CLASS 9 Science CHAPTER 3 Atoms And Molecules (NCERT Notes)
Antoine L. Lavoisier laid the foundation of chemical sciences by establishing two important laws of chemical combination.
LAW OF CONSERVATION OF MASS
Law of conservation of mass states that mass can neither be created nor destroyed in a chemical reaction.
LAW OF CONSTANT PROPORTIONS
Lavoisier, along with other scientists, noted that many compounds were composed of two or more elements and each such compound had the same elements in the same proportions, irrespective of where the compound came from or who prepared it.
In a compound such as water, the ratio of the mass of hydrogen to the mass of oxygen is always 1:8, whatever the source of water. Thus, if 9 g of water is decomposed, 1 g of hydrogen and 8 g of oxygen are always obtained. Similarly in ammonia, nitrogen and hydrogen are always present in the ratio 14:3 by mass, whatever the method or the source from which it is obtained.
Law of constant proportions is also known as the law of definite proportions. This law was stated by Proust as “In a chemical substance the elements are always present in definite proportions by mass”. John Dalton provided the basic theory about the nature of matter. He took the name ‘atoms’ as given by the Greeks and said that the smallest particles of matter are atoms. His theory was based on the laws of chemical combination. Dalton’s atomic theory provided an 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.
How big are atoms?
Atoms are very small, they are smaller than anything that we can imagine or compare with. Atomic radius is measured in nanometres. 1/10 9 m = 1 nm and 1 m = 109 nm
SYMBOLS OF ATOMS OF DIFFERENT ELEMENTS
Dalton was the first scientist to use the symbols for elements in a very specific sense. When he used a symbol for an element he also meant a definite quantity of that element, that is, one atom of that element. Berzilius suggested that the symbols of elements be made from one or two letters of the name of the element. Now-a-days, 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) aluminium, 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, potassium is K from kalium. Therefore, each element has a name and a unique chemical symbol.
ATOMIC MASS
The atomic mass of an element is the average mass of the atoms of an element measured in atomic mass unit(amu, also known as daltons ,D). The atomic mass unit (earlier abbreviated as ‘amu’, but according to the latest IUPAC recommendations, it is now written as ‘u’ – unified mass) as equal to the mass of one carbon atom. Or the atomic mass is a weighted average of all of the isotopes of that element.
While searching for various atomic mass units, scientists initially took 1/16 of the mass of an atom of naturally occurring oxygen as the unit. This was considered relevant due to two reasons:
• Oxygen reacted with a large number of elements and formed compounds.
• This atomic mass unit gave masses of most of the elements as whole numbers.
However, in 1961 for a universally accepted atomic mass unit, carbon-12 isotope was chosen as the standard reference for measuring atomic masses. One atomic mass unit is a mass unit equal to exactly one- twelfth (1/12th) the mass of one atom of carbon-12. The relative atomic masses of all elements have been found with respect to an atom of carbon-12.
HOW DO ATOMS EXIST?
Atoms of most elements are not able to exist independently. Atoms form molecules and ions. These molecules or ions aggregate in large numbers to form the matter that we can see, feel or touch.
What is a Molecule?
A molecule is a group of two or more atoms that are chemically bonded together, that is, tightly held together by attractive forces. Or a molecule can be defined as the smallest particle of an element or a compound that is capable of an independent existence and shows all the properties of that substance. Atoms of the same element or of different elements can join together to form molecules.
MOLECULES OF ELEMENTS
The molecules of an element are constituted by the same type of atoms. Molecules of many elements, such as argon (Ar), helium (He) etc. are made up of only one atom of that element.
But this is not the case with most of the non- metals.
For example, a molecule of oxygen consists of two atoms of oxygen and hence it is known as a diatomic molecule, O2. If 3 atoms of oxygen unite into a molecule, instead of the usual 2, we get ozone. The number of atoms constituting a molecule is known as its atomicity.
MOLECULES OF COMPOUNDS
Atoms of different elements join together in definite proportions to form molecules of compounds.
WHAT IS AN ION?
Compounds composed of metals and non-metals contain charged species. The charged species are known as ions. 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.
Writing Chemical Formulae
The chemical formula of a compound is a symbolic representation of its composition. 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.
The rules that you have to follow while writing a chemical formula are as follows:
• the valencies or charges on the ion must balance.
