Chemistry

Atoms and Molecules


1. Molecules of Elements


The molecule of an element contains two (or more) similar atoms chemically combined together.For example, a molecule of hydrogen element contains 2 hydrogen atoms combined together,and it is written as H2 [see Figure 22(a)]. Hydrogen gas consists of H2 molecules and not of single atoms H.Hydrogen molecule (H2) is a diatomic molecule because it contains 2 atoms per molecule.Similarly, nitrogen gas exists as N2 molecules, oxygen gas as O2 molecules and chlorine gas as Cl2 molecules[see Figures 22(b),(c) and (d)].Bromine element, which is a liquid, consists of Br2 molecules and iodine element, which is a solid, consists of 12 molecules.
Ozone gas has 3 oxygen atoms combined together, so ozone exists in the form of O3 molecules.Phosphorus element has 4 phosphorus atoms combined together, so phosphorus exists in the form of P4 molecules.Solid sulphur element has 8 sulphur atoms joined together, therefore, sulphur exists in the form of S8 molecules. The noble gases like helium,neon, argon and krypton, etc.,exist as single atoms He,Ne,Ar and Kr, respectively. So, their atoms and molecules are just the same. Most of the elements which exist as solids, consist of a large cluster of atoms which can be considered to be 'giant molecules' or 'very big molecules'.For example, all the metals consist of giant molecules and they are represented by their symbols. For example, the metal elements like sodium, magnesium, aluminium and iron, etc., are represented by their symbols Na, Mg, A1 and Fe, etc., they do not have any separate formulae. Carbon element is a solid non-metal which is also represented by its symbol C. The properties of solid elements are not due to their single atoms but due to big cluster of atoms. We will now discuss atomicity of elements.
The number of atoms present in one molecule of an element is called its atomicity.The atomicity of some of the common elements is discussed below.
(i) Noble gases (helium, neon, argon, krypton, etc.) have one atom each in their molecules such as He, Ne, Ar and Kr. So,the atomicity of noble gases is 1.Noble gases are said to be monoatomic (having 1-atom molecules).The atomicity of metal elements like sodium (Na), magnesium (Mg), aluminium (Al), copper(Cu) and iron (Fe), etc., is also taken to be 1.Thus, metals are considered to be monoatomic.The atomicity of carbon element is also 1.So, carbon is monoatomic.
(ii) Hydrogen (H2), nitrogen (N2), oxygen (O2), chlorine (Cl2), bromine (Br2) and iodine (l2), all have 2 atoms each in their molecules. So,the atomicity of hydrogen, nitrogen, oxygen, chlorine, bromine and iodine is 2 each.In other words, the elements hydrogen, nitrogen, oxygen, chlorine, bromine and iodine are diatomic (having 2-atom molecules).
(iii) Ozone (O3) has 3 atoms in its molecule, so the atomicity of ozone is 3.Ozone is said to be triatomic (having 3-atom molecules).
(iv) Phosphorus (p4) has 4 atoms in its molecule, so the atomicity of phosphorus is 4.Phosphorus is said to be tetra-atomic (having 4-atom molecules).
(v) Solid sulphur (S8) has 8 atoms in its molecule, so the atomicity of sulphur is 8.Sulphur is said to be octa-atomic (having 8-atom molecules).

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Notes


Atoms and Molecules - Notes
1. Monovalent Cations (Cations Having a Valency of 1+)
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2. Drawbacks of Dalton’s Atomic Theory
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3. Divalent Cations (Cations Having a Valency of 2+)
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4. Trivalent Cations (Cations Having a Valency of 3+)
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5. Dalton’s Symbols of Elements
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6. an example, let us give the significance of symbol C
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7. Formulae of Some Molecular Compounds
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8. An Important Discussion
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9. Molecules
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10. Simple Ions and Compound Ions (Polyatomic Ions)
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11. Explanation of the Law of Conservation of Mass
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12. Molecular Formulae of Some Common Elements
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13. 2. A Negatively Charged Ion is Known as Anion
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14. As an example, let us give the significance of the formula H2O
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15. Molecules of Compounds
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16. Formulae of Elements
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17. Chemical Formulae
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18. Symbols of Elements
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19. Some Ionic Compounds
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20. Gram Atomic Mass And Gram Molecular Mass
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21. Formulae of Compounds
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22. Molecular Mass
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23. Atoms
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24. Formula Mass
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25. Atoms, Molecules and Ions
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26. 1. Molecules of Elements
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27. Mole of Atoms
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28. 1. A Positively Charged Ion is Known as Cation
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29. Ions
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30. Mole of Molecules
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31. Dalton's Atomic Theory
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32. Laws Of Chemical Combination
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33. Calculation of Molecular Mass
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34. Divalent Anions (Anions Having a Valency of 2-)
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35. Some Common Ions
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36. Molecular Masses of Some Common Elements
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37. John Dalton
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38. Significance of the Formula of a Substance
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39. Explanation of the Law of Constant Proportions
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40. Formula Unit of Ionic Compounds
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41. Ionic Compounds
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42. Symbols Derived from Latin Names of the Elements
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43. Modern Symbols of Elements
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44. Atomic Mass Of An Element
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45. Valencies of Some Common Metal Elements Valencies of Some Common Non-Metal Elements
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46. Atomic Masses of Some Common Elements
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47. Laws Of Conservation of Mass
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48. Writing Of Formulae Of Molecular Compounds
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49. Atoms And Molecules
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50. Writing Of Formulae Of Ionic Compounds
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