Saturday, October 19, 2013

Dalton’s atomic theory,Atomic mass and Relative Atomic mass (9th CBSE Ch.Atoms and Molecules)

In 1803, A British school teacher John Dalton provided the basic theory about the nature of matter which provides explanation for the law of conservation of mass and the law of definite proportions.
According to Dalton’s atomic theory, all matter, whether an element, a compound or a mixture is composed of small particles called atoms

The postulates of this theory may be stated as follows:
 (i) All matter is made of very tiny particles called atoms.
(ii) Atoms are indivisible particles, which cannot be created or destroyed in a chemical reaction.
(iii) Atoms of a given element are identical in mass and chemical properties.
(iv)  Atoms of different elements have different masses and chemical properties.
(v)  Atoms combine in the ratio of small whole numbers to form compounds.
(vi) The relative number and kinds of atoms are constant in a given compound.

Drawbacks of Dalton’s Atomic Theory:
ð  The atom is further subdivided into protons, neutrons and electrons.
ð  The atoms of same elements are not similar in all respect. They may vary in mass and density. These are known as isotopes. For example: chlorine has two isotopes having mass numbers 35 a.m.u and 37 a.m.u.
ð  Atoms of different elements are not different in all respects. Atoms of different elements that have the same atomic mass are called isobar.
ð  According to Dalton atoms of different elements combine in simple whole number ratio to form compounds. This is not seen in complex organic compounds like sugar C12H22O11.
ð  The theory fails to explain the existence of allotropes like Diamond and Graphite which having different properties even these are made up of same kind ot atom namely Carbon.
The introduction of matter wave concept by de Broglie, the principle of uncertainty by Heisenberg etc., paved the way for modern atomic theory [Modification in Dalton’s atomic theory]

Modifications in Dalton’s atomic theory - Modern atomic theory are as follows.

ð  Atom is considered to be a divisible particle.
ð  Atoms of the same element may not be similar in all respects. eg: Isotopes (17Cl35,17Cl37 )
ð   Atoms of different elements may be similar in some respects eg. Isobars (18Ar 40 , 20Ca 40 )
ð   Atom is the smallest particle which takes part in chemical reactions.
ð  The ratio of atoms in a molecule may be fi xed and integral but may not be simple e.g., C12H22O11 is not a simple ratio (Sucrose)
ð  Atoms of one element can be changed into atoms of other element by transmutation.
ð  The mass of an atom can be converted into energy. This is in accordance with Einstein’s equation E = mc2

Atom: It is the smallest particle of an element which may or may not have independent existence. The atoms of certain elements such as hydrogen, oxygen, nitrogen, etc .do not have independent existence whereas atoms of helium, neon, argon, etc. do have independent existence. Thus we can say that all elements are composed of atoms.

Q. How do we know the presence of atoms if they do not exist independently for most of the elements?
Answer: Atom join in different way to form matter (neutral molecules or ion) that we are able to touch, feel and see.

How big are atoms?
Atoms are extremely small. They are so small, that you cannot see them with most microscopes. Now, Scan tunneling Microscope (STM) is the modern instrument that made it possible to take photograph of atom. The size of an isolated atom can 't be measured because we can 't determine the location of the electrons that surround the nucleus. We can estimate the size of an atom, however, by assuming that the radius of an atom is half the distance between adjacent atoms in a solid. This technique is best suited to elements that are metals, which form solids composed of extended planes of atoms of that element. The results of these measurements are therefore often known as metallic radii.

Q. What is the unit of measurement of atomic radius?
Ans: Picometers (pm) or Angstroms (Å)

Q.The size of sodium atom is bigger than that of hydrogen atom. Why?
Answer: Size of atom is the distance between the nucleus and outermost shell ( valence shell ) of an atom. The atomic number of sodium is greater than that of hydrogen. So, it needs more number of shells to fill electrons and hence will have more number of shells than hydrogen. Hence, atomic size of sodium is bigger than that of sodium.

