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ATOM AND MOLECULES

 

Introduction

  • The idea of divisibility of matter was considered long back in India around 500 BC.
  • An Indian philosopher Maharishi Kanad, postulated that if we go on dividing matter (padarth), we shall get small and smaller particles. he named these particles parmanu.
  • Another Indian philosopher, Pakudha Katyayama, elaborate on this doctrine and said that these particles normally exist in a combined form which gives us various forms of matter.
  • Around the same era, ancient Greek philosopher- Democritus and Leucippus suggested that if we go on dividing matter, a stage will come when particles obtained cannot be divided further. Democritus called these indivisible particles atoms.
  • By the end of the 18th century, scientists recognized the difference between elements and compounds and naturally become interested in finding out how and why elements combine and what happens when they combine.
  • Antoine L. Lavoisier laid the foundation of chemical sciences by establishing two important laws of chemical combination.

Laws of Chemical Combination

Lavoisier and Joseph gave two laws of chemical combination after much experimentation.

1. Law of Conservation of Mass

Antoine L. Lavoisier proposed the law of conservation of mass in 1774.
" In a chemical reaction, mass of reactants remains conserved or in a chemical reaction, total mass of products is equal to total mass of reactants."
Example: If mass of reactants is A and B and mass of products is C and D.
mass of A and B = mass of C and D
A+B = C+D
* " Matter can neither be created nor be destroyed in the course of a chemical reaction"

2. Law of Constant Proportion

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. 
According to this law," all pure samples of the same chemical compound contain the same elements combined together in the same proportions by mass."
Example:  
A. CO2 prepared from any o the following methods is found to contain carbon and oxygen in the same ratio of 3:8 by mass.
  • by burning coal.
  • by the action of dilute HCl on marble pieces.
  • by heating limestone.
B. Water obtained by any of the following methods is found to contain hydrogen and oxygen in the same ratio of 1:8 by mass.
  • by distillation of sea water.
  • by combining hydrogen and oxygen.
  • water obtained from a well or hand pump or river.

C. In ammonia molecules, nitrogen, and hydrogen are always found to be in the same ratio of 14:3.
D. Cupric oxide samples can be made by;
  • heating powdered copper in oxygen.
  • treating copper with concentrated nitric acid and heating the resulting cupric nitrate.
  • heating copper carbonate.

Dalton's Atomic Theory

  • In 1803, John Dalton developed a theory of the structure of matter, which is known as Dalton's atomic theory.
  • 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 are:
  1. All the matter is made of very tiny particles called atoms.
  2. Atoms are indivisible particles, which cannot be created or destroyed in a chemical reaction.
  3. Atoms of a given element are identical in mass and chemical properties.
  4. Atoms of different elements have different masses and chemical properties.
  5. Atoms combine in the ratio of small whole numbers to form compounds.
  6. The relative numbers and kinds of atoms are constant in a given compound.

Atom

Atom is the smallest particle of an element, which may or may not have independent existence and which maintains its chemical identity throughout all chemical and physical changes.
Example: Copper atom exists independently while oxygen atom does not exist independently. Oxygen exists in the form of a molecule containing two atoms of oxygen.

Size of Atoms 

Atomic radius is measured in nanometers.

1/109 m = 1 nm

1 m = 109 nm

Sizes of Different Atoms

                                              Radii (in m)

Atoms of hydrogen-             10-10

Molecules of water-              10-9

Molecule of hemoglobin-     10-8

Grain of sand-                       10-4

Ant-                                       10-2

Watermelon-                         10-1


Symbols of Atoms of Different Elements

  • Dalton was the first scientist to use the symbols for the 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 the element be made from one or two-letter of the name of the element.
  • In the beginning, the names of the elements were derived from the name of the place where that element was found. Example: The name of copper was taken from Cyprus
  • some names were taken from specific colors. Example: Gold was taken from the English word meaning yellow.
  • Now a days, IUPAC approves the 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 and the second letter as a small letter. Example: Hydrogen (H), Aluminium (Al), Cobalt (Co)
  • Symbols of some elements are formed the first letter of the names and a letter appearing later in the name. Example: Chlorine (Cl), Zinc (Zn)
  • Other symbols have been taken from the names of the element in Latin, German or Greek. Example:  Iron (Fe)- from its Latin name Ferrum.
                             Sodium (Na)- from Natrium
                             Potassium (K)- from Kalium

Atomic Mass

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 the elements have been found with respect to an atom of carbon-12.
Example: Atomic mass if magnesium is 24, it means that a magnesium atom is 4 times heavier than 1/12th of the C-12 atom.
* Atomic mass of magnesium is 24 u.

