3. Atoms and Molecules | Laws of Chemical Combination Science class 9
3. Atoms and Molecules | Laws of Chemical Combination Science class 9
Laws of Chemical Combination
3. Atoms And Molecules
Element: The basic substance of matter that can not be simplified is an element.
Example: Hydrogen, Carbon, Oxygen, Iron, Silver and Gold etc.
Atom: The smallest tiny particles of matter which cann't be divided further is called atom (Parmanu).
Molecules: A group of two or more than two atoms of the same element or different elements that chemically bonded together is called molecules.
Example: O2, H2, N2, H2O, CO2, MgCl2 etc.
Compound: A molecule that contains more than one element is compound.
Example: H2O, CO2, NH3, BrCl2, CH4 etc.
The smallest tiny particles of element is atom. like atom of Hydrogen (H), atom of oxygen (O), atom of Carbon (C), atom of Magnesium (Mg) etc.
So all matters are made by these tiny particles of elements i.e atoms.
Formation of Matter
Atoms of elements
⇓
Molecules Or atoms
⇓
Compounds
⇓
Matters
Laws of Chemical Combination:
The laws of chemical combination were established by Lavoisier and
Joseph L. Proust.
The laws of chemical combination are two;
1. Law of conservation of mass: Law of conservation of mass states that
mass can neither be created nor destroyed in a chemical reaction.
2. Law of constant proportions: In a chemical substance the elements are
always present in definite proportions by mass.
Dalton's Atomic Theory:
British chemist John Dalton provided the basic theory about the nature of matter.
Dalton picked up the idea of divisibility of matter, which was till then just a philosophy. 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.
The postulates of dalton's 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.
The name of elements and its symbol:
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).
Example:
Hydrogen: H
Carbon : C
Aluminium: Al
Magnesium: Mg
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.
Each element has a name and a unique chemical symbol.
Some examples of elements and their symbol.
Atomic Mass: An atomic mass unit or amu is one twelfth of the mass of an unbound atom of carbon -12 isotope. This is approximetely 1.67377 x 10 -27 kilogram (kg).
The relative atomic mass of the atom of an element is defined as the average mass of the atom, as compared to 1/12th the mass of one carbon-12 atom.
Atomic mass unit is abbreviated as 'amu' but on IUPAC latest recommendation, it is writen as 'u'.
Existance of atoms:
Atoms never exist independly. They form molecules or ions.
We can say, atoms exist in group.
Ions And Atoms
3. Atoms and Molecules
Ions: The charged atoms are known as ions.
Ions have two types of charge.
(1) Anion : A negatively charged ion is called an ‘anion’.
Eg: Cl-, O-2, S2-, N3-, Br-, H- etc.
These all atoms have negative charged so they are ions with anoin.
Note: All non-metals form anoin.
(2) Cation : The positively charged ion is called a ‘cation’.
Eg. Na+, K+, Ag+, Cu+, Mg2+, Ca2+, Zn2+, Fe2+, Al3+ etc.
These all atoms have positive charged so they are ions with cation.
Note: All metals form 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.
Polyatomic ion: A group of atoms carrying a charge is known as a polyatomic ion.
Valency: The combining power (or capacity) of an element is known as its valency.
Writing Chemical Formulae
The chemical formula of a compound is a symbolic representation of its composition.
First many questions might have risen in the mind.
(i) How are atoms charged?
(ii) How different aions have different charges like -, 2-, 3- or +, 2+, and 3+ etc ?
(iii) What is combining power or capacity of an element ?
(iv) In waht manner compounds form?
For understanding this ion and its type anion and cation as well as valancy we need to learn more about the structure of atom before learning writing chemical formula.
Structure of atom:
Always rememer some points these are as follow:
- An atom has three non-atomic particles i.e electrons, protons and neutrons.
- All electrons moves around the nucleus of an atom in different shells.
- Protons and neutrons are present in nucleus of an atom.
- All electrons have same negative charge and no mass.
- All protons have same positive charge and mass as 1 unit.
- Neutrons have no charge but have mass as 1 unit.
- The numbers of electrons and protons are equal in an atom.
- The numbers of neutrons may be greater, equal or less than protons.
- The number of protons present in nucleus of an atom is also calculated as atomic number of the same atom.
- The sum of protons and neutrons present in nucleus of an atom is calculated as atomic mass of the same atom.
- Hydrogen has no any neutron in its nucleus. There is only 1 proton.
- Each orbit is also known as shell and they are called energy levels.
- These orbits or shells are represented by the letters K,L,M,N,… or the numbers, n=1,2,3,4,….
Distribution of electrons in different shells or orbits.
The maximum number of electrons present in a shell is given by the formula 2n2, where ‘n’ is the orbit number or energy level index, 1,2,3,….
For example:
It means the first shell can have only maximum 2 electrons, second can have maximum 8 electrons, third shell can have maximum 18 electrons and forth shell can have maximum 32 electrons.
