ATOMS AND MOLECULE. part 2nd
Chemical Formulae
Each chemical compound is known by a specific name. Writing the full name of a compound repeatedly is time-consuming and inconvenient. Therefore, in chemistry each substance is denoted by its chemical formula.
There are two types of chemical formulae. These are
(i) Molecular formula of a compound.
The symbolic representation of a molecule of a compound, representing the actual number of various atoms present in it is called its molecular formula. The molecule of a compound contains two or more than two types of elements. If a molecule of a compound contains two different kinds of elements only, it is called binary compound. For example, water (H2O), sodium chloride (NaCl), iron sulphide (FeS), etc. are binary compounds.
Information Conveyed by a Chemical Formula
By looking at the chemical formula of a substance, we can gather the following information :
- Name of the substance
- Name of various elements present in that substance
- Chemical formula of a substance represents one molecule of that substance
- Relative number of atoms of various elements present in one molecule of that compound
- Relative masses of various elements in the compound
- It represents one mole of that substance
- We can calculate the gram molecular mass of that substance
For example, the chemical formula of CO2 conveys the following information:
- The substance is carbon dioxide.
- Carbon dioxide is composed of the elements: carbon and oxygen.
- In a molecule of carbon dioxide, carbon and oxygen atoms combine together in the ratio 1:2 by number.
- The ratio of mass of carbon and oxygen is 3:8.
- It represents one mole of carbon dioxide molecule.
- The molecular mass of carbon dioxide is 44 a.m.u.
Writing Chemical Formula of Compounds
Chemical formula of a molecular compound represents the actual number of atoms of different elements present in one molecule of the compound, e.g., chemical formula of water is H2O, that of ammonia is NH3, while that of carbon dioxide is CO2 and so on.
Chemical formula of an ionic compound simply represents the ratio of the cations and anions present in the structure of the compound, e.g., the formula Na+ Cl- simply represents that sodium chloride contains Na+ and CI-ions in the ratio of 1:1 (though the actual crystal of sodium chloride consists of very large but equal number of Na+ ions and CI- ions). Similarly, the formula Mg2+(C1-)2, simply tells that it contains
Mg2+ and CI– ions in the ratio of 1: 2 and so on.
Valency of an Element
Valency of an element is defined as the combining capacity of the element. It is equal to the number of hydrogen atoms or number of chlorine atoms or double the number of oxygen atoms with which one In addition to the atoms, the ions which are charged species, also have some valences. Positive ions or cations have positive valences. Negative ions or anions have negative valences. The valences of the polyvalent ions are expressed by enclosing them in bracket and putting the positive or negative signs outside it. Let us write the valences of some commonly used positive and negative ions.
Valencies of Positive lons
Positive ions may be monovalent, bivalent, trivalent, tetravalent etc. depending upon the charge present on them. As shown in fig
Some elements show more than one valences i.e., they show variable valency. In such cases, Roman Numerals are used to denote the valences. These are put in brackets. For example, copper (I) and copper (II); similarly, iron (II) and iron (III).
Valencies of Negative lons
Like positive ions, negative ions may also be monovalent, bivalent, trivalent, etc. in nature. These are shown in the fig. Below
 |
| List of some common Negative ions (Anions) |
Formulae of Tonic Compounds
One of the most important points to remember while writing the formula of a chemical compound is that it is always electrically neutral. In other words, the positive and negative valences of the ions present in the chemical compound add up to zero. To write a formula, follow the steps given below. This method of writing formula is called the criss-cross method. As shown in fig below and the steps involved in writing formula as as under.
- Step 1: Write the symbol of the cation showing the charge on it. Write the symbol of the anion showing the charge on it, on the right hand side of the cation.
- Step II : If a compound contains polyatomic ions, the formula of the ion is enclosed within brackets before criss-crossing the valences.
- Step III : Divide the valency number by common factor, if any, to get simple ratio. Now ignore the (+) and (-) symbols.
- Step IV : Now, write the valency of each atom/radical below its symbol, then cross-over the valences. Thus, the symbol of cation is subscribed with the charge number of the anion and the anion is subscribed with the charge number of the cation. This is called the criss-crossing of valences.
- Step V : If the subscript is 1, it is not written in the final stoichiometric formulae.
Formulae of Molecular Compounds
While writing the chemical formulae of the molecular compounds, we write the constituent elements and their valences as shown below. Then, we cross-over the valences of the combining atoms.as shown in Fig
Mass Percentage Composition of an Element in a Compound.
