Chapter 1: Atoms and Molecules
Sugar,salt,sand, diamond ,graphite, copper, silver, gold,iron, aluminum, wood,rocks, minerals, food and fabrics are all different kinds of matter. Matter is called 'padarth' in Hindi.An Indian philospher, Maharishi Kanad, said that if we go on dividing matter, we will get smaller and smaller particles of matter. Ultimately, we will get the smaller particle of matter, which can't be divided any further. Based on this philosophy, Kanad was one of the first persos to propose that matter is made up of very small particles called ''parmanu''. John Dalton called these particles by the name of Atom. The word 'atom' means 'indivisible'- something which cannot be divided further.
Another Indian philospher Pakudha Katyayama went a step further and proposed that the particles of matter normally exist in a combined form and various combination of particles give us various kinds of matter. This combined form of atoms is now called ' molecules'. We now know that all matter is made up of small particles called atoms and molecules. Different kinds of atoms and molecules have different properties due to which different kinds of matter also show different properties. Thus, the properties of matter depend on the properties of atom and molecules from which it is made.
We will now describe the laws of chemical combination which confirmed the idea of atom being the smallest particles of matter.
Laws of chemical combination
There are three important laws of chemical combination. These are:
1. Law of conservation of mass
2. Law of constant proportions
3. Law of multiple proportions
The laws of chemical combination are the experiment laws which have been formulated by scientists after performing a large number of experiments involving various types of chemical reactions. These experimental laws ultimately led to the idea of atoms being the smallest unit of matter. In fact , the laws of chemical combination played a significant role in the development of Dalton's atomic theory of matter.In this chapter we will study only two laws of chemical combination: the law of conservation of mass, and the law of constant proportions. The third law of chemical combination, the law of multiple proportions will be studied in another chapter.
Law of conservation of mass
In the 18th century, scientist noticed that if they carried out a chemical reaction led to the formulation of the law of conservation of mass. Law of conservation of mass was given by Lavoisier in 1774. According to the law of conservation of mass: Matter is neither created nor destroyed in a chemical reaction. The substances which combine together in a chemical reaction are known as ' reactants' whereas the new substances formed as a result of chemical reaction are called products. The law of conservation of mass means that in a chemical reaction, the total mass of products is equal to the total mass of reactants.There is no change in mass during a chemical reaction. There is no change in mass during a chemical reaction. Since, there is no gain or loss in mass in a chemical reaction, the mass remains conserved. Please note that the term total mass of reactants and products includes solids, liquid and gases- including air - that are a part of reaction. The law of conservation of mass will become clear from the following example.
When calcium carbonate is heated, a chemical reaction takes place to form calcium oxide and carbon dioxide. It has been found by experiments that if 100 grams of calcium oxide are decomposed completely then 56 grams of calcium and 44 gram of carbon dioxide are formed.
Laws of Constant Proportions
The law of constant proportions was given by Proust in 1779. He anyalsed the chemical composition of a large number of compounds and came to the conclusion that the proportion of each element in compound is constant.Based on these observation,Proust formulated the law of constant proportions. According to the law of constant proportions: A chemical compound together in the same elements combined together in the same propertion by mass.This law means that whatever be the source from which it is obtained a pure chemical compound is always made up of the same elements in the same mass percentage. For example, water is a compound which always consists of the same proportion by mass. This law method by which always made up of same elements in the same mass percentage. For example, water is a compound which always made up of the same two elements, hydrogen and oxygen, combined together in the same Proportion of 1:8 by mass. Let us discuss this in a little more detail.
We know that water is a compound. If we decompose 100 grams of pure water by passing electricity through it, then 11 grams of hydrogen and 89 gram of oxygen are obtained. Now, if we repeat this experiment by taking pure water from different sources, the same masses of hydrogen and oxygen elements are obtained in every case. This experiment shows that water always consists of the same two elements, hydrogen and oxygen, combined together in the same constant proportion of 11:89 or 1:8 by mass. And this is the law of constant proportions.
John Dalton
John Dalton was born in 1776 in a poor Weaver's family in England. He received his early education from his father and at the village school.Dalton started teaching in the village at the age of 12.In 1793, Dalton left for Manchester to teach physics, chemistry and mathematics in a college. In 1794, he described color blindness.In, 1808, Dalton presented his atomic theory to explain the properties of matter.Dalton's atomic theory of matter became one of the foundation of modern chemistry.Dalton was the first to calculate the masses of the atom of several elements.He died in 1844.
