Atoms and Molecuels

Antoine Luarent de Lavoisier and Joseph Louis Proust were French scientist gave theories of Laws of Chemical Combination after many series of experiments. They define the knowhow of formation of matter. Their Laws were accepted universally.

There are mainly two Laws of Chemical Combination: (i) Law of Conservation of Mass and (ii) Law of Constant Proportions

(i) Law of Conservation of Mass

Law of Conservation of Mass states that mass can neither be created nor destroyed in a chemical reaction.

A series of experiments done by scientists and established facts that in a chemical reaction only form of matter changed and not created nor destroyed.

(ii) Law of Constant Proportions

Law of Constant Proportions states that In a chemical substance the elements are always present in definite proportions by mass.

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.

Example: Water is composed of hydrogen and oxygen. A water molecule always consists of one part of oxygen and 2 part of hydrogen. And by mass a water molecule consists of mass of hydrogen to the mass of oxygen in the ratio of 1:8 whatever the source of water.

And, thus, when 9 g of water is decomposed, 1 g of hydrogen and 8 g of oxygen are always obtained.

Similarly, ammonia which is formed by the chemical combination of nitrogen and hydrogen, contains nitrogen and hydrogen in the ratio of 14:3 by mass.

Explanation of Law of Constant Proportion: Dalton's Atomic Theory

British scientist John Dalton gave the appropriate explanation of Law of Constant Proportion. This is known as Dalton's Atomic Theory.

Postulates of Dalton's Atomic Theory

(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.

Indivisible Particles of Matter-Atoms

What is an Atom?

Atom is the smallest particle of matter. All elements are made up of atoms. Atoms are the building block of matter.

Definition of Atom

The smallest particle of matter, which contains all the properties of matter and cannot be divided further, is called Atom.

"Atom = a + tomos"

The word "Tomos" is a Greek word which means "to cut" and "a" is a negative prefix which means "not". Thus full meaning of "A-tomos" is which cannot be cut or divided further. Thus, "Atom" means "Indivisible".

Atom of most of the elements does not exist in free state in nature. Since an atom is chemically very much reactive and hence does not exist in free state in nature.

Example of atoms: hydrogen atom, oxygen atom, nitrogen atom, chlorine atom, etc.

Size of an Atom

Atom is so small that we cannot see an atom with naked eyes rather a very powerful microscope is needed to see the atom.

Atoms are spherical in shape. Thus, size of an atom is measured by its radius.

Radius of an atom is measured in nanometers. Nanometer is small unit of meter.

1 m = 10⁹ nm

Or, `1/(10^9)\ m` = 1 nm

Example of sizes of some of the atoms/molecule

The size (radius) of hydrogen atom = `10^(-10)` m

The size (radius) of water molecule = `10^(-9)` m

The size (radius) of grain of sand = `10^(-4)` m

The size of an ant = `10^(-2)`

Atoms and their Symbol

So far more than 115 elements are discovered. For convenience every elements are denoted by their symbols.

It was John Dalton who started to use the symbols for elements. Dalton used some pictures as symbol to denote an element.

Latter Berzilius suggested that symbols of elements be made from one or two letters of the name of the element.

In the beginning the names of element were derived from the name of place where they were found for the first time. For example: The name of copper was taken from Cyprus, where it was found first.

Name of some elements are taken from their specific colour, for example Gold which means yellow.

The first or first two letters of the name of elements are taken as symbol to denote them.

Symbol and Name of Some Elements

H (Hydrogen), O(Oxygen), He (Helium), Li (Lithium), Be (Beryllium), B (Boron), C (Carbon), N (Nitrogen), Ne (Neon), Na (Sodium), Mg (Magnesium), Al (Aluminium), Si (Silicon), P (Phosphorous), S (Sulphur), Cl (Chlorine), Ar (Argon), K (Potassium), Ca (Calcium), Br (Bromine), F (Fluorine), I (Iodine), Co (Cobalt), Ni (Nickel) etc.

Atomic Mass

Atomic Mass of elements was the most remarkable concept of Dalton's Atomic Theory. According to Dalton's Atomic Theory, each element had a characteristic Atomic Mass.

Determining of Atomic Mass

To determine the atomic mass of an element was a relatively difficult task, thus Relative Atomic Masses were determined using the Law of Chemical Combinations and the compounds formed.

Unit of Atomic Mass

Atomic Mass unit was abbreviated as 'amu'. Thus, in early days 'amu' was used to denote atomic mass unit.

But latter according to suggestion of IUPAC (International Union of Pure and Applied Chemistry), simply 'u' is started to used to denote the Atomic Mass Unit. Where 'u' stands for 'unified mass'.

Initially Oxygen was taken as standard to determine the relative atomic mass of other elements because of following two reasons:

(a) Oxygen reacted with a large number of elements and formed compounds.

(b) This atomic mass unit gave masses of most of the elements as whole number.

The atomic mass unit (amu) of Oxygen assigned equal to '1.33 u' and atomic mass unit (amu) of Carbon assigned equal to '1 u'.

Latter in 1961 'carbon-12' an isotope of carbon was chosen as the standard reference for measuring atomic mass.

