Classification of elements

Introduction: Classification is one of the important aspect in science. It isn’t practiced only in science, it is equally important in other areas.

Until mid 17th century, only a small no. of elements were known and ideas about chemical informations were limited. Hence, there was no realization of classification of elements. During 19th century when larger no of elements were known and it was possible to gather larger informations relating to them, need of classifications of elements was felt. When John Dalton put forward atomic theory and it was possible to measure atomic weight. The scientist tried to classify elements based on atomic weights. Several attempts were made to classify based on atomic weight. Following are some of the notable attempts mode in earlier days.

Prout’s hypothesis: Prout believed that all elements are made up of some basic elements. That basic element being Hydrogen. Atomic weight of all elements is certain multiple of atomic wt. of Hydrogen. When atomic wt. of several elements were found in fraction then his assumption couldn’t be accepted

Dobereiner triad: Dobereiner believed that elements existed in the form of triad and all elements in a triad are similar. They don’t simply bear common properties but there’s a kind of relationship between their atomic weight. Atomic weight of middle element of a triad is average of 1st and 3rd elements. His idea couldn’t be accepted because such triads weren’t existing in several places. triad Na k 23 39 As 75 5 sob 120 7 81

Lothar Meyor Curve: Lothar Meyor plotted graph of various properties of elements such as its volume, melting point, boiling point against atomic weight and observe that similar elements were occupying similar position on the graph.

Newlands octaves:  Newland arranged the elements in a horizontal row in the order of increasing atomic weight and observed the repetition of elements after a regular interval. He stated “when elements are put in order of increasing atomic weight in a horizontal row, there is repetition of elements with similar properties at regular interval i.e. every 8th element.

Mendeleev’s Periodic table: The tabular arrangement of elements based on their properties where the repetition of elements with similar properties occur at regular interval is called periodic table. The most significant towards the classification of elements was made by a Russian chemist D.I Mendeleev.

Mendeleev arranged the elements in the order of increasing atomic weight so that the elements with similar properties fall under same vertical column. He termed the vertical column as ‘group’ and horizontal rows as ‘periods’ or ‘series’. He classified only 63 elements in his first published periodic table. He also put forward a law called ‘Mendeleev’s periodic law’ which states the physical and chemical properties of elements are the periodic function of their atomic weight. It means when elements are arranged in the order of increasing atomic weight, there is recurrence of the elements with similar properties at regular interval.

Main features of Mendeleev’s periodic table

1. In the Mendeleev’s periodic table elements are put in the order of increasing atomic weight in such a way so that elements with similar properties are repeated in regular interval.
2. There are 8 groups and 7 periods: Group 0 elements were added later.
3. There were blank places left for undiscovered element. Some of the elements were named as the follows.

*Eka – Boron

*Eka – Aluminum

*Eka – Silicon

*Eka – Manganese

Defect of Mendeleev’s periodic table

1. The position of Hydrogen isn’t justified because it is kept in Group 1 along with the metals.
2. If elements are to be classified on the basis of atomic weight, a separate place needs to be assigned for isotopes but there’s no separate place for them. [Note: Isotopes are elements with some atomic number but with different atomic weight due to different no. of neutrons]
3. Elements aren’t put in the order of increasing atomic weight in some of the places of his tables.
4. Eg, Ar with high atomic weight comes before K, Co comes before Ni etc.
5. Dissimilar elements are kept in same group. Eg, Reactive metals like Li, Na, K etc are kept along with inactive metals.
6. Similarly, Mn which is a metal is kept in Group ‘7’ along with non-metals (halogens).
7. 3 elements are kept in a single cell in group 8.

Advantages of Mendeleev’s Periodic table.

1. Although there are defects in Mendeleev’s periodic table, but his classification is advantageous to the scientists who worked in this field.
2. He systematised the elements in the form of table. Mendeleev summarised properties of elements in a table for 1st time.
3. He had left blank spaces in his periodic table. He hadn’t simply left the blank spaces but also predicted properties of undiscovered elements. It helped the scientists later to find out new elements and to determine their placements.
4. Correction of faulty atomic weight of elements like Be, Cs, In, U, Pt etc were corrected after the Mendeleev’s periodic table.