Dalton's atomic theory
1. All the matter is made up of very small particles called atom.
2.Atoms cannot be divided.
3.Atoms can neither be created nor destroyed.
4.Atoms are of various kinds. There are as many kinds of atoms as are elements.
5. Atoms of different elements differ in mass,size and chemical properties.
6.The number and kind of atom in a given compound is fixed.
7.Atoms of the same elements can combine in more than one ratio to form more than one compound.
8.All the atoms of a given elements are identical in every respect, having the same mass, size and chemical proporties.
9.Chemical combination between two elements consists in the joining together of atoms of these elements to form molecules of compounds.
10.During chemical combination in more than one ratio to form more than one compound.
Drawbacks of Atomic Theory
1. Under special circumstances, atoms can be further divided into still smaller particles called electrons, protons and neutrons.
2.Dalton's Atomic Theory says that all the atoms of an element have exactly the same mass.It is, however,now know that atoms of the same element can have slightly different masses.
3.Dalton's atomic theory said that atoms of different elements have different masses.It is, however, now known that even atoms of different elements can have the same mass.
Atoms
Just as all the houses made up a brick,in the same way,all the matter is made up of atoms.Thus, atom are the building blocks of all the matter around us.In chemistry, atom is defined as follow: An atom is the smallest particle of an element that can take part in a chemical reaction.Atoms of the most of the elements are very reactive and don't exist in the free state.They exist in combination with the atoms of the same element or another element.There are as many kinds of atoms as are elements.
Atoms are very, very small in size.An idea of the extremely small size of atoms can be had from the fact that 35,000,000 copper atoms arranged end to end in a line would cover a distance of about 1 centimeter.Tge size of an atom is indicated by its radius which is called atomic radius.Atomic radius is measured in nanometres.The symbol of a nanometres is nm.
Hydrogen atom is the smallest atom of all.The Atomic radius of a hydrogen atom is 0.037 nanometre.If we express the radius of a hydrogen atom in metres, it will be 0.037×10^-9 metre.It is really, very, very small.
Molecules
A molecule is an electrically netrual group of two atoms chemically bonded together.The forces which hold the atoms together in a molecule are called covalent bonds.Thus,a combination of atoms is called a molecule.We know that atoms of most of the elements are very reactive and can't exist in the free state.This gives us another definition of molecule which can be written as follows:A molecule is the smallest particle of a substance which has the properties of that substance and exist in the free state.
Molecules vcan be formed either by the combination of atoms of the atoms of the same element or of different elements. Depending on this,there are two types of molecules: molecules of elements and molecules of compound.
Ions
An ions is a positively or negatively charged atom. Example of the ions are: sodium ion,Na^+. An ion is formed by the loss or gain of electrons by an atom,so it contains an unequal number of electrons and protons. There are two types of ions : cations and anions.
1.A positively charged ion is known as Cation
A cation is formed by the loss of one or more electrons by an atom.
2.A negatively charged ion is known as Anions
An anions is formed by the gain of one or more electrons by an atom.
Simple ions and Compound ions
1. Those ions are formed from single atoms are called simple ions
2.Those ions which are formed from groups of joined atoms are called compound ions.
Mole concept
Banks have extremely large number of coins of various denominations.Counting of such a large number of coins is a difficult job.So, in banks, they weigh coins, rather than count them.The banks people know that a fixed number of a particular coin will always have the same mass.This means that from the mass of a particular type of coins,they can find the number of coins.Thus, bank people link the mass of coins to the number of coins present.In chemistry also, scientist link the mass of an element or compound to the number of atoms or molecules present in them.This is done through mole.
A mole is represented by Avogadro number (6.022×10^23) of particles of a substance. These particles may be atoms, molecules or ions.
The Italian scientist Amendo Avogardo in whose honour the number of particles in 1mole of a substance, 6.022 ×10^23, is known as Avogardo number.
A group of 6.022 ×10^23 particles of a substance is called a mole of that substance.
Moles of atoms
1 mole of atoms of an element has a equal to the gram atomic mass of the element.
Moles of molecules
1 mole of molecules of a substance has a mass equal to the gram molecular mass of the substance.
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