Basic Concepts of Chemistry

1.BASIC CONCEPTS 1.1 ATOM Long time ago, it was thought that matter is made up of simple, indivisible particles. Greek philosophers thought that, matter could be divided into smaller and smaller particles to reach a basic unit, which could not be further sub-divided. Democritus (460-370 B.C.) called these particles atomos, derived from the word "atomos" means indivisible. However, the ideas of Greek philosophers were not based on experimental evidences. In the late 17th century, the quantitative study of the composition of pure substances disclosed that a few elements were the components of many different substances. It was also investigated that how, elements combined to form compounds and how compounds could be broken down into their constituent elements. In 1808, an English school teacher, John Dalton, recognized that the law of conservation of matter and the law of definite proportions could be explained by the existence of atoms. He developed an atomic theory; the main postulate of which is that all matter is composed of atoms of different elements, which differ in their properties. Atom is the smallest particle of an element, which can take part In a chemical reaction. For example, He and Ne, etc. have atoms, which have independent existence while atoms of hydrogen, nitrogen and oxygen do not exist independently. The modern researches have clearly shown that an atom is further composed of subatomic particles like electron, proton, neutron, hypron, neutrino, anti-neutrino, etc. More than 100 such particles are thought to exist in an atom. However, electron, proton and neutron are regarded as the fundamental particles of atoms. 1.BASIC CONCEPTS etesrs.ponisli A Swedish chemist.). Berzelius- (1779-1848) determined the atomic masses of elements. A number of his values are close to the modern values of atomic masses. Berzelius also developed the system of giving element a symbol. 1.1.1 Evidence of Atoms It is not possible actually to see the atoms but the nearest possibility to its direct evidence is by using an electron microscope. A clear and accurate image of an object that is smaller than the wavelength of visible light, cannot be obtained. Thus an ordinary optical microscope can measure the size of an object upto or above 500 nm (Inm = 109m). However, objects of the size of an atom can be observed in an electron microscope. It uses beams of electrons instead of visible light, because wavelength of electron is much shorter than that of visible light. 1.BASIC CONCEPTS Fig. (1.1) shows electron microscopic photograph of a piece of a graphite which has been magnified about 15 millions times. The bright band in the figure are layers of carbon atoms. In the 20. century, X-ray work has shown that the diameter of atomsa re ofthe order2x10.'m which is 0.2 nm.Massesofatoms range from 10-0 to 10,5 kg. They are often expressed in atomic mass units (amu)when 1 amu is= 1.661 x 10-0 kg. The students e can havean idea abouttheamazinglysmall size of an atom from the factthat a full stop may havetwo million atoms present in it. Molecule A molecule Is the smallest particle of a pure substance which can exist Independently. It may contain one or more atoms. The number of atoms present in a molecule determines its atomicity. Thus molecules can be monoatomic, diatomic and triatomic, etc., if they contain one, two and three atoms respectively. Molecules of elements may contain one, two or more same type of atoms. For example, He, CI,, 5,. On the other hand, molecules of compounds consist of different kind of atoms. For example, HCI, NH,„ The sizes of molecules are definitely bigger than atoms. They depend upon the number of atoms present in them and their shapes. Some molecules are so big that they are called macromolecules. Haemoglobin is such a macromolecule found in blood. It helps to carry oxygen from our lungs to all parts of our body. Each molecule of haemoglobin is made up of nearly 10,000 atoms and it is 68,000 times heavier than a hydrogen atom. 1.1.3 Ion Ions are those species which carry either positive or negative charge. Whenever an atom of an element loses one or more electrons, positive ions are formed. 1.BASIC CONCEPTS A sufficient amount of energy is to be provided to a neutral atom to ionize it Think is called a cation. A cation may cony +1, +2, +3, etc.charge or charges. The number of charges present on an ion depends upon the number of electrons lost bythe atom. Anyhow, energy is always required to do so. Hence the formation of the positive ions is an endothermic process. The most common positive ions are formed by the metal atoms such as Na', K+, Ca2', , Fe., Se, etc. The chapter on chemical bonding will enable us to understand the feasibilities of their formation. When a neutral atom picks up one or more electrons, a negative ion is produced, which is called an anion. Energy is usually released when an electron is added to the isolated neutral atom, Therefore, the formation of an uninegative ion is an exothermic process.The most common negative ions are r,or,ax-,s'etc. The cations and anions possess altogether different properties from their corresponding neutral atoms. There are many examples of negative ions which consist of group of atoms like OH-, CO3', PO„*, MniV, Cr,072- etc. The positive ions having group of atoms are less common e.g. NH,' and some carbocations in organic chemistry. 