table 2.3. Although there is only one compound with the formula CH4, there are 75 constitutional isomers with the formula C10H22 (all 75 have been prepared in the lab and identified) and more than 4 billion with the formula C30H62 ... 

For compounds, entropy increases with chemical complexity, that is, with the number of atoms in a formula unit or molecule of the compound. This trend holds for both ionic and covalent substances, as long as they are in the same phase ... 

Molar mass is important when we need to know the number of atoms in a sample. It would be impossible to count out 6 X ID23 atoms of an element, but it is very easy to measure out a mass equal to the molar mass of the element in grams. Each of the samples shown in Fig. E.2 was obtained in this way each sample contains the same number of atoms of the element (6.022 X 1023), but the masses vary because the masses of the atoms are different (Fig. E.4). The same rule applies to compounds. Flence, if we measure out 58.44 g of sodium chloride, we obtain a sample that contains 1.000 mol NaCl formula units (Fig. E.5). 

Berzelius method of assigning letters to represent elements was also applied to compounds. Berzelius used superscripts to denote the number of atoms in a compound. Thus, water would be O and carbon dioxide CO. Later these superscripts were changed to the current practice of designating the number of atoms using subscripts. The absence of a subscript implies the subscript one. Using symbols we can write the chemical formula for ammonia ... 

Note that the subscripts in a molecular formula represent the number of atoms in a molecule. Since a molecule of CuS04 has four oxygen atoms, the relative mass of oxygen must be multiplied by four and added to the relative mass of one copper atom and one sulfur atom to find the relative mass of a mole of CuS04, copper sulfate molecules. Two atoms of potassium, four atoms of oxygen, and one atom of chromium must be accounted for in potassium chromate, K2Cr04. Students should calculate the mass of one mole of each of the molecules needed, convert each to 0.1 mole (multiply by... 

Dalton proposed a series of chemical symbols corresponding to each element or, in different contexts, to one atom of each element, and drew groups of these symbols to represent molecules. This meant that chemical formulas could represent the number of atoms in a compound, not merely the relative combining weights. 

Although a molecular formula shows the number of atoms in a compound, it does not show the type of bonding between atoms. We can show how atoms are bonded to each other by using structural formula. 

You make unit conversions everyday when you determine how many quarters are needed to make a dollar or how many feet are in a yard. One unit that is often used in calculations in chemistry is the mole. Chapter 11 shows you equivalent relationships among mole, grams, and the number of representative particles (atoms, molecules, formula units, or ions). For example, one mole of a substance contains 6.02 X 10 representative particles. Try the next example to see how this information can be used in a conversion factor to determine the number of atoms in a sample of manganese. 

An empirical formula gives the simplest whole number ratio of atoms in a chemical compound. In contrast, a molecular formula specifies the number of atoms in a molecule. Two or more substances may have the same empirical formula but different... 

The chemical formula for a substance shows its chemical composition. This represents the elements present as well as the ratio in which the atoms of the elements occur. The formula for a single atom is the same as the symbol for the element. Thus, Na can represent a single sodium atom. It is unusual to find such isolated atoms in nature, with the exception of the noble gases (He, Ne, Ar, Kr, Xe, and Rn). A subscript following the symbol of an element indicates the number of atoms in a molecule. For instance, F2 indicates a molecule containing two fluorine atoms, and a molecule containing four phosphorus atoms. 

These numbers approximate the numbers of atoms in a molecule, the small deviations from integral values resulting from the approximate nature of the molar mass measurement. The molecular formula, C4H6O2, is obtained without going through the intermediate evaluation of an empirical formula. 

I determine the number of atoms in a molecule from its chemical formula. 

The mole is often referred to as a chemist s unit of quantity. Counting atoms is a difficult process and beyond the scope of most calculators, but measuring the mass of a sample is easy when we can relate the number of atoms in a sample to its mass. This is the unique purpose of the mole. A mole of any substance is its molecular formula weight expressed in grams. Avogadro s number s a universal constant that states the number of molecules in a mole Nq = 6.023 x 10 molecules/mole. One mole (abbreviated mol) of any element (chemical compound) has the same number of chemical particles as one mole of another element (chemical compound). In other words, 1 mole of any compound contains 6.02 x 10 molecules. Review the following problem using the mole concept. 