Modern periodic table: When several defects were seen in Mendeleev’s periodic table, various attempts were made to remove them by classifying elements differently. Finally, Henry Moseley concluded that atomic no. is the periodic function instead of atomic weight. Based on it, he proposed a periodic law called Modern periodic law which states “The physical and chemical properties of an element are periodic functions of their atomic number.”

It means when elements are put in the order of increasing atomic number, there is recurrence of elements with similar properties of definite interval. In the modern periodic too, elements with similar properties are put in the same vertical column called ‘group’. There is a gradual variation in the properties among the elements in horizontal row with the increase in atomic number.

Main feature of Modern Periodic table.

1. Elements are arranged on the basis of increasing atomic number so that there is a recurrence of elements with similar characteristics at regular interval.
2. There are 18 vertical columns or “groups” according to IUPAC.
3. There are 7 horizontal rows called periods in modern periodic table.
4. Lanthanides and actinides are kept separately below main body of Periodic table.
5. Elements with zero valency called inert gas which are kept on extreme right side of periodic table in Group O or Group 15.
6. Series of 10 elements with Group III B or (3) to II B or (12) which are kept between groups of reactive metals and non-metals

Grouping of elements in Modern periodic table.

In modern periodic table, elements are not simply arranged in the number of increasing atomic number but elements are also grouped according to their properties.

1. Positions of Metals, non-metals and metalloids:

Elements can be categorized into metals, non-metal and metalloids on the basis of their electrical property. Metals ore good conductor of electricity, non metals are bad conductor of electricity and metalloids are semi-conductors of electricity. In the modern periodic table metals lie on left side and non-metals are kept on right side. Metolloids are sandwiched between group of metals and non-metals along a diagonal.

a. Metals

There are different types of metals as listed below:

1. Alkali metal – IA (Most reactive)
2. Alkaline earth metal – lI A
3. Transition metals (d-block) – less reactive
4. Inner transition metals (f-block metals)
5. Poor metals (p-block metals)

b. Non-metals

Non-metals are kept on the right side of modern periodic table. They include Carbon, Nitrogen oxygen, Phosphorous, sulphur etc. They are of 4 types:

1. Inert gases(Group 0)
2. Halogens(VIIA)
3. Other nan metals

Similarly group VIIA elements are called halogens which is a group of most reactive non-metals. Inert gases (He, Ne, Ar, Kr, Xe, Rn) are also non metals, they are called noble gases.

c. Metalloids

B, Si, Ge, As, Sb, Te, Po are metalloids. Metalloids are kept along a diagonal between the group of metals and non-metals.

2. Position of Hydrogen: In modern periodic table Hydrogen is put in group IA along with alkali metals because of the similar electronic configuration, although it shows its resembles with Halogens (Group VIIA)

3. Position of Lanthanides and Actinides: A series of 14 elements after Lanthanum-57(La) i.e Ce(58) to Lu(71) are called Lanthanides. They are also called rare earth elements. Similarly, a series of 14 elements after Actinium-89 i.e. Th-90 to Lr-103 are called Actinides. Actinides are radioactive in nature. Both are kept separately below the main table.

4. Grouping of elements into blocks: In the modern periodic table elements are grouped into 4 blacks ie s, p, d and f in accordance to their electronic configuration. A block of an element is determined where the lost electron has entered.

s-block element: If the last election of an element has entered into s-sub shell of a valence shell, it is called s-block element. Group IA and IIA elements come under this block.

p-block element: If the last electron of an element enters into p-block of valence shell, then it is called p-block elements. Group IIIA to Group 0 elements come under this block. There are non-metals, metalloids and metals in this block.

B-5: 1s², 2s², 2p¹

Ne-10: 1s², 2s², 2p⁶

Cl-17: 1s², 2s², 2p⁶, 3s², 3p⁵

d-block elements: If the last electron of an element enters into d-block sub shell of the penultimate shell (next to the outer mast shell), they are called d-block element.