See the table:
Electronic Configuration And Valency Table | |||||||
Atomic No. | Symbol (Atom) | No. of electrons | k shell | L shell | M shell | N shell | Valency |
1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17. 18. 19. 20. |
H He Li Be B C N O F Ne Na Mg Al Si P S Cl Ar K Ca |
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 |
1 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 |
0 1 2 3 4 5 6 7 8 8 8 8 8 8 8 8 8 8 8 |
1 2 3 4 5 6 7 8 8 8 |
0 1 2 |
1 0 1 2 3 4 3 2 1 0 1 2 3 4 3 2 1 0 1 2 |
Formation of Simple Molecules
3. Atoms and Molecules
The rules that you have to follow while writing a chemical formula are as follows:
(i) The valencies or charges on the ion must balance.
(ii) 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.
(iii) 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.
(iv) In compounds formed with polyatomic ions, the ion is enclosed in a bracket before writing the number to indicate the ratio.
Binary Compounds: The simplest compounds, which are made up of two different elements are called binary compounds.
Writing Formula of Simple Compounds:
While writing the chemical formulae for compounds, we write the constituent
elements and their valencies as shown below. Then we must crossover the valencies of the combining atoms.
There are two non-metals which participate in molecule
Example 1:
Formula of Hydrogen Compound:
Therefore formula: HCl
Example 2:
Formula of Hydrogen Sulphide:
Therefore formula:H2O
Example 3:
Formula of Carbon Tetrachloride:
Therefore formula: CCl4
Formula of Ionic Compound: When the First element is Metal which forms cation (+) and Second element is non-metal which form anion (-).
Formula of Magnesium Chloride:
For finding the the formula of Magnesium Chloride, we write symbol of cation with element like this (Mg+), After this we write the symbol of anion of (Cl-). Then we crossover the charges we obtain formula.
Example 4:
Therefore formula: MgCl2
Example 5:
Formula of Alumunium Oxide:
Therefore formula:Al2O3
The Compound of Polyatomic Ions:
Example 6:
Formula of Sodium Nitrate:
Therefore formula: NaNO3
Example 6:
Therefore formula: (NH4)2SO4
Molecular Mass
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. E.g- Molecular Mass of water(H2O).
Atomic mass of Hydrogen = 1 u
Atomic mass of Oxygen = 16 u
There are two atoms of hydrogen and one atom of oxygen in a water molecule.
∴ Molecular mass of water = 2 x 1 + 16 = 18 u
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.
Example -
NaCl, CaCl2, Al2O3, MgCl2 etc.
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.
- The amount of substances can be calculated by its mass or by the number of atoms.
Mole:
Mole is the amount of substance in which the number of particles of any species (atom, molecules, ions or particles) are equal to the atoms present in carbon-12 in 12 gram. This amount of species is known as mole.
Mole is a heap of many atoms. In which numbers of atoms, molecules or ions of any element is equal to 6.022 x 1023.
Avogadro Constant or Avogadro Number: The number of particles (atoms, molecules or ions) present in 1 mole of any substance is fixed, with a value of 6.022 x 1023. This is an experimentally obtained value. This number is called the Avogadro Constant or Avogadro Number (represented by N0).
Example:
Atom of Element | Atomic Mass(u) | Quantity in (g) | No. of Mole | Number of atoms |
Hydrogen | 1 u | 1 g | 1 | 6.022 x 1023 |
Carbon | 12 u | 12 g | 1 | 6.022 x 1023 |
Nitrogen | 14 u | 14 g | 1 | 6.022 x 1023 |
Oxygen | 16 u | 16 g | 1 | 6.022 x 1023 |
Sodium | 23 u | 23 g | 1 | 6.022 x 1023 |
See the table and try to understand the following things:
- There is 1 mole in 1 g of hydrogen atoms and 6.022 x 1023 atoms in that of 1 mole.
-- There is 1 mole in 12 g of Carbon atoms and 6.022 x 1023 atoms in that of 1 mole.
-- There is 1 mole in 16 g of Oxygen atoms and 6.022 x 1023 atoms in that of 1 mole.
Similarily you can find the mole of every element and you can also find the number of atoms in this mole.
Now understand this:
-- There is 1 atom in 1 u of hydrogen.
-- There is 1 atom in 12 u of Carbon.
-- There is 1 atom in 14 u of Nitrogen.
-- There is 1 atom in 16 u of Oxygen.
Similarly, you can find atom of all elements.
Molar mass : The Mass of one mole of atoms of that element is known as molar mass. 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. and 1 g of hydrogen has 1 mole. 1 mole of hydrogen has 6.022 x 1023 atoms.
Similarly; The molar mass of carbon is 12 g.
Molar mass of 1 mole of oxygen is 16 g.
Molar mass of 1 mole of sodium is 23 g.
Similarily:
In 1 u of Hydrogen has only 1 Hydrogen atom.
In 4 u of Helium has only 1 Helium atom.
In 12 u of Carbon has only 1 Carbon atom.
Assignment
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Science Chapter List
1. Matter in Our Surroundings
2. Is Matter around us Pure
3. Atoms and Molecules
4. Structure of The Atom
5. The Fundamental Unit of Life
6. Tissues
7. Diversity in Living Organisms
8. Motion
9. Force and Laws of Motion
10. Gravitation
11. Work and Energy
12. Sound
13. Why Do We Fall ill
14. Natural Resources
15. Improvement in Food Resources
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