ASHAQ HUSSAIN BHAT
TEACHER SCHOOL
EDUCATION DEPARTMENT
JAMMU AND KASHMIR
ATOMS AND MOLECULE.
Introduction.
The idea of tiniest unit of matter, viz. "anu" and "parmanu", was put forth by Maharishi Kanad in Vedic period in our country. Another Indian philosopher, Pakudha Kaccayana, elaborated this doctrine and said that these particles normally exist in combined form, which gives us various forms of matter. Democritus, a Greek philosopher, also proposed that matter is made up of extremely small particles, "the atoms”. The name comes from Greek word 'atomos' means "indivisible". However, all these theories proposed by the Indian and Greek philosophers were based on abstract thinking and not on experiments. For about 2000 years, the atomic theory remained a mere speculation till John Dalton, an English school teacher, propounded his theory on atoms. Dalton's theory was based on the chemical laws of combination known at that time. These laws of chemical combination were given by A.L. Lavoisier and Joseph L. Proust and were based upon extensive study of chemical reactions.
Laws of Chemical Combination.
By studying the quantitative measurements of many reactions, it was observed that the chemical reactions taking place between various substances are governed by certain laws. These laws are called the laws of chemical combination. These laws formed the basis of Dalton's atomic theory. There are two main laws of chemical combination which are as follows:
(1) Law of Conservation of Mass
(2) Law of Constant Proportions (also known as the law of definite composition)
Law of Conservation of Mass.
This law deals with the relation between the mass of the reactants and the products during the chemical changes. It was stated by a French chemist, A. Lavoisier in 1774. The law of conservation of mass states that during any chemical change, the total mass of the products is equal to the total mass of the reactants. In other words, the law of conservation of mass means that mass can neither be created nor destroyed during any chemical combination. This law is also known as law of indestructibility of matter. From a large number of experiments, it was concluded that although substances may undergo chemical changes, the total mass of the products of the reaction is exactly equal to the total mass of the reactants. For example, 12 g of carbon combines with 32 g of oxygen to form 44 g of carbon dioxide,
C + 02 ------------------>CO2
12g 32g 44g
Law of Constant Proportions.
This law states that in a pure chemical substance the elements are always present in definite by mass.
This law is also known as the law of definite proportions. This means that, whatever, may be the source from which a compound is obtained, it is always made up of the same elements in the same proportion by mass.
For example, carbon dioxide can be prepared by any one of the following methods :
- By burning charcoal in air (C + O2 —> CO2).
- By heating limestone (CaCO3 -> CaO + CO2).
- By adding dilute hydrochloric acid to any carbonate (Na2CO3 + 2HCl → 2NaCl + H2O + CO2),
- By heating sodium bicarbonate (2NaHCO3 ------> Na2CO3 + H2O + CO2)
It is found that in each case, carbon dioxide is found to be made up of the same elements, i.e., carbon and oxygen combined together in the same fixed ratio, i.e., 12:32 or 3 : 8 by mass.
As the law of constant proportions is true, it helps us to calculate the percentage of any element in the given compound, using the following expression :
% of an element in the compound=Mass of that element/Mass of the compound x 100
Limitations of law of constant proportions.
- The law is not applicable if the elements involved in the formation of a compound have two or more isotopes. For example, in the formation of carbon dioxide : By using C-12 isotope, the ratio of C:0 12:32By using C-14 isotope, the ratio of C:0= 14:32
This means that the two ratios are not the same,
- The elements may combine in the same ratio but the compounds formed may be different. For example, in the compounds C,H,OH and CHOCH, (both having the same molecular formula) 1.e., C2H6O the ratio of C:H:0 - 24:6:16 or 12:3:8 by mass.
Dalton's Atomic Theory.
John Dalton provided the basic theory about the nature of matter. He took the name, 'atoms', for the smallest particle of matter as given by the Greek. Based on the laws of chemical combination, he proposed a model of atom known as Dalton's atomic theory.
The main postulates of the Dalton's atomic theory are :
- All matter, whether an element, a compound or a mixture is made up of extremely small particles called Atoms (1.e., same name was used for the smallest indivisible particles as used by Greek philosophers).
- An atom can neither be created nor destroyed, i.e., atom is indestructible Atom is the smallest particle that takes place in a chemical reaction. In other words, whole atoms rather than fractions of atoms take part in a chemical reaction.
- Atoms of the same element are identical in all respects, i.e., size, shape, mass and properties.
- Atoms of different elements have different sizes and masses and also possess different properties
- Atoms of the same or different elements combine together to form molecules or compounds.
- When atoms of different elements combine together to form compounds, they do so in a simple whole number ratio such as 1:1, 2:1, 2:3 etc.
- Atoms of two different elements may combine in different ratios to form more than one compound. For example, carbon and oxygen may combine to form carbon monoxide (CO) and carbon dioxide (CO2) in which the ratios of the combining atoms (C and O) are 1:1 and 1:2 respectively,
- The number and kind of atoms in a given compound is always fixed.
Explanation of Laws of Chemical Combination by Dalton's Atomic Theory.