• when a compound consists of a metal and a non-metal, the name or symbol of the metal is written first. For example: calcium oxide (CaO), sodium chloride (NaCl), iron sulphide (FeS), copper oxide (CuO) etc., where oxygen, chlorine, sulphur are non-metals and are written on the right, whereas calcium, sodium, iron and copper are metals, and are written on the left.
• in compounds formed with polyatomic ions, the ion is enclosed in a bracket before writing the number to indicate the ratio.
FORMULAE OF SIMPLE COMPOUNDS
The simplest compounds, which are made up of two different elements are called binary compounds.
While writing the chemical formulae for compounds, we write the constituent elements and their valencies. Then we must crossover the valencies of the combining atoms.
Examples
1. Formula of hydrogen chloride
Formula of the compound would be HCl.
2. Formula of hydrogen sulphide
3. Formula of carbon tetrachloride
The formulae of ionic compounds are simply the whole number ratio of the positive to negative ions in the structure.
For magnesium chloride, we write the symbol of cation (Mg2+) first followed by the symbol of anion (Cl-
). Then their charges are criss-crossed to get the formula.
4. Formula of magnesium chloride
Thus, in magnesium chloride, there are two chloride ions (Cl-
) for each magnesium ion (Mg2+). The positive and negative charges must balance each other and the overall structure must be neutral. Note that in the formula, the charges on the ions are not indicated.
Molecular Mass and Mole Concept
MOLECULAR MASS
The molecular mass of a substance is the sum of the atomic masses of all the atoms in a molecule of the substance. It is therefore the relative mass of a molecule expressed in atomic mass units (u).
FORMULA UNIT MASS
The formula unit mass of a substance is a sum of the atomic masses of all atoms in a formula unit of a compound. Formula unit mass is calculated in the same manner as we calculate the molecular mass. The only difference is that we use the word formula unit for those substances whose constituent
particles are ions. For example, sodium chloride as discussed above, has a formula unit NaCl. Its formula unit mass can be calculated as–
1 × 23 + 1 × 35.5 = 58.5 u
MOLE CONCEPT
Example of the reaction of hydrogen and oxygen to form water:
2H2+ O2 → 2H2O.
The above reaction indicates that
(i) two molecules of hydrogen combine with one molecule of oxygen to form two molecules of water,
(ii) 4 u of hydrogen molecules combine with 32 u of oxygen molecules to form 36 u of water molecules.
The mole is the amount of substance that contains the same number of particles (atoms/ ions/ molecules / formula units etc.) as there are atoms in exactly 12 g of carbon-12. One mole of any species (atoms, molecules, ions or particles) is that quantity in number having a mass equal to its atomic
( From NCERT Book )
or molecular mass in grams. The number of particles (atoms, molecules or ions) present in 1 mole of any substance is fixed, with a value of 6.022 × 1023. This is an experimentally obtained value. This
number is called the Avogadro Constant or Avogadro Number (represented by N0),named in honour of the Italian scientist, Amedeo Avogadro. 1 mole (of anything) = 6.022 × 1023 in number. 1 mole of a particular substance is also fixed.
The mass of 1 mole of a substance is equal to its relative atomic or molecular mass in grams. The atomic mass of an element gives us the mass of one atom of that element in atomic mass units (u). To get the mass of 1 mole of atom of that element, that is, molar mass, we have to take the same numerical
value but change the units from ‘u’ to ‘g’. Molar mass of atoms is also known as gram atomic mass. For example, atomic mass of hydrogen=1u. So, gram atomic mass of hydrogen = 1 g.
1 u hydrogen has only 1 atom of hydrogen
1 g hydrogen has 1 mole atoms, that is,
6.022 × 1023 atoms of hydrogen.
Similarly,
16 u oxygen has only 1 atom of oxygen,
16 g oxygen has 1 mole atoms, that is,
6.022 × 1023 atoms of oxygen.
To find the gram molecular mass or molar mass of a molecule, we keep the numerical value the same as the molecular mass, but simply change units as above from u to g. For example molecular mass of water (H2O) is 18 u. From here we understand that 18 u water has only 1 molecule of water, 18 g water has 1 mole molecules of water, that is, 6.022 × 1023 molecules of water.
1 mole = 6.022 × 1023 number
= Relative mass in grams.