Sc-21: 1s², 2s², 2p⁶, 3s², 3p⁶, 3d¹, 4s²

Zn-30: 1s², 2s², 2p⁶, 3s², 3p⁶, 4s², 3d¹⁰

Transition metals come under this block.

f-block elements: If the last electron of an element enters into f sub shell of the anti penultimate shell (n-2), inner transition metals like Lanthanides and Anthanides also come under the block.

Solved exercise

Q1. State Mendeleev’s and Modern periodic law.

Mendeleev’s periodic law states that “The physical and chemical properties of elements are the periodic functions of their atomic weight.”

Modern periodic law states that “The physical and chemical properties of elements are periodic functions of their atomic number.”

Q2. What are groups and period in periodic table?

The vertical column in periodic table where elements with similar properties are kept are called groups and the horizontal rows where variation in properties of elements occur is called period.

Q3. When did Mendeleev publish his periodic table?

Mendeleev published his periodic table on 1869 AD.

Q4. Who was Mendeleev?

Mendeleev was the son of Russian teacher who classified the elements in his Mendeleev’s table.

Q5. Name the scientist who put forward the modern periodic law.

Henry Moseley put forward the modern periodic law.

Q6. How many groups and periods are there in modern periodic table?

There are 7 periods and 9 groups in modern periodic table.

Q7. How many groups are there in modern periodic table according to IUPAC?

There are 18 groups in modem periodic table according to IUPAC.

Q8. Differentiate between Mendeleev’s periodic table and Modern Periodic table.

Q9. What is transition metal?

Elements of Group IIIB(3) to IIB(12) lying between groups of metals and non-metals that have lower reactivity is called transition metal.

Q10. Why are group ‘0’ elements called inert gas?

Group 0 elements are called inert gas because they have valency ‘0’ i.e. they have stable electronic configuration.

Q11. How was modern periodic table born?

Modern periodic table was born to remove defects of Mendeleev’s periodic table i.e, to correct the defects appeared in this periodic table.

Q12. Write two defects of Mendeleev’s periodic table.

Defects of Mendeleev’s periodic table are:

1. Position of Hydrogen isn’t justified as it’s kept in Group 1 along with metals.
2. 3 elements are kept in a single cell in group VIII.
3. Dissimilar elements are kept in same group.
4. No separate place assigned for isotopes.

Q13. Write 2 advantages of Mendeleev’s periodic table.

Advantages of Mendeleev’s periodic table are:

1. He systemized elements in farm of table.
2. He summarised properties of element in table for 1st time.
3. Correction of faulty atomic weight of elements like Be, Cs, In etc were corrected after his table.

Q14. What is alkali metal? Where are they kept in modern periodic table?

Metals in group IA like Li, Na, K that are highly reactive is called alkali metal. They are kept in left side or Group IA of modern periodic table.

Q15. Write two reasons why Hydrogen is put in Group IA along with alkali metals.

Reasons why Hydrogen is put in Group IA along with alkali are:

1. It has similar electronic configuration like of alkali metals.
2. It has one electron in its valence shell.

Q16. What are metalloids? Where are they kept in modern periodic table?

Metalloids are the elements sandwiched between metals and non-metals like B, Si, Ge, Te etc They are kept in Group IIIA to Group VIA.

Q17. Write 3 features of Mendeleev’s periodic table.

Features of Mendeleev’s periodic table are:

1. Elements are arranged on the basis of increasing atomic weight.
2. There are 8 groups and 7 periods.
3. Some gaps were left for undiscovered elements.

Q18. How are elements divided in modern periodic table in accordance with their Valency.

Elements are divided into metals, non-metals and metalloids.

Electronic configuration: Elements are divided into s, p, d and f block elements based on electronic configuration.

Metals and non-metals: Elements are divided into alkali metals, alkaline earth metals, transition metal, inner transition metal, poor metal, inert gases, halogen and other non-metals.