• Explanation of law of conservation of mass : According to Dalton's Atomic Theory, matter is made up of atoms which can neither be created nor destroyed. As atoms can neither be created nor destroyed and the atoms have definite masses, therefore, the total mass remains unchanged during a chemical reaction.
• Explanation of law of constant proportions: According to Dalton's Atomic Theory, atoms combine elements combine in a fixed ratio by mass. in the ratio of small whole numbers to form compounds. It means that in the compound,
Drawbacks of Dalton's Atomic Theory.
• Atom is no longer considered as the smallest indivisible particle. This is because recent studies have shown that atom is made up of still smaller particles called electrons, protons and
•• Atoms of the same element may have different masses. For example, there are two types of atoms of chlorine with masses 35 and 37. Such atoms of the same element with different mass numbers are called isotopes.
••• Atoms of different elements may have same masses. For example, atoms of potassium and calcium are known with same mass number (40). Such atoms of different elements with same mass numbers are known as isobars.
•••• Substances made up of the same kind of atoms may have different properties. For example, charcoal, graphite and diamond are all made up of carbon atoms but have different physical properties.
•••••• The ratio in which different atoms combine to form compound may be fixed and integral but may not be simple. For example, sugar molecule (C12H2011) contains C, H and in the ratio 12:22:11 which is integral and fixed but not simple.
What is an Atom?
The smallest unit of an element, which may or may not exist independently, but always takes part in a chemical reaction, is called an atom.
Atoms are the building blocks of matter. They are smaller than anything you can imagine or compare with. More than a million atoms when stacked upon one another would make a layer as thick as the sheet of paper of your science book. The size of an atom is indicated by its radius which is called "atomic radius'. It is measured in' nano meters
1 nanometre (nm) = 10-9metre (m) or 1 metre (m) = 10 9 nanometre (nm)
The given table shows the relative size of the radius of an atom of hydrogen with respect to other objects
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| Some elements along with Atomic radius |
Symbols Used to Represent Atoms of Different Elements.
In case of elements, symbol means a short method of representing the full name of an element, Dalton was the first scientist to suggest specific symbols in terms of figures for different elements known at that time (which were limited in number). In fact, the symbol used by him also represented the quantity of the element, i.e., one atom of the element. A few of these symbols, as proposed by Dalton are given. Fig
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| Symbols of some elements used by Dalton |
Different elements have been named either after the name of the place where they were first discovered or after the name of the scientist who discovered it or on the basis of some important property of the element. However, as more and more elements were discovered, an International Committee was set up, called International Union of Pure and Applied Chemistry (IUPAC), which approved the names of the different elements.
The symbol of an element is the "first letter" or "first letter and another letter” of the English name or Latin name of the element. However, in all cases, the first letter is always capital and another letter (if added) is always a small letter.
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| Symbols of Elements Derived from English |
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| Symbols derived from Latin Names |
Significance of a Symbol.
The symbol of an element has both qualitative as well as quantitative significance. These are,
- The symbol stands for the name of the element.
- The symbol stands for one atom of the element.
- The symbol represents quantity of the element equal in mass to its atomic mass, or gram-atomic mass.
- The symbol also represents mass of the element which contains one Avogadro's number of atoms of that element. For example, the symbol O stands for---—the element oxygen —one atom of oxyge — the mass of oxygen equal to its gram-------atomic mass, i.e., 16 g of oxygen —the mass of oxygen which contains one Avogadro's number (6.022 x 1023) of oxygen atoms.
Atomic Mass.
Atoms are extremely small particles. Hence, the actual masses of the atoms are so small that it is difficult to determine the actual masses of individual atoms.
In 1961, IUPAC recommended the use of an isotope of carbon with mass number 12 as the standard reference for measuring atomic masses. It is called carbon-twelve (C-12) and is represented as 12C. The atomic mass (or earlier called as "atomic weight") is now defined as follows :
The atomic mass of an element is the relative mass of its atoms as compared with the mass of an atom of carbon-12 isotope taken as 12 units.
Atomic mass unit (amu) may be defined as 1/12 th of the mass of an atom of carbon-12 isotope on the atomic scale, i.e., 1 amu = 1/12th of mass of C-12 isotope.
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| Atomic mass of some common elements |
Gram Atomic Mass.
Atomic mass expressed in grams is called gram atomic mass of that element. For example,
Atomic mass of hydrogen = 1.0 u
Gram atomic mass of hydrogen = 1.0 g
. Atomic mass of oxygen = 16.0 u
Gram atomic mass of oxygen = 16.0 g
The amount of an element having mass equal to gram atomic mass is called one "gram atom" (or g atom) of the element. For example,
1 g atom of hydrogen = 1.0 g
1 g atom of oxygen = 16.0 g
The relationship between gram atoms, mass and atomic mass of a substance is,
Number of gram atoms =Mass in grams/Gram atomic mass
Average Atomic Mass.
It has been found in a number of cases that most elements consist of atoms having different masses, ie they exist in the form of 'isotopes'. In such cases average value is taken. For example, chlorine consists of two isotopes with mass number 35 and 37 in the ratio of 3 : 1. Hence, average relative atomic mass of chlorine is
Chlorine=3 x 35+1 x 37/3+1=35.5
How do Atoms Exist?