1.1.4 Molecular Ion When an atom loses or gains an electron, it forms an ion. Similarly, a molecule may also lose or gain an electron to form a molecular ion, e.g., CH„., CO., N2. Cationic molecular ions are more abundantthan anionic ones. These ions can be generated by passing high energy electron beam or a-particles or X-rays through a gas. The break down of molecular ions obtained from the natural products can give important information about their structure. 1.2 RELATIVE ATOMIC MASS Relative atomic mass is the mass of an atom of an element as compared to the mass of an atom of carbon taken as 12. The unit used to express the relative atomic mass is called atomic mass unit (amu) and it is 1/12 th of the mass of one carbon atom, On carbon -12 scale, the relative atomic mass of is 12.0000 amu and the relative atomic mass of 1H is 1.008 amu. The masses of the atoms are extremely small. We-don't have any balance to weigh such an extremely small mass, that is why we use the relative atomic mass unit scale. The relative atomic masses of some elements are given in the following Table (1.1). Table 1.1 Relative atomic masses of a few elements Element Relative Atomic Mass (amu) Element Relative Atomic Mass (amu) H 0 Ne 1.008 15.9994 20.1797 CI Cu U 35.453 63.546 238.0289 These element have atomic masses in fractions and will be expl fined in the following article on isotopes. 1.3 ISOTOPES In Dalton's atomic theory, all the atoms of an element were considered alike in all the properties including their masses. Later on, it was discovered that atoms of the same element can possess different masses but same atomic numbers. Such atoms of an element are called isotopes. So isotopes are different kind of atoms of the same element having same atomic number, but different atomic masses. The isotopes of an element possess same chemical properties and same position in the periodic table. This phenomenon of isotopy was first discovered by Soddy. Isotopes have same number of protons and electrons but they differ in the number of neutrons present in their nuclei. Carbon has three isotopeswritten as , , ':O and expressed as C-12, C-13 and C-14. Each of these have 6-protrons and 6 electrons. However, these isotopes have 6, 7 and 8 neutrons respectively. Similarly, hydrogen has three isotopes :It, It , called protium, deuterium and tritium. Oxygen has three, nickel has five, calcium has six, palladium has six, cadmium has nine and tin has eleven isotopes. 1.3.1 Relative Abundance of Isotopes The isotopes of all the elements have their own natural abundance. The properties of a particular element, which are mentioned in the literature, mostly correspond to the most abundant isotope of that element. The relative abundance of the isotopes of elements can be determined by mass spectrometry. First of all, Aston's mass spectrograph was designed to identify the isotopes of an element on the basis of their atomic masses. There is another instrument called Dempster's mass spectrometer. This was designed for the identification of elements which were available in solid state. The substance whose analysis for the separation of isotopes is required, is converted into the vapour state. The pressure of these vapours is kept very low, that is, 10, to 10, torr. These vapours are allowed to enter the ionization chamber where fast moving electrons arc thrown upon them. The atoms of isotopic element present in the form of vapours, are ionized. These positively charged ions of isotopes of an element have different masses depending upon the nature of the isotopes present in them. The positive ion of each isotope has its own (m/e) value. When a potential difference (E) of 500-2000 volts is applied between perforated accelerating plates, then these positive ions are strongly attracted towards the negative plate. In this way, the ions are accelerated. These ions are then allowed to pass through a strong magnetic field of strength (H), which will separate them on the basis of their (m/e) values. Actually, the magnetic field makes the ions to move in a circular path. The ions of definite m/e value will move in the form of groups one after the other and fall on the electrometer. The mathematical relationship for (m/e) is = Where H is the strength of magnetic field, E is the strength of electrical field, r is the radius of circular path. If E is increased, by keeping H constant then radius will increase and positive ion of a