Naming of an element
Dalton was the first scientist to use the symbols for elements in a very specific sense.

Q. Why are Dalton’s symbols not used in chemistry?
Answer: Dalton was the first scientist to use the symbol for the name of the elements a specific sense but it was difficult to memorize and in uses so Dalton's symbol are not used in chemistry
Berzelius suggested that the symbols of elements be made from one or two letters of the name of the element.

IUPAC (International Union of Pure and Applied Chemistry) approves names of elements. Many of the symbols are the first one or two letters of the element’s name in English. The first letter of a symbol is always written as a capital letter (uppercase) and the second letter as a small letter (lowercase)
For example:  (i) hydrogen, H (ii) aluminum, Al and not AL (iii) cobalt, Co and not CO.
Symbols of some elements are formed from the first letter of the name and a letter, appearing later in the name. Examples are: (i) chlorine, Cl, (ii) zinc, Zn etc.
Other symbols have been taken from the names of elements in Latin, German or Greek. For example, the symbol of iron is Fe from its Latin name ferrum, sodium is Na from natrium, and potassium is K from kalium. Therefore, each element has a name and a unique chemical symbol.

Molecule: A molecule is the smallest or the simplest structural unit of an element (or) a compound which contains one (or) more atoms. It retains the characteristics of an element. A molecule can exist freely and it is a combined form of bonded units whereas an atom is a singular smallest form of non bonded unit.
Molecules are of two types, namely homo atomic molecules and hetero atomic molecules.

Homo atomic molecules: These are the molecules which are made up of atoms of the same element. For example hydrogen gas consists of two atoms of hydrogen (H2).Similarly oxygen gas consists of two atoms of oxygen (O2).

HETERO ATOMIC MOLECULES : The hetero atomic molecules are made up of atoms of different elements. They are also classified as diatomic, triatomic, or polyatomic molecules depending upon the number of atoms present. H2O, NH3, CH4, etc., are the examples for hetero atomic molecules.

Atomicity:The number of atoms present in one molecule of an element is called the atomicity of an element. Depending upon the number of atoms in one molecule of an element, molecules are classified into monoatomic, diatomic, triatomic or poly atomic molecules containing one, two, three, or more than three atoms respectively.
Mono atomic molecules: Helium (He) Neon (Ne) Metals ;  Di atomic molecules:   Hydrogen H2 Chlorine Cl2
Tri atomic molecules: Ozone (O3) ; Poly atomic molecules: phosphorous P4 Sulphur S8

Atomicity = Molecular Mass/Atomic mass

Isotopes ⇒ These are the atoms of same element with same atomic number (Z) but different mass number (A). Example (17Cl35,17Cl37 )
Isobars These are the Atoms of the different element with same mass number but different atomic number. Example (18Ar40, 20Ca 40 )
 Isotones ⇒ These are the atoms of different elements with same number of neutrons Example ( 6C13, 7N14 )

AVOGADRO’S HYPOTHESIS: Amedeo Avogadro put forward hypothesis and is based on the relation between number of molecules and volume of gases that is “volume of a gas at a given temperature and pressure is proportional to the number of particles”.

Avogadro’s Law: Equal volumes of all gases under the same conditions of temperature and pressure. contain the equal number of molecules.

 (a) Find the atomicity of chlorine if its atomic mass is 35.5 and its molecular mass is 71 
(b) Find the atomicity of ozone if its atomic mass is 16 and its molecular mass is 48


An ion is a charged particle and can be negatively or positively charged.
A negatively charged ion is called an ‘anion’ and the positively charged ion, a ‘cation’. For example, sodium chloride (NaCl). Its constituent particles are positively charged sodium ions (Na+) and negatively charged chloride ions (Cl–).
Ions may consist of a single charged atom or a group of atoms that have a net charge on them. A group of atoms carrying a charge is known as a polyatomic ion e.g. Calcium oxide (Ca+2 O-2)

Atomic mass and Relative Atomic mass (RAM):

Q. Each element had a characteristic atomic mass even then we are using Relative Atomic mass. Give reason?

Answer: Since determining the mass of an individual atom was a relatively difficult task due to extremely smaller size, relative atomic masses were determined using the laws of chemical combinations and the compounds formed.