A mole is used to measure the mass of a sample by relating the number of atoms in a sample to its mass. A mole of any substance is its molecular formula weight expressed in grams. Avogadro s number is a universal constant that states the number of molecules in a mole Nq = 6.023 x 10 molecules/mole. 

As the number of atoms in a molecule increases, molecular weight calculations like this become more and more complex and time-consuming. Fortunately, however, these calculations can be done readily with computers, and tables are now available that give relative values for the + 1 and M + 2 peaks for all combinations of common elements with molecular formulas up to mass 500. Part of the data obtained from one of these tables is given in Table 9.5. Use Table 9.5 to check the results of our example (Fig. 9.43). 

All this led Mulder to the formula C400H620N100O120P1S1 for two of his proteins, and the same for the third, blood serum albumin, but with one more sulphur atom. Now, since no compound can contain less than one atom of any element (unless it is not there at all), this sets a lower limit for the number of atoms in a molecule, and thus for its minimum molecular weight, which for Mulder s three proteins comes out at about 14,400. It is the minimum molecular weight, because the analysis delivers only the proportions of the constituent elements, the true value possibly being 28,800 or 43,200 and so on—assuredly, in any event, a... 

In Chapter 2 (Section 2.9), we saw that an element s molar mass—the mass in grams of one mole of its atoms—is numerically equivalent to its atomic mass. We then used the molar mass in combination with Avogadro s number to determine the number of atoms in a given mass of the element. The same concept applies to compounds. The molar mass of a compound—the mass in grams of 1 mol of its molecules or formula units—is numerically equivalent to its formula mass. For example, we just calculated the formula mass of CO2 to be 44.01 amu. The molar mass is, therefore ... 

The system of indexing molecules is, first, according to the number of atoms in the molecule. Then, with the chemical formula written in what seems a natural way, they are ordered alphabetically in order of the atoms as they appear in the formula. 

Because atoms are neither created nor destroyed, chemists regard each elemental symbol as representing one atom of the element (with the subscripts giving the number of each type of atom in a formula) and then multiply formulas by factors to show the same numbers of atoms of each element on both sides of the arrow. The resulting expression is said to be balanced and is called a chemical equation. For example, there are two H atoms on the left of the preceding skeletal equation but three H atoms on the right. So, we rewrite the expression as... 

Empirical formula — simplest ratio of atoms in a molecule Molecular formula — actual number of atoms in a molecule Structural formula — shows the order of atom linkage... 

Structural formulas also provide information on the way the atoms are arranged and bonded to one another within a molecule. The structural formula of substances not only specifies the type of atoms and how many atoms of each type there are in the molecule of a compound it also provides an outline of the structure of the molecule, pinpointing exactly where each atom is located. Each element in a structural formula is represented by its symbol, and the bonds between atoms are indicated by lines connecting the symbols (see Fig. 60). Thus, structural formulas not only provide information on the type and number of atoms in a molecule of a substance but also depict the internal structure of the molecule of the substance. 

The site and its occupancy are represented by two right lower indexes separated by a comma. The first index indicates the type of site the second one indicates the number of atoms in this site. (Aa, for instance, means an atom A on a site occupied by A in the ideal structure, whereas AB represents an atom A in a site normally (ideally) occupied by B). A formula such as MM)1 x v or (Mi 1 Jv represents a disordered alloy, whereas... 

Dalton went on to explain how these weights can be obtained. It is clear that if one knew the number of atoms in the molecule of a compound, the relative weights of the different atoms could be calculated from the empirically measured composition. Daltons procedure was to provide a set of rules of simplicity by which the formula could be reasonably assumed. 

Molecular formulas merely include the kinds of atoms and the number of each in a molecule (as C4H , for butane). Structural formulas show the arrangement of atoms in a molecule (see Fig. 1-1). When unshared electrons are included, the latter are called Lewis (electron-dot) structures [see Fig. 1-1(/)]. Covalences of the common elements—the numbers of covalent bonds they usually form—are given in Table 1-1 these help us to write Lewis structures. Multicovalent elements such as C, O. and N may have multiple bonds, as shown in Table 1-2. In condensed structural formulas all H s and branched groups are written immediately after the C atom to which they are attached. Thus the condensed formula for isobutane [Fig. l-l(f>)) is CH,CH(CH,)... 

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