The percentage composition of an element in a compound can be determined, if we know the molecular mass or unit formula mass and the atomic mass of the element.
If M,is the molecular mass (or unit formula mass) of a compound and A, the atomic mass of the atoms of some particular element, then
Mass percentage composition of the element =
Sum of atomic masses of the atoms of the element /Molecular mass×100
=A/M×100
However, if instead of molecular mass and atomic mass, the mass of compound and the mass of element is given, then percentage composition can be calculated as follows:
If 'W, is the mass of the compound which contain 'w' as the mass of an element then,
Mass of element
Mass percentage composition of the element =Mass of element/Mass of Compound×100
w/W×100
Mole Concept
Quite commonly, we use different units for counting such as dozen for 12 articles, score for 20 articles and gross for 144 articles irrespective of their nature. In a similar way, chemists use the unit 'mole' for counting atoms, molecules ions, etc.. A mole is a collection of 6.022 × 1023 particles. Thus, a mole represents 6.022 x 1023 particles.
The number 6.022 x 1023 is called Avogadro number and is symbolised as NA or No. In other words, a mole is an Avogadro number of particles. For example,
- 1 mole of hydrogen atoms = 6.022 x 10²³ hydrogen atoms
- 1 mole of hydrogen molecules = 6.022 x 10²³ hydrogen molecules
- 1 mole of sodium ions = 6.022 x 10²³ sodium ions
- 1 mole of electrons = 6.022 ×10²³ electrons
A mole of particles may be defined as the amount of substance that contains the same number of entities
(atoms, molecules, ions or other particles), as the number of atoms present in 0.012 kg (or 12 g) of the carbon-12 isotope.
Mole in Terms of Mass
A mole of atoms is defined as the amount of the substance (element) which has mass equal to gram atomic mass (i.e., atomic mass expressed in grams). In other words, it is equal to one gram atom of the element. A mole of molecules is defined as that amount of the substance (element or compound) which has mass equal to gram molecular mass (i.e., molecular mass expressed in grams). In other words, it is equal to one gram molecule of the substance. For example,
If the number of chemical entities of a substance is known, then the number of moles of that substance can be calculated as follows:
Number of chemical entities of a substance(N)
Number of moles (n) =Number of chemical entities of a substance(N)/6.022 x 10²³ (i.e., Avogadro's number, Na)
Or n=N/NA
Mole in Terms of Volume
In case of gaseous substances, it is found that Avogadro's number of molecules (i.e., one mole of molecules) of any gas under standard conditions of temperature and pressure, i.e., STP conditions (0°C and one atmospheric pressure) occupy the same volume which is equal to 22400 mL of 22.4 litres. Hence, A mole of a gaseous substance is defined as that amount of the substance which has volume equal to 22400 mL at STP conditions.
Thus,
- 1 mole of CO2 gas = 22400 mL of CO2 at STP
- 1 mole of NH3 gas = 22400 mL of NH3 at STP
- 1 mole of SO2 gas = 22400 mL of SO2 at STP
Molar Mass and Molar Volume
The mass of 1 mole of the substance (i.e., Avogadro's number of particles) is called molar mass of that substance. If the substance is atomic, its molar mass is equal to gram atomic mass. If the substance is molecular, its molar mass is equal to gram molecular mass. As it is mass of one mole of the substance, its units are gram per mole (g mol-l) or kilogram per mole (kg mol–1). Thus,
- Molar mass of C-12 isotope = 12 g mol-1 or 0.012 kg mol-1
- Molar mass of iron (Fe) = 56 g mol-1
- Molar mass of magnesium (Mg) = 24 g mol-1
- Molar mass of H2 = 2.0 g mol-1
- Molar mass of O2 = 32.0 g mol-1
- Molar mass of H²0 = 18.0 g mol-1
- Molar mass of NH3 = 17.0 g mol-1
- Molar mass is usually represented by the symbol 'M'.
- Thus, MH²,0 = 18 g mol-1 and so on.
The volume occupied by one mole of any gas at STP is always same and equal to 22400 mL or 22.L or 22.4 dm³.This volume is called molar volume or gram molecular volume (G.M.V.).
In general, under STP conditions
Gram molecular mass of gas = Gram molecular volume
= Molar volume 22. 4 L
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