Relative Atomic mass (RAM): In 1961 IUPAC selected an isotope of carbon (12C) as a slandered for comparing atomic and molecular mass of element and compound.

Relative atomic mass of an element is the ratio of mass of one atom of element to the 1/12th part of mass of one atom of carbon. Relative atomic mass is a pure ratio and has no unit. If the atomic mass of an element is expressed in grams, it is known as gram atomic mass. e.g. 

Gram atomic mass of hydrogen = 1g where as gram atomic mass of carbon = 12g

Atomic mass is expressed in atomic mass unit (amu). One atomic mass unit is defined as 1/12th part of the mass of one atom of carbon.

Q. The atomic mass of an element is in fraction .What does it mean?
Ans If the atomic mass of an element is in fraction, this mean that it exists in the form of isotopes. The atomic mass is the average atomic mass and is generally fractional.

Chemical Formulae: The chemical formula is a symbolic representation of a compound of its composition.
For writing Chemical Formulae the name or symbol of the metal is written first then non-metals with their valencies. Then we must crossover the valencies of the combining atoms. For example:
 (a) Formula for aluminium oxide: Al3+ O-2 Þ Al2O(b) calcium hydroxide :  Ca+2 OH-1 Þ Ca(OH)2

Valency: The combining power (or capacity) of an element is known as its valency. Valency can be used to find out how the atoms of an element will combine with the atom(s) of another element to form a chemical compound.

RELATIVE MOLECULAR MASS (RMM) : The relative molecular mass of an element or a compound is the ratio of mass of one molecule of the element or a compound to the mass of 1/12 th part of mass of one atom of carbon. Relative Molecular mass is a pure ratio and has no unit. If the molecular mass of a given substance is expressed in gram, it is known as gram molecular mass of that substance.
Molecular mass is the sum of the masses of all the atoms present in one molecule of the compound or an element.

Test your numerical skill:
Problem:    Find the gram molecular mass of water (H2O)
Solution:  Þ 2(H) = 2 x 1 = 2  and   1(O) = 1 x 16 = 16    ;  Gram molecular mass of H2O = 2 + 16 = 18g

Problem:  Find the gram molecular mass of carbon dioxide
Solution:  Þ  (CO2) 1(C) = 1 x 12 = 12 and  2(O) = 2 x 16 = 32;  Gram molecular mass of CO2 = 12 + 32 = 44 g

Calculate the percent by mass (weight) of sodium (Na) and chlorine (Cl) in sodium chloride (NaCl)
Relative molecular mass of NaCl  = 22.99 + 35.45 = 58.44
The total mass of Na present:
1 Na is present in the formula, mass = 22.99
Tthe percent by mass (weight) of Na in NaCl: 
%Na = (22.99 ÷ 58.44) x 100 = 39.34%
The total mass of Cl present:
Cl is present in the formula, mass = 35.45
The percent by mass (weight) of Cl in NaCl:
%Cl = (35.45 ÷ 58.44) x 100 = 60.66%
 Here,   39.34 + 60.66 = 100.
The answers above are probably correct if %Na + %Cl = 100, that is, 

Empirical formula: The empirical formula is the simplest formula for a compound in which atoms of different elements are present in simple ratio. It shows the relative number of atoms of each element. For example CH2O is the empirical formula of Glucose C6H12O6

Molecular formula: It is the formula in which the actual number of atoms of  different element are present. For example, if the empirical formula of benzene is CH where as molecular formula is C6H6, etc.

9th CBSE Science Chap 3 Atoms and Molecules Self ...: 1. Fill in the blanks (a) In a chemical reaction, the sum of the masses of the reactants and products remains unchanged. This is called —...

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