In the previous standard we have learnt that compounds are formed by chemical combination of elements. We have also learnt that an important principle of Dalton’s atomic theory is that molecules of a compound are formed by joining atoms of different elements to each other.
Laws of chemical combination
The composition of a substance changes during a chemical change. The fundamental experiments in this regard were performed by scientists in the 18th and 19th century. While doing this, they measured accurately, the substances used and formed and discovered the laws of chemical combination. Scientists could then write the molecular formulae of various compounds on the basis of Dalton’s atomic theory and the laws of chemical combination. Here we shall verify the laws of chemical combination by means of known molecular formulae.
Apparatus : Conical flask, test tubes, balance, etc.
Chemicals : Calcium chloride (CaCl2 ), sodium sulphate (Na2 SO4 ), calcium oxide (CaO), Water (H2 O)
Law of conservation of matter
In the above activities, the mass of the original matter and the mass of the matter newly formed as a result of the chemical change are equal. In 1785, the French Scientist Antoine Lavoisier inferred from his research that ‘there is no rise or drop in the mass of the matter during a chemical reaction.’ In a chemical reaction the total mass of the reactants is same as the total mass of the products formed due to the chemical reactions and this is called the law of conservation of matter.
Law of constant proportion
In 1794 the French scientist J. L. Proust stated the law of constant proportions as “The proportion by mass of the constituent elements in the different samples of a compound is fixed,” e.g., the proportion by mass of hydrogen and oxygen in water is obtained from any source 1:8. This means that 9 g water is formed by chemical combination of 1 g hydrogen and 8 g oxygen. Similarly, the proportion by mass of carbon and oxygen in carbon dioxide obtained from any source is 3:8. This means that in 44 g of carbondioxide there is 12 g of carbon and 32 g of oxygen so that the proportion by mass of carbon and oxygen is 3:8.
Verification of the law of constant proportion
Many compounds can be made by different methods. For example, two samples of the compound copper oxide, CuO, were obtained, one by decomposition of copper carbonate, CuCO3 , and another by decomposition of copper nitrate, Cu(NO3 )2 . From each of these samples, a mass of 8g of copper oxide was taken and each was treated independently with hydrogen gas. Both gave 6.4 g copper and 1.8 g water. Let us see how does this is a verification of the law of constant proportion.
The reaction of copper oxide with hydrogen yielded two known substances, namely, the compound water and the element copper. It is known that, in the compound water, H2 O, the elements H and O are in the proportion 1:8 by mass. This means that in 9g water there are 8g of the element oxygen. Therefore, 1.8g water contains (8×1.8/9 = 1.6)g oxygen. This oxygen has come from 8g copper oxide. It means that 8g of both the samples of copper oxide contained 6.4g copper and 1.6g oxygen; and the proportion by mass of copper and oxygen in it is 6.4:1.6, that is, 4:1. Thus, the experiment showed that the proportion by mass of the constituent elements in different samples of a compound is constant. Now let us see what the expected proportion by mass of the constituent elements of copper oxide would be from its known molecular formula CuO. To find out this, we need to use the known atomic masses of the elements. The atomic masses of Cu and O are 63.5 and 16 respectively. This means that the proportion by mass of the constituent elements Cu and O in the compound CuO is 63.5 : 16 which is 3.968:1, or approximately 4:1.
The experimental value of proportion by mass of the constituent elements matched with the expected proportion calculated from the molecular formula. Thus, the law of constant proportion is verified.
Atom : size, mass and valency
We have learnt that at the centre of an atom is the nucleus and that there are moving electrons in the extra-nuclear part. The electrons are negatively charged elementary particles while the elementary particles that make up the nucleus are positively charged protons and electrically neutral neutrons.
The size of an atom is determined by its radius. The atomic radius of an isolated atom is the distance between the nucleus of an atom and its outermost orbit. Atomic radius is expressed in nanometres.
The mass of an atom
The mass of an atom is concentrated in its nucleus and it is due to the protons (p) and neutrons (n) in it. The total number (of protons and neutrons) in the atomic nucleus is called the atomic mass number. Protons and neutrons are together called nucleons. An atom is very tiny. Then how do we determine its mass? Scientists too, faced this question. It was not possible for scientists of the 19th century to measure atomic mass accurately. Therefore, the concept of ‘relative mass of an atom’ was put forth. A reference atom was required for expressing the relative mass of an atom. The hydrogen atom being the lightest was initially chosen as the reference atom. The relative mass of a hydrogen atom which has only one proton in its nucleus was accepted as one (1). Therefore, the magnitude of the relative atomic masses of various atoms became equal to their atomic mass number (p+n).
Let us see how to express the relative mass of a nitrogen atom, having accepted the relative atomic mass of hydrogen as 1.
The mass of one nitrogen atom is fourteen (14) times that of a hydrogen atom. Therefore, the relative mass of a nitrogen atom is 14. This is how the relative atomic masses of various elements were determined. On this scale, the relative atomic masses of many elements came out to be fractional. Therefore, in the course of time, some other atoms were chosen as reference atoms. Finally in 1961, the carbon atom was selected as the reference atom. In this scale, the relative mass a carbon atom was accepted as 12. The relative atomic mass of one hydrogen atom compared to the carbon atom becomes 12 x 1/12, that is 1. The mass of one proton and of one neutron on the scale of relative atomic masses is approximately one.