Thus, One atomic mass unit or '1 u' is the mass unit equal to exactly one-twelfth (`1/12th`) the mass of one atom of 'carbon-12`

Or, `1\ u = 1/12\ th` of the mass of one carbon-12 atom.

Thus, relative atomic mass of all elements have been found with respect to an atom of carbon-12.

Definition of Relative Atomic Mass :

The relative atomic mass of the atom of an element is defined as the average mass of the atom, as compared to `1/12^(th)` of the mass of one carbon-12 atom.

The relative atomic mass of Hydrogen = 1 u

This means that hydrogen atom is 1 times heavier than `1/12\ th` of the carbon-12 atom.

How do Atoms Exist?

Atoms are very much reactive, so atoms of most of the elements do not exist in free state in nature. Atoms form molecules and ions. These molecules or ions aggregate in large numbers to form the matter that we can see, feel or touch.

Example:

Oxygen (O₂), Hydrogen (H₂), Sodium (Na), Potassium (K), Chlorine (Cl₂), etc.

What is a Molecule?

Definition of 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 that substance.

A molecule is chemically neutral.

Example:

Nitrogen molecule: nitrogen molecule consists of two nitrogen atom (N₂), water molecule: water molecule consists of two hydrogen and one oxygen atom (H₂O), molecule of oxygen consists of two oxygen atoms (O₂).

Types of molecules

Atoms of the same element or of different elements can join together to form molecules. Molecules can be divided into two types: (a) Molecules of Elements (b) Molecules of compounds

(a) Molecules of Elements

Molecules formed by the atoms of same type of element are called Molecules of Elements.

(i) Molecules of metals

Molecules of most of the metals are formed by only one atom of elements. For example: Copper (Cu), Aluminium (Al), Iron (Fe), Gold (Au), Silver (Ag), etc.

(ii) Molecules of Non-metals

Molecules of most of the non-metals are formed by the combination of two or more atoms of same elements. For example: Molecule of Oxygen (O₂), Molecule of hydrogen (H₂), Molecule of Chlorine (Cl₂), Molecule of Fluorine (F₂),

However, molecules of some non-metals are formed by only one atom of that element. For example: molecule of carbon (C).

Atomicity

Molecules of elements are formed by the chemical combination of two or more than two atoms of same elements. The number of atoms constituting a molecule is known as its Atomicity.

On the basis of number of atoms combined together, molecules of element can be divided into five types: (a) Monoatomic (b) Diatomic (c) Triatomic (d) Tetraatomic and (e) Polyatomic

(a) Monoatomic Molecule

Molecules formed by only one atom, are called Monoatomic Molecules or simply Monoatomic.

Example: Molecule of Argon (Ar), Molecule of Helium (He), etc.

(b) Diatomic Molecule

Molecules formed by two atoms of same elements are called Diatomic Molecule or simply Diatomic.

Example: Molecule of oxygen (O₂), Molecule of chlorine (Cl₂), Molecule of hydrogen (H₂), etc.

(c) Triatomic Molecule

Molecules formed by three atoms are called Triatomic Molecule or simply Triatomic.

Example:

Ozone (O₃): Molecule of ozone (O₃) is formed by the chemical combination of three oxygen atom, and hence is called triatomic molecule.

(d) Tetra-atomic Molecule

Molecules formed by chemical combination of four atoms, are called Tetra-atomic Molecule.

Example:

Phosphorous (P₄): Molecule of phosphorous (P₄) is formed by the chemical combination of four atoms of phosphorous and hence is called tetra-atomic molecule.

(e) Poly Atomic Molecule

Molecules formed by the chemical combination of more than two atoms are called Poly atomic moleucles.

Example:

Ozone (O₃): Molecule of ozone (O₃) is formed by the chemical combination of three oxygen atom, and hence is called polyatomic molecule.

Phosphorous (P₄): Molecule of phosphorous (P₄) is formed by the chemical combination of four atoms of phosphorous and hence is called poly atomic molecule.

Molecules of Compounds

Example:

Molecule of Water (H₂O): Molecule of water is formed by the combination of hydrogen and Oxygen atoms in the simple ratio of 2:1. And a water molecule is formed by the combination of hydrogen and oxygen atom in the ratio by mass of 1:8.

Combination of simple ratio of hydrogen and oxygen to form one molecule of water means, two atoms of hydrogen and one atoms of oxygen, i.e. in the ratio of 2:1, combined together to form one molecule of water.

And combination of ratio by mass means ratio of atomic mass of hydrogen atoms to that of mass of oxygen atom.

Here mass of hydrogen is equal = 1

And mass of oxygen = 16

And the ratio of hydrogen and oxygen to combine to form a water molecule = 1:8

This means that, one atom of hydrogen is combined with half of the oxygen atom.

Since, it is not possible to combine hydrogen with half of the oxygen atom, thus two hydrogen atom combined with one oxygen atom to form one molecule of water.

This is the cause that, the formula of water is H₂O.

Molecule of Ammonia (NH₃): Molecule of ammonia is formed by the combination of nitrogen and hydrogen atoms in the ratio of 14:3 by mass.

Molecule of Carbon dioxide (CO₂): Molecule of carbon dioxide is formed by the combination of carbon and oxygen in the ration of 3:8 by mass.