Classification of Elements

 

Chapter 14
Classification of Elements 

1 Classification of Elements

·      An element is a pure substance made of only one kind of atom. Different elements have different atoms and different properties.

·      The chapter states that 118 elements have been discovered, among which 92 are natural and 26 are artificial.

·      Classification means arranging elements into groups according to similarities and differences in their properties.

2 Periodic Table

A periodic table is a scientific table prepared for the study of elements by keeping elements with similar properties in the same group and elements with different properties in different groups.

·      Vertical columns are called groups.

·      Horizontal rows are called periods.

·      A group contains elements with similar outer electronic arrangement and similar chemical properties.

·      A period contains elements arranged in increasing atomic number from left to right.

3 Mendeleev’s Periodic law

·      Mendeleev’s periodic law states that the physical and chemical properties of elements are periodic functions of their atomic weights.

Limitations of Mendeleev’s Table

·      It was based on atomic weight, but atomic weight is not the most fundamental property of elements.

·      It could not properly explain isotopes. For example, C-12, C-13 and C-14 are isotopes of carbon with different atomic weights but the same atomic number.

·      If atomic weight were the main basis, isotopes would need separate positions, but they are forms of the same element and occupy one position.

·      Some elements did not fit perfectly when arranged only by atomic weight.

 

4 Modern Periodic Table

·      Henry Moseley discovered that the properties of elements depend on atomic number rather than atomic weight.

·      Modern periodic law states: The physical and chemical properties of elements are periodic functions of their atomic numbers.

Characteristics of the Modern Periodic Table

Characteristic	Description
Basis of arrangement	Elements are arranged according to increasing atomic number.
Periods	There are 7 periods. Elements in the same period have the same number of valence shells.
Groups	There are 18 groups according to the IUPAC system.
Metals, nonmetals, metalloids	Metals are mainly on the left, nonmetals on the right and metalloids between them.
Important families	Group IA: alkali metals; Group IIA: alkaline earth metals; Group VIIA: halogens; Group 18/0: noble gases.
Transition metals	Groups IB to VIIB and three columns of VIIIB form transition metals.
Lanthanides and actinides	Lanthanides and actinides are kept separately below the main body of the table.
Blocks	Elements are classified into s, p, d and f blocks based on electronic configuration.

 

Period	No. of elements	Nature
1st	2	Very short
2nd	8	Short
3rd	8	Short
4th	18	Long
5th	18	Long
6th	32	Very long
7th	32	Very long

 

5 Electronic Configuration Based on Subshells

·      Electrons revolve around the nucleus in shells or orbits.

·      A shell may contain one or more subshells.

·      The K shell has 1s; the L shell has 2s and 2p; the M shell has 3s, 3p and 3d; the N shell has 4s, 4p, 4d and 4f.

·      s, p, d and f subshells can accommodate a maximum of 2, 6, 10 and 14 electrons respectively.

·      Aufbau’s principle states that electrons are filled in subshells in the increasing order of energy.

·      Order of filling: 1s, 2s, 2p, 3s, 3p, 4s, 3d, 4p, 5s, 4d, 5p, 6s, 4f, 5d, 6p.

Shell	Subshells	Maximum electrons
K	1s	2
L	2s, 2p	8
M	3s, 3p, 3d	18
N	4s, 4p, 4d, 4f	32

 

6 Electronic Configuration of Elements 1-20

Atomic no.	Element	Symbol	Shell configuration	Subshell configuration
1	Hydrogen	H	1	1s1
2	Helium	He	2	1s2
3	Lithium	Li	2,1	1s2 2s1
4	Beryllium	Be	2,2	1s2 2s2
5	Boron	B	2,3	1s2 2s2 2p1
6	Carbon	C	2,4	1s2 2s2 2p2
7	Nitrogen	N	2,5	1s2 2s2 2p3
8	Oxygen	O	2,6	1s2 2s2 2p4
9	Fluorine	F	2,7	1s2 2s2 2p5
10	Neon	Ne	2,8	1s2 2s2 2p6
11	Sodium	Na	2,8,1	1s2 2s2 2p6 3s1
12	Magnesium	Mg	2,8,2	1s2 2s2 2p6 3s2
13	Aluminium	Al	2,8,3	1s2 2s2 2p6 3s2 3p1
14	Silicon	Si	2,8,4	1s2 2s2 2p6 3s2 3p2
15	Phosphorus	P	2,8,5	1s2 2s2 2p6 3s2 3p3
16	Sulphur	S	2,8,6	1s2 2s2 2p6 3s2 3p4
17	Chlorine	Cl	2,8,7	1s2 2s2 2p6 3s2 3p5
18	Argon	Ar	2,8,8	1s2 2s2 2p6 3s2 3p6
19	Potassium	K	2,8,8,1	1s2 2s2 2p6 3s2 3p6 4s1
20	Calcium	Ca	2,8,8,2	1s2 2s2 2p6 3s2 3p6 4s2

 

 

7 Metals, Nonmetals and Metalloids

Type	Position	Main features	Examples
Metals	Left side and central region of the periodic table	Good conductors of heat and electricity; generally lose electrons; form positive ions.	Na, Mg, Al, Fe, Cu, Ag, Au
Nonmetals	Right side of the periodic table	Poor conductors; generally gain or share electrons; many form acidic oxides.	C, N, O, S, Cl, F
Metalloids	Between metals and nonmetals	Show some properties of metals and some properties of nonmetals; often semiconductors.	Si, Ge, Bi

 

Important Groups

·      Alkali metals: Group IA elements such as Li, Na and K. They have one valence electron and form strong bases or alkalis in water. General valence configuration: ns1.