Atomic Mass of few Elements

Elements                               Atomic mass (u)
Hydrogen                               1
Carbon                                   12
Nitrogen                                14
Oxygen                                  16
Sodium                                  23
Magnesium                            24
Sulphur                                  32
Chlorine                                 35.5
Calcium                                 40

Molecule

  • A molecule is a group of two or more atoms that are chemically bonded together, that is, tightly held together by attractive forces.
  • Atoms of the same element or of different elements can join together to form molecules.
  • 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 the substances.

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.
  • Atomicity: The number of atoms constituting a molecule is known as its atomicity.
           Example: Diatomic: hydrogen (H2), nitrogen (N2), oxygen (O2), chlorine (Cl2), etc.

                             Triatomic: oxygen (O3)

                             tera-atomic: phosphorus (P4)

                             octa-atomic: Sulphur (S8)


Molecules of Compound 

Atoms of different elements join together in definite proportions to form molecules of the compound.
Example: 


Ions

An ion is a charged particle. It can be of two types:
  • A negatively charged ion is called an anion.
  • A positively charged ion is called a cation.
Example: Sodium Chloride (NaCl)- Its constituent particles are positively charged sodium ion (Na+) and negatively charged chloride ion (Cl-).
  • Polyatomic ion: A group of atoms carrying a charge is known as a polyatomic ion.

Writing Chemical Formula 

  • The chemical formula of a compound is a symbolic representation of its composition.
  • 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 atoms of another element to form a chemical compound.
  • The valency of the atom of an element can be thought of as the hands of that atom.
  • The rules that you have  to follow while writing a chemical formula are as follows:
  1. The valencies or charge on the ion must balance:
  2. When a compound consists of a metal and a non-metal, the name or symbol of the metal is written first. Example: Calcium oxide (CaO), Sodium Chloride (NaCl), Iron sulphide (FeS), Copper oxide (CuO).
  3. In compounds formed with polyatomic ions, the ion is enclosed in a bracket before writing the number to indicate the ratio. In case the number of polyatomic ion is one, the bracket is not required. Example: NaOH

Formula of Simple Compounds

  • The simplest compounds which are made up of two different elements are called binary compounds.
  • while writing the chemical formula for compounds, we write the constituent elements and their valencies. then we must crossover the valencies of the combining atoms.
Example:
1. Hydrogen Chloride
The formula of the compound would be HCl.

2. Hydrogen sulphide

The formula of the compound would be H2S.

3. Carbon tetrachloride

The formula of the compound would be CCl4.

4. Magnesium chloride

The formula of the compound would be MgCl2.

5. Aluminium Oxide 
The formula of the compound would be Al2O3.

6. Calcium oxide
The formula of the compound would be CaO.

7. Sodium nitrate

The formula of the compound would be NaNO3.

8. Calcium hydroxide
The formula of the compound would be Ca(OH)2.

9. Sodium carbonate

The formula of the compound would be Na2CO3.
 
10. Ammonium sulphate
The formula of the compound would be (NH4)SO4.

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.

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.
Example: Calculate the relative mass of H2O and HNO3.
The atomic mass of hydrogen = 1u
The atomic mass of oxygen = 16u
The atomic mass of nitrogen = 14u
Molecular mass of H2O = 2x1+1x16 = 2+16 = 18u
The molecular mass of HNO3  = 1x1+1x14+3x16 = 1+14+48 = 63u

Q: Calculate the formula unit mass of CaCl2.
Atomic mass of Ca + (2 x atomic mass of Cl)
40+2x35.5 = 40+71 = 111u

Mole Concept

  • One mole of any species (atoms, molecules, ions, or particles) is that quantity in number having mass equal to its atomic or molecular mass in grams.
  • The number of particles present in 1 mole of any substance is fixed, with a value of 6.022x1023. This number is called the Avogadro constant or Avogadro number. It is represented by N0.

  • 1 mole = 6.022x1023
  • 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 unit (u). To get the mass of 1 mole of an atom of that element, that is, molar mass, we have to take the same numerical value but change the units from 'u' to 'g'.
  • The molar mass of atoms is also known as gram atomic mass. For example, the atomic mass of hydrogen is 1u. So, the gram atomic mass of hydrogen is 1g.
  • 1u hydrogen has only 1 atom of hydrogen. 1g hydrogen has only 1-mole atoms that is, 6.022x1023atoms of hydrogen.
  • Similarly, 16 u oxygen has only 1 atom of oxygen. 16 g oxygen has 1-mole atoms that is, 6.022x1023atoms of oxygen.
  • The word 'mole' was introduced around 1896 by Wilhelm Ostwald who derived the term from the Latin word moles meaning a 'heap' or 'pile'.
  • The unit mole was accepted in 1967 to provide a simple way of reporting a large number - the massive heap of atoms and molecules in a simple.
  • Formula 
    Number of moles = given mass/molar mass

    n = m/M
and 

Number of moles = given number of particles/Avogadro constant

n = N/N0

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