particular m/e will fall at a different place as compared to the first place. This can also be done by changing the magnetic field. Each ion sets up a minute electrical current. Electrometer is also called an ion collector and develops the electrical current. The strength of the current thus measured gives the relative abundance of ions of a definite m/e value. Similarly, the ions of other isotopes having different masses are made to fall on the collector and the current strength is measured. The current strength in each case gives the relative abundance of each of the isotopes. The same experiment is performed with C-12 isotope and the current strength is compared. This comparison allows us to measure the exact mass number of the isotope Fig. (1.2), shows the separation of isotopes of Ne. Smaller the (m/e)of an isotope, smaller the radius of curvature produced by the magnetic field according to above equation. O We know at present above 280 different isotopes occur in nature. They include 40 radioactive isotopes as well. Besides these about 300 unstable radioactive isotopes have been produced through artificial disintegration. The distribution of isotopes among the elements is varied and complex as it is evident from the Table (1.2). The elements like arsenic, fluorine, iodine and gold, etc have only a single isotope. They are called mono-isotopic elements. In general, the elements of odd atomic numberalmost never possess morethan two stable isotopes. The elements of even atomic number usually have larger number of isotopes and isotopes whose mass numbers are multiples of four are particularly abundant. For example, lfir),.Mg, 3851, `')Ca and .Fe form nearly 50% of the earth's crust. Out of 280 isotopes that occur in nature, 154 have even mass number and even atomic number. 1.3.2 Determination of Relative Atomic Masses of Isotopes by Mass Spectrometry Mass spectrometer is an instrument which is used to measure the exact masses of different isotopes of an element. In this technique, a substance is first volatilized and then ionized with the help of high energy beam of electrons. The gaseous positive ions, thus formed, are separated on the basis of their mass to charge ratio (m/e) and then recorded in the form of peaks. Actually mass spectrum is the plot of data in such a way that (m/e) is plotted as abscissa (x-axis) and the relative number of ions as ordinate (y-axis). First of all, Aston's mass spectrograph was designed to identify the isotopes of an element on the basis of their atomic masses. There is another instrument called Dempster's mass spectrometer. This was designed for the identification of elements which were available in solid state. The substance whose analysis for the separation of isotopes is required, is converted into the vapour state. The pressure of these vapours is kept very low, that is, 10, to 10, torr. These vapours are allowed to enter the ionization chamber where fast moving electrons arc thrown upon them. The atoms of isotopic element present in the form of vapours, are ionized. These positively charged ions of isotopes of an element have different masses depending upon the nature of the isotopes present in them. The positive ion of each isotope has its own (m/e) value. When a potential difference (E) of 500-2000 volts is applied between perforated accelerating plates, then these positive ions are strongly attracted towards the negative plate. In this way, the ions are accelerated. These ions are then allowed to pass through a strong magnetic field of strength (H), which will separate them on the basis of their (m/e) values. Actually, the magnetic field makes the ions to move in a circular path. The ions of definite m/e value will move in the form of groups one after the other and fall on the electrometer. The mathematical relationship for (m/e) is = Where H is the strength of magnetic field, E is the strength of electrical field, r is the radius of circular path. If E is increased, by keeping H constant then radius will increase and positive ion of a particular m/e will fall at a different place as compared to the first place. This can also be done by changing the magnetic field. Each ion sets up a minute electrical current. Electrometer is also called an ion collector and develops the electrical current. The strength of the current thus measured gives the relative abundance of ions of a definite m/e value. Similarly, the ions of other isotopes having different masses are made to fall on the collector and the current strength is measured. The current strength in each case gives the relative abundance of each of the isotopes. The same experiment is performed with C-12 isotope and the current strength is compared. This comparison allows us to measure the exact mass number of the isotope Fig. (1.2), shows the separation of isotopes of Ne. Smaller the (m/e)of an isotope, smaller the radius of curvature produced by the magnetic field according to above equation. In modern spectrographs, each ion strikes a detector, the ionic current is amplified and is fed to the recorder. The recorder makes a graph showing the relative abundance of isotopes plotted against the mass number. •F*12)Magramol a maple Mass The above Fig (1.3) shows a computer plotted graph for the isotopes of neon. The separation of isotopes can be done by the methods based on their properties. Some important methods are as gaseous diffusion, thermal diffusion, distillation, ultracentrifuge, electromagnetic separation and laser separation. 