Chemical symbols of elements
Dalton used certain signs to represent elements. For example for hydrogen, © for copper. Today we use the symbols determined by IUPAC (International Union of Pure and Applied Chemistry). These are official names and symbols and are used all over the world. The current method of choosing chemical symbols is based on the method invented by Berzelius. According to this method the symbol of an element is either the first letter or the first and second/another specific letter in its name. Of the two letters, the first is written as capital letter and the second is small.
Molecules of elements and compounds
Atoms of some elements such as helium, neon have independent existence. It means that these elements are in a mono-atomic molecular state. Sometimes, two or more atoms of an element combine to form molecules of that element. Such elements are in a polyatomic molecular state. For example, the elements oxygen, nitrogen are in a diatomic molecular state as O2 , N2 respectively. When atoms of different elements combine with each other, the molecules of compounds are formed. In other words, compounds are formed by chemical attraction between different elements.
Molecular mass and the concept of mole
The molecular mass of a substance is the sum of the atomic masses of all the atoms in a single molecule of that substance. Like atomic mass, molecular mass is also expressed in the unit Dalton (u). How to deduce the molecular mass of H2 O?
When elements and compounds take part in chemical reactions, it is their atoms and molecules that react with each other, and therefore it is necessary to know the numbers of their atoms and molecules. However, while carrying out a chemical reaction it, is convenient to measure out quantities that can be handled instead of counting the numbers of atoms and molecules. The concept of ‘mole’ is useful for this purpose.
A mole is that quantity of a substance whose mass in grams is equal in magnitude to the molecular mass of that substance in Daltons. Thus, the molecular mass of oxygen is 32u, and therefore 32g oxygen is 1mole of oxygen. The molecular mass of water is 18u. Therefore, 18g of water make 1 mole of water.
The number of molecules in one mole of any substance is constant. The Italian scientist Avogadro did fundamental work in this context. Therefore this number is called Avogadro’s number and is denoted by the symbol NA. Later scientists demonstrated experimentally that the value of Avogadro’s number is 6.022 x 1023. A mole of any substance stands for 6.022 x 1023 molecules. Just as a dozen has 12 items, a century has 100 or a gross has 144, a mole means 6.022 x 1023. For example, a mole of water, that is, 18g of water contains 6.022 x 1023 molecules of water.
How many molecules are there in 66 g of CO2 ?
Method : molecular mass of CO 2 is 44
The capacity of an element to combine is called its valency. The valency of an element is indicated by a specific number. It is the number of chemical bonds formed by one atom of that element with other atoms. In the 18th and 19th century, the laws of chemical combination were used to find out the valencies of elements. In 20th century, the relationship of the valency of an element with its electronic configuration was recognised.
A sodium atom gives away one electron and a cation of sodium is formed, hence, the valency of sodium is one. A chlorine atom takes up one electron and forms an anion of chlorine (chloride) and thus, the valency of chlorine is 1. After the give and take of electrons is over, the electronic configuration of both the resulting ions has a complete octet. Due to the attraction between the unit opposite charges on the two ions, one chemical bond is formed between Na+ and Cl- and the compound NaCl is formed.
Thus, a sodium atom has the capacity to give away one electron while a chlorine atom has the capacity to take up one electron. This means that the valency of both the elements sodium and chlorine is 1. From this the electronic definition of valency is as follows : “The number of electrons that an atom of an element gives away or takes up while forming an ionic bond, is called the valency of that element.”
Under different conditions the atoms of some elements give away or take up different numbers of electrons. In such cases those elements exhibit more than one valency. This property of elements is called variable valency.
Compounds with ionic bonds have two constituents. These are cations (positively charged ions) and anions (negatively charged ion).They take part independently in chemical reaction and are therefore, called radicals. It is seen from the above chart that different bases such as NaOH, KOH are formed when various cationic radicals are paired with the different anionic radical, hydroxide. Hence the cationic radicals are also called basic radicals. Different bases are distinguished from each other by the basic radicals in them. On the other hand, different acids, such as HCl, HBr are formed when different anionic radicals are paired with the cationic radical H+ . Therefore, the anionic radicals are called acidic radicals. The difference in the composition of various acids becomes clear from the acidic radicals present in them.
Chemical formulae of compounds : A recapitulation.
The characteristic of a compound formed by ionic bonds is that its molecule has two parts. These are a cation and an anion, that is, a basic radical and an acidic radical. These two parts are oppositely charged. The force of attraction between them constitutes the ionic bond. The name of an ionic compound contains two words. The first word is the name of the cation and the second is the name of the anion. For example, the formula of a compound such as sodium chloride is written by writing the symbol of the cation on the left and adjoined to it on the right is the symbol of the anion. The charges are not shown though the number of the ions is written as a subscript on the right of the symbol of the ion. In the case of composite radical the number is written as subscript outside the bracket enclosing its symbol. This number is obtained easily by the method of cross multiplication of the valencies. The steps for writing a chemical formula are shown on the next page.