·      Alkaline earth metals: Group IIA elements such as Mg and Ca. They have two valence electrons. General valence configuration: ns².

·      Halogens: Group VIIA/17 elements such as F, Cl, Br and I. They have seven valence electrons and are very reactive nonmetals. General valence configuration: ns² np⁵.

·      Noble gases or inert gases: Group 18/0 elements such as He, Ne, Ar, Kr, Xe and Rn. They have complete valence shells and normally do not react. General valence configuration: ns² np⁶, except helium as 1s².

·      Transition metals: Elements between group IIA and IIIA, including Fe, Co, Ni, Cu, Zn, Ag, Au and Hg.

8 Characteristics of Period and Group

A. Valency

·      The combining capacity of an element in chemical reaction is called Valency.

B. Electropositivity and Electronegativity

·      Electropositivity is the tendency of an atom to lose valence electrons and form positive ions.

·      Electronegativity is the tendency of an atom to gain or attract electrons and form negative ions or polar bonds.

·      Across a period, electropositivity decreases and electronegativity increases.

·      Down a group, electropositivity increases while electronegativity decreases.

 

 

 

No.	Question	Correct answer
1	In which group do elements with configuration 1s2 2s2 2p1 lie?	Group IIIA / 13
2	What are the elements between groups IIA and IIIA called?	Transition metals
3	Which nonmetal is the most reactive among F, Cl, Br and I?	Fluorine
4	Which metal is the most reactive among Li, Na, K and Cs?	Cesium
5	Which group contains inert gases?	Group 0 / 18
6	Correct increasing order of reactivity of group IIA metals?	Be < Mg < Ca

 

9 Short Answer Questions

Q. Why is classification of elements necessary?

Ans. Classification is necessary because many elements exist. Grouping similar elements together makes their study easier and helps predict their physical and chemical properties.

Q. State modern periodic law.

Ans. Modern periodic law states that the physical and chemical properties of elements are periodic functions of their atomic numbers.

Q. How many groups and periods are there in the modern periodic table?

Ans. There are 18 groups and 7 periods in the modern periodic table.

Q. Why is sodium called an alkali metal?

Ans. Sodium is placed in group IA, has one valence electron, is highly reactive and forms a strong base or alkali when it reacts with water.

Q. Write the position of sodium in the modern periodic table.

Ans. Sodium has atomic number 11 and electronic configuration 2,8,1. It is in group IA/1 and period 3.

Q. Write the electronic configuration and position of sulphur.

Ans. Sulphur has atomic number 16 and electronic configuration 2,8,6 or 1s2 2s2 2p6 3s2 3p4. It is in group VIA/16 and period 3.

Q. Why are inert gases kept in group 0 or 18?

Ans. They have complete valence shells and normally do not gain, lose or share electrons. Their valency is zero, so they are kept in group 0/18.

Q. Why does atomic size decrease from left to right in a period?

Ans. The number of protons increases while electrons are added to the same shell. Stronger nuclear attraction pulls the electron cloud closer, so atomic size decreases.

Q. Why does atomic size increase down a group?

Ans. A new shell is added at each step down a group, so the outermost electrons are farther from the nucleus and atomic size increases.

Q. Why is potassium more reactive than sodium?

Ans. Potassium has a larger atomic size than sodium, so it loses its valence electron more easily. Therefore, potassium is more reactive.

Q. Why is fluorine more reactive than chlorine?

Ans. Fluorine has a smaller atomic size and attracts electrons more strongly than chlorine, so it gains electrons more easily and is more reactive.

Q. Why does nonmetallic character increase from left to right?

Ans. Atomic size decreases and the attraction for electrons increases across a period, so nonmetallic character increases.

10. Differences

Basis	Group	Period
Meaning	Vertical column	Horizontal row
Number	18 groups	7 periods
Valence electrons	Generally same in a group	Increase from left to right
Number of shells	Increases down the group	Same within a period
Properties	Elements have similar properties	Properties change gradually across a period

 

Basis	Sodium	Chlorine
Type	Metal	Nonmetal
Group	IA / 1	VIIA / 17
Valence electrons	1	7
Ion formed	Na+	Cl-
Reactivity nature	Loses electron	Gains electron

 

Basis	Electropositivity	Electronegativity
Meaning	Tendency to lose electrons	Tendency to attract or gain electrons
Ion formed	Positive ion / cation	Negative ion / anion
Trend across period	Decreases	Increases
Trend down group	Increases	Decreases
Common in	Metals	Nonmetals