Chapter - 3 Atoms and Molecules

Laws of chemical combination

Law of conservation of mass
According to the law of conservation of mass, mass can neither be created nor destroyed in a chemical reaction. It can only change its form from one form to another.

Law of constant proportion
The law of constant proportion is also known as the law of Definite proportions. It states that in a chemical substance the constituent elements are always present in definite proportion by mass.

Dalton's Atomic theory
According to this theory element a compound or a mixture consist of small particles called atoms.

The theory was further explained as .

Matters are made up of tiny particles called atom.
Atoms are indivisible particles and cannot be created or destroyed.
Atoms of a particular element r identical in mass and chemical property.
Combine in the ratio of small whole numbers to form compound.
Relative number and kind of atoms R constant in a given compound.

Atom

The smallest particle of matter is an atom. Atoms are the building blocks of matter. Millions of atoms stacked together to form a thin piece of paper. The radius of atoms is measured in NM. Since the atom size is very small we might think that is insignificant, but they are there and also affecting whatever we do.

Modern day symbols of atom of different elements:
Dalton designed specific symbols for each element in a very specific sense. Each symbol was meant for a particular element also for a definite quantity of that element that is a single atom.

Berzelius suggested that the symbol of element can be made from one or two letters of the name of the elements. nowadays IUPAC (International Union of Pure and Applied Chemistry) is an international scientific organisation which approves names for elements symbols and units.
For example: Hydrogen as H, aluminium as Al and Cobalt as Co.

Atomic mass
Dalton told in his atomic theory about the atomic mass, each element has a characteristic atomic mass. Those days determining the mass of an individual atom was relatively difficult so atomic masses were determined by using the law of chemical combination and the compounds formed. Similarly, relative atomic mass of the atom of an element is defined as the average mass of the atom.

Example: 3 grams of carbon combines with 4 grams of oxygen to form CO.

Atoms Existence
Atoms of different element cannot exist independently. A number of atoms combines together to form molecules or ions. These molecules or ions aggregates together to form matter which we can see, feel or touch.

Molecules of elements
Molecules of an element are formed by the accumulation of same type of atoms. molecules of elements like Argon helium are made up of only one atom of that element. Molecules can be monoatomic, diatomic, polyatomic and so on. The total number of atoms constituting a molecule is known as its atomicity.

Molecules of compounds
When atoms of different elements join together in definite proportion they form the molecules of a compound. As water molecule consists of hydrogen and oxygen in the ratio of 1:8.

Ions
Compound composed of metals and nonmetals contains charged species known as ions. Ions may consist of a single charged atom or a group of atoms that have a net charge on them. A negatively charged ion is called as "anion" and a positively charged ion is called as "cation". In the case of (Sodium Chloride) NaCl, sodium is a positively charged Ion and chloride is a negatively charged ion.

Writing chemical formulae
The symbolic representation of the composition of a compound is the chemical formula of that particular compound. The combining power of an element is known as its valency. With the help of valency, we can understand that how the atom of a particular element will combine with the atoms of another element to form a compound.

Formula of simple compounds
Simplest compound made up of only two different elements are called as binary compounds. For writing the chemical formula of a compound we write the constituent elements and their valencies and then crossovering the valency of the combining atoms helps forming the compound.

Molecular mass
Molecular mass of a substance is the sum of the atomic masses of all the atoms in a molecule of the substance. The relative mass of a molecule is expressed as atomic mass units(u).

Formula unit mass
Formula unit mass of a substance is the sum of the atomic masses of all the atoms in a formula unit of a compound. This calculation is similar to that of the molecular mass but the only difference is that the substance constitutes of ion particles.

Mole concept
Atoms and molecules are too small to count so the numbers are expressed in terms of Avogadro number. 1 mole is the number equal to Avogadro's number.
Mole can be defined as a unit which represents 6.023 *10²³. mass of one mole of a substance is called its molar mass.

Example
One mole of carbon atom is equals to 12 grams of carbon atom equals to 6.023*10²³ atoms of carbon.
One mole of hydrogen atom is equals to 1 gram of hydrogen atom which is equals to 6.022*10²³ atoms of hydrogen.

Numerical on Mole concept