Elements in the Modern Periodic Table

Types of Elements

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

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 III^rd 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.

 

Element	Atomic number	Electronic configuration	Valency	Formula of oxide	Formula of hydroxide
Lithium	3	2, 1	+1	Li2O	LiOH
Sodium	11	1, 8, 1	+1	Na2O	NaOH
Potassium	19	2, 8, 8, 1	+1	K2O	KOH
Rubidium	37	2, 8, 18, 8, 1	+1	Rb2O	RbOH
Caesium	55	2, 8, 18, 18, 8, 1	+1	Cs2O	CsOH
Francium	87	2, 8, 18, 32, 18, 8, 1	+1	-	-

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.

Element	Atomic number	Electronic configuration	Valency	State	Colour
Fluorine	9	2, 7	-1	Gas	Greenish but more yellow
Chlorine	17	2, 8, 7	-1	Gas	Greenish yellow
Bromine	35	2, 8, 18, 7	-1	Liquid	Dark red liquid
Iodine	53	2, 8, 18, 18, 7	-1	Solid	Dark purple
Astatine	85	2, 8, 18, 32, 18, 7	-1	-	-

 

Remember

Bromine is the only Liquid non-metal. Iodine when heated undergoes sublimation.

 

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.

Remember

Palladium though a transition element, has an electronic configuration that does not agree with the general rule.

Inner-transition Elements

The 6^th period consists of elements that have atomic numbers 58 to 71. They are called Lanthanides. The 7^th 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 7^th 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.

 

Period	Valency shell	Type of Period	Number of elements	Atomatic number of the elements
1st Period	n = 1	Short period	2	Atomic number 1 and 2
2nd Period	n = 2	Short period	8	Atomic number 3 to 10
3rd Period	n = 3	Long period	8	Atomic number 11 to 18
4th Period	n = 4	Long period	18	Atomic number 19 to 36
5th Period	n = 5	Long period	18	Atomic number 37 to 54
6th Period	n = 6	Long period	32	Atomic number 55 to 86
7th Period	n = 7	Incomplete	23	Atomic number 87 to 109

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, 1^st period = 2 elements.

L Shell - 2 x (2)² = 8, 2^nd 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.

Group - > Period	1A	2A	3A	4A	5A	6A	7A	0
Period 1	1H 1							2He 2
Period 2	3Li 2,1	4Be 2,2	5B 2,3	6C 2,4	7N 2,5	8O 2,6	9F 2,7	10Ne 2,8
Period 3	11Na 2,8,1	12Mg 2,8,2	13Al 2,8,3	14Si 2,8,4	15P 2,8,5	16S 2,8,6	17Cl 2,8,7	18Ar 2,8,8
Period 4	19K 2,8,8,1	20Ca 2,8,8,2

 

Remember

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.