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On the basis of electronic configuration, the elements of the periodic table are classified into:
- Noble gases
- Normal elements
- Transition elements
- Inner-transition elements
- Alkali metals
- Halogens
Noble Gases
Noble gases are also known as inert gases and do not take part in
chemical reactions. They have their outermost shell complete and thus remain
stable. They do not generally combine with other substances, nor are they
affected by oxidising agents or by reducing agents. They are placed in the 18 or VIIIA group. Since, the outermost shell is complete, the valency is zero, hence VIIIA group is also referred to as zero group.
Normal Elements
In the case of these elements, all shells except the outermost shell are completely filled. Elements belonging to 1 (IA), 2 (IIA), 3 (IIIA), 4 (IVA), 5 (VA), 6 (VIA) and 7 (VIIA) are normal elements. Elements of the second period are known as typical elements [Li (Z = 3) to Ne (Z = 10)] because each element is placed in a group whose number matches with the number of valence electrons. The elements of the IIIrd period are representative elements [Na (Z = 11) to Ar (Z = 18)] as each of them is a representative of its group. Groups 1 (IA) and 2 (IIA) are strongly metallic and are called group of 'alkali metals and alkaline earth metals', while group 7 (VIIA) are halogens.
Alkali Metals
Elements of group I A of the periodic table constitute a family of very reactive metals called alkali metals. They are lithium, sodium, potassium, rubidium, caesium and francium. All of them have one electron in the valence shell. They are called alkali metals because their hydroxides are strong alkalis. These metals are soft, light and easily fusible. In fact, sodium and potassium are lighter than water. At room temperature they readily get oxidised in air and so are preserved under kerosene in the laboratory.
| Lithium
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3
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2, 1
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+1
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Li2O
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LiOH
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| Sodium
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11
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1, 8, 1
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+1
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Na2O
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NaOH
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| Potassium
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19
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2, 8, 8, 1
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+1
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K2O
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KOH
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| Rubidium
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37
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2, 8, 18, 8, 1
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+1
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Rb2O
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RbOH
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| Caesium
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55
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2, 8, 18, 18, 8, 1
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+1
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Cs2O
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CsOH
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| Francium
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87
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2, 8, 18, 32, 18, 8, 1
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+1
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-
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-
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Halogens
The elements placed in group 7 (VIIA) of the periodic table are called halogens or salt producers. All these elements form salts called halides, e.g. NaCl, NaI, KCl, KI etc. Halogen is an ancient Greek word meaning 'salt producer'. Halogens have seven electrons in their valence shell and so are monovalent.
| Fluorine
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9
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2, 7
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-1
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Gas
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Greenish but more yellow
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| Chlorine
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17
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2, 8, 7
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-1
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Gas
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Greenish yellow
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| Bromine
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35
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2, 8, 18, 7
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-1
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Liquid
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Dark red liquid
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| Iodine
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53
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2, 8, 18, 18, 7
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-1
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Solid
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Dark purple
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| Astatine
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85
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2, 8, 18, 32, 18, 7
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-1
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-
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-
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| Bromine is the only Liquid non-metal. Iodine when heated undergoes sublimation.
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Transition Elements
All the elements belonging to 3 to 12 groups are called transition elements. They resemble each other in several physical and chemical properties. They are all metals. They are called transition elements because they are placed between the most reactive metals on the left and non-metals on the right. Their compounds are coloured. They exhibit variable valency.
| Palladium though a transition element, has an electronic configuration that does not agree with the general rule.
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Inner-transition Elements
The 6th period consists of elements that have atomic numbers 58 to 71. They are called Lanthanides. The 7th period consists of elements that have atomic numbers 90 to 105. They are called Actinides. Both of them are called inner transition elements. Lanthanides and actinides are not accommodated in the main body of the periodic table but are placed in separate rows in form of two series at the bottom of the modern periodic table. The 7th period is an incomplete period as it has only 23 elements.
The position of hydrogen is not certain. Thus it can be placed in both group 1 (IA) and group 7 (VIIA).
In a period, the number of valence shell remains the same for all elements. However, the number of electrons in the valence shell increases from left to right.
The number of elements present in each period is given in the following table.
The number of elements in these periods is based on the way electrons are filled into various shells. The maximum number of electrons that can be accommodated in a shell depends on the formula 2n2 where 'n' is the number of the given shell from the nucleus.
For example,
K Shell - 2 x (1)2 = 2, 1st period = 2 elements.
L Shell - 2 x (2)2 = 8, 2nd period = 8 elements.
M Shell - 2 x (3)2 = 18, but the outermost shell can have only 8 electrons, so the third period also has only 8 elements.
The position of an element in the Periodic table tells us about its chemical reactivity.
Representative periodic table for eight groups up to calcium (atomic number 20) with their electronic configuration is given in the table.
| The position of an element in the periodic table is generally determined by its electronic configuration e.g. electronic configuration of sodium is 2, 8, 1 i.e., it has three shells and one electron in the outermost shell. Hence, it is placed in period number 3 and group number 1.
However, in the case of transition elements this pattern is not followed.
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