1.3.3 Average Atomic Masses Table (1.1) of atomic masses of elements shows many examples of fractional values. Actually the atomic masses depend upon the number of possible isotopes and their natural abundance. Following solved example will throw light on this aspect. Example (1): A sample of neon is found to consist of ,;1,1e and :Ne in the percentages of 90.92%, 0.26%, 8.82% respectively. Calculate the fractional atomic mass of neon. Solution: The overall atomic mass of neon, which is an ordinary isotopic mixture, is the average of the de-termined atomic masses of individual isotopes. The above results tell us that in one hundred grams of the given compound, there are 60.26 grams of carbon, 11.11 grams of hydrogen and 28.62 grams of oxygen. Percentage composition of a compound can also be determined theoretically if we knowtheformula mass of the compound. The following equation can be used for this purpose. pnrcnnnann an nk.n, Mass of dm element in one mole of the compound x. Formula mass of the compound L4.1 Empirical Formula It is the simplest formula that gives the small whole number ratio between the atoms of different elements present in a compound. In an empirical formula of a compound, Aff,, there are x atoms of an element A and y atoms of an element B. The empirical formula of glucose (C6H,206) is CI-1,0 and that of benzene (C6Hd is CH. Empirical formula of a compound can be calculated following the steps mentioned below: 1. Determination of the percentage composition. 2. Finding the number of gram atoms of each element. For this purpose divide the mass of each element (% of an element) by its atomic mass. 3. Determination of the atomic ratio of each element. To get this, divide the number of moles of each element (gram atoms) by the smallest number of moles. 4. If the atomic ratio is simple whole number, it gives the empirical formula, otherwise multiply with a suitable digit to get the whole number atomic ratio. Example (3): Ascorbic acid (vitamin C) contains 40.92% carbon, 4.58% hydrogen and 54.5% of oxygen by mass. What is the empirical formula of the ascorbic acid? Solution: From the percentages of these elements, we believe that in 100 grams of ascorbic acid, there are 40.92 grams of carbon, 4.58 grams of hydrogen and 54.5 grams of oxygen. KEY POINTS 1. Atoms are the building blocks of matter. Atoms can combine to form molecules. Covalent com-pounds mostly exist in the form of molecules. Atoms and molecules can either gain or lose elec-trons, forming charged particles called ions. Metals tend to lose electrons, becoming positively charged ions. Non-metals tend to gain electrons forming negatively charged ions. When X-rays or a -particles are passed through molecules in a gaseous state, they are converted into molec-ular ions. 2. The atomic mass of an element is determined with reference to the mass of carbon as a standard element and is expressed in amu. The fractional atomic masses can be calculated from the relative abundance of isotopes. The separation and identification of isotopes can be carried out by mass spectrograph. 3. The composition of a substance is given by its chemical formula. A molecular substance can be represented by its empirical or a molecular formula. The empirical and molecular formula are related through a simple integer. 4. Combustion analysis is one of the techniques to determine the empirical formula and then the molecular formula of a substance by knowing its molar mass. 5. A mole of any substance is the Avogadro's number of atoms or molecules or formula units of that substance. 6. The study of quantitative relationship between the reactants and the products in a balanced chemical equation is known as stoichiometry. The mole concept can be used to calculate the relative quantities of reactants and products in a balanced chemical equation. 7. The conceptof molarvolume of gases helpsto relate solids and liquidswith gases in a quantitative manner. 8. A limiting reactant is completely consumed in a reaction and controls the quantity of products formed. 9. The theoretical yield of a reaction is the quantity of the products calculated with the help of a balanced chemical equation. The actual yield of a reaction is always less than the theoretical yield. The efficiency of a chemical reaction can be checked by calculating its percentage yield.