The atoms of only a few elements called noble gases (such as helium, neon, argon, etc.) are chemically unreactive and exist in free state or single atoms. Atoms of most of the elements are chemically reactive and do not exist in free state or single atom. Atoms usually exist in two ways:
(i) in the form of molecules
(ii) in the form of ions
Molecules.
A molecule is a group of two or more atoms which are held together strongly by some kind of attractive forces Such an attractive force holding the atoms together is called a chemical bond.
We may also define a molecule as follows: A molecule is the smallest particle of an element or a compound which can exist freely under ordinary conditions and shows all the properties of that element or compound.
Molecules of an Element.
A molecule of an element consists of the same type of atoms bonded together. For example, a molecule of oxygen is formed when 2 atoms of oxygen combine together. Oxygen atom alone cannot exist independently. It exists as a diatomic molecule. This means that two atoms of oxygen combine together to form a molecule. The formula of oxygen molecule is 02.
Atomicity.
The number of atoms present in one molecule of an element is called its atomicity. The atomicity of an element is indicated by writing the number as a subscript on the right hand side bottom of the symbol. For example, H2 shows that the atomicity of hydrogen is 2. P4 shows that the atomicity of phosphorus is 4. He shows that the atomicity of helium is 1.
On the basis of their atomicity's, the elements may be classified as monatomic, diatomic, triatomic, tetratomic, etc. As shown in Fig below
Morecules or compounds.
In the molecules of compounds, the atoms of different elements are combined or bonded together by chemical bonds. These are present in a definite proportion by mass according to the law of constant proportions. The molecules of compounds may also be diatomic, triatomic, tetratomic and polyatomic in nature depending upon the number of the atoms linked or combined by chemical bonds. For example, as shown in fig
The molecules of elements are homoatomic in nature which means that the atoms present in them are the same. The molecules of the compounds are heteroatomic in nature in the sense that different atoms are present in them. For example, a molecule of nitrogen element is homoatomic in nature (N2). A molecule of methane (CH4) which is a compound is heteroatomic in nature.
lons.
An ion may be defined as an atom or group of atoms having positive or negative charge.
- The ion which has one or more positive charges is called a cation. At the same time, the ion carrying one or more negative charges is known as an anion.
- A few positively charged ions or cations are :Na+ (sodium ion), K+(potassium ion), Ca2+ (calcium ion), Al3+ (aluminium ion).
- A few negatively charged ions or anions are CI–(chloride ion), S2– (sulphide ion), OH– (hydroxide ion), SO⁴–(sulphate ion).
- All the metals form cations. Only the hydrogen ion (H') and ammonium ion (NH) are the cations formed from non-metals.
- All the non metals form anions (except H* and NHA).
Molecular Mass.
molecular mass of a substance (element or compound) is the average relative mass of its molecules compare with that of an ate of C-12 isotope taken as 12. In other words, molecular mass of a substance represent the number of times the molecule of that substance is heavier than 1/12th of the mass of an atom of C-12 isotope.
Calculation of Molecular Mass.
As molecules are made up of two or more atoms of the same or different elements, and each atom has a definite atomic mass, therefore, molecular mass of a molecule of a substance can be calculated by adding atomic masses of all the atoms present in one molecule of the substance. For example,
Molecular mass of H2 = 2 x atomic mass of hydrogen
= 2 (1) = 2 a.m.u.
Molecular mass of H2O = 2(atomic mass of hydrogen) + atomic mass of oxygen
=2(1) + 16 = 18 a.m.u.
Molecular mass of CO2 = atomic mass of C +2 (atomic mass of oxygen)
= 12 + 2(16) = 12 +32 = 44 a.m.u.
Molecular mass of NH3 = atomic mass of nitrogen + 3 (atomic mass of hydrogen)
= 14 + 3 (1) = 17 a.m.u.
Formula Mass.
In case of the compounds formed by ions (ionic compounds), formula of the compound does not represent its molecule, but only represents the ratio of different ions in the compounds. This is called formula unit of the ionic compound,
Formula mass of an ionic compound is obtained by adding atomic masses of all the atoms in a formula unit of the compound,
For example,
••Formula mass of calcium oxide (Cao) = atomic mass of calcium + atomic mass of oxygen
= 40 + 16 = 56 u
••Formula mass of sodium chloride (NaCl) = atomic mass of sodium + atomic mass of chlorine
= 23 + 35.5 = 58.5 u
Gram Molecular Mass.
Molecular mass expressed in grams is called gram molecular mass of that substance.
For example,
- Molecular mass of H2 = 2.0 u
- Gram molecular mass of H2 = 2.0 g
- Molecular mass of O2 = 32.0 u
- Gram molecular mass of O2 = 32.0 g
The amount of the substance having mass equal to its gram molecular mass is called one “gram molecule" (Or g molecule) of the substance. Thus,
Gram Formula Unit Mass.
Formula unit mass expressed in grams is called gram formula unit mass. This amount is called one gram formula unit.
For example, Formula unit mass of NaCl = 23 + 35.5 = 58.5 u
Gram formula unit mass of NaCl = 58.5 g
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