### Atoms, Moles, Formulae and Equations

Atom is the smallest particle of an element, which can take part in chemical reaction.

Molecule is the smallest particle of an element which can exist on its own and still retain the chemical properties of that element.

Ion is an atom or group of atoms which are either positively or negatively charged due to loss or gain of electrons.

First 20 elements
1. Hydrogen
2. Helium
3. Lithium
4. Beryllium
5. Boron
6. Carbon
7. Nitrogen
8. Oxygen
9. Flourine
10. Neon
11. Sodium
12. Magnesium
13. Aluminum
14. Silicon
15. Phosphorus
16. Sulphur
17. Chlorine
18. Argon
19. Potassium
20. Calcium
Relative atomic mass of an element is the number of times the average mass of one atom of an element is greater than one twelfth of the mass of carbon 12.
Relative Molecular mass of a compound is the number of times the average mass of one Molecule of the substance is greater than one-twelfth of the mass of carbon 12.

Mole
One mole of a substance is the amount containing as many elementary entities as the number of atoms in exactly 12 grams of carbon 12.
Note: Avogadro number = 6.02 ×10²³
Molar mass of any substance is the relative molecular mass expressed in grams.

Example 1: Calculate the molar mass of KOH (k=39, O=16, H=1)
Solution
K=39, O=16, H=1
KOH= 1×39+1×16+1×1
=39+16+1
=56g/mol

Laws of Chemical Combination
There are four laws of chemical combination:
1. Law of conservation of Mass: this states that matter is neither created nor destroyed but can be transformed into different states.
2. Law of definite proportions: this states that all pure sample of a particular chemical compound contain the same number of elements mixed in the same proportion by mass.
3. Law of multiple proportions: this law states that when two elements x and y combine to form more than one compound, then the several masses of element x, which combine with a fixed ratio of the other element y, are in a simple multiple proportion.
4. Law of reciprocal proportion: this law states that the masses of several elements A,B,C which combine separately with a fixed mass of another element D, are the same as the masses in which A, B, C, themselves combine with one another.
Note: Valency is the combining power of an element. Scroll below to take quiz

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