Reactivity of elements: There are mainly 2 types of elements: metals and non-metals. The reactively of them are influenced by different factors. Metals have tendency to lose electrons, to obtain stable electronic configuration. Similarly. non-metal tend to gain electrons to obtain stable electronic configuration. Reactivity of metals depends on how easily it’ll gain electrons and reactivity of non-metals depend on how easy it gains electrons to get octet or duplet condition. Among the metals group IA(Alkali metals) are most reactive. Reactivity of metals decrease on moving right side of a period. Among the metals of a particular group, the reactivity increases while moving down a group. Among the non-metals halogens (Group VIIA) are most reactive. The reactivity of non-metals decrease while moving down a group. Similarly, the reactivity of non-metals towards the left of the period.

Reactivity of elements in periodic groups: Reactivity of elements increases or decreases on moving down a group. The creativity of metals increases while moving down a group but the reactivity of non metals decreases while moving down a group.

Reactivity of metals: As we know the reactivity of metals Increase on moving downward. When we down a group, the atomic radius increases because of the additional shell. The metal with larger atomic radius loses electron more easily than the smaller atomic radius. Therefore, reactivity increases on moving down a group of metals.

If we compare reactivity of Na and K, K is more reactive than Na although both of them have one electron in their valence shell. The atomic radius of K is larger than Na and can lose its valence electron more easily than Na. Hence, K is more reacttve than Na.

Reactivity of Non-metals: Reactivity of Non-metals decrease while moving down a ground of non-metals. As we go down a group, the atomic radius increases. The atom of smaller atomic radius con gain electron more easily than that of atom with larger atomic radius. Therefore, reactivity of non-metals decreases on going downward.

The reactivity of Fluorine is called higher than Chlorine because Fluorine has smaller atomic radius than Chlorine and gain electron more easily. Thus, Fluorine is more reactive than Chlorine. Among the non-metals, halogens (VIIA) are most reactive. The reactivity order of halogen lies as F>Cl>Br>I

Reactivity of elements along a period: Reactivity of metals decrease from Group IA to IIIA. Similarly the reactivity of non-metals increases on moving right side from Group VA to VIIA.

Q1. How does metallic property of elements on moving left to right on a period?

The metallic property of elements goes on decreasing on moving left to right on a period.

Q2. What happens to the reactivity of element on moving from top to bottom in metallic group of modern periodic table. Why? 

Reactivity of elements goes on increasing on moving from top to bottom in metallic group of modern periodic table because due to increase in size of atoms.

Q3. What happens to atomic size of elements on moving from left to right in a period of modern periodic table. why?

Atomic size of elements decrease on moving from left to right in a period of modern periodic table because nucleus increases with increase in proton but not the outer shell because the shell is pulled towards the centre by nucleus.

Q4. What happens to electro positive character of elements on moving from

a. Top to bottom in a group: Electro positive character of elements go on increasing from top to bottom in a group.

b. Left to right in a period: Electro positive character of elements go on decreasing from Left to right in a period due to increase in size of nucleus while shell’s size remains same, i.e its radius decreases.

Q5. What kinds of elements ore kept in p-block? 2 Write 3 examples of p-black elements with different nature.

Elements whose last electron entered the p-block of valance shells are kept in p-block i.e. metals, non-metals and metalloids are kept in p-block. 3 examples of it are:

Boron (Metalloids)

Fluorine (Non-metals)

Aluminium (Metals)

Q6. Study and answer

a. Which element from A-H represents.

1. Alkali metals – A and B
2. Noble gas – H
3. One with valency – C and E
4. Halogen – G anf F

b. Write Formula of oxide of A and C

c. Molecular formula of compound when combination between B and E occurs.

d. Name the elements

E – Oxygen

G – Fluorine

F – Chlorine

e. Which is the most reactive non metal?

G or Fluorine is the most reactive non-metal.

f. Which element doesn’t take part in chemical combination. Why?

10 (Neon) and 14 (Argon) don’t take part in chemical combination as they are already stable.

g. What will be no. of valence electrons in “G”?

There be 7 valence electrons in “G”.

h. Write the group, period, valency and block of elements from A to H.

A (Lithium)

Group-IA

s-block

Valency (1)

Period (2)

C (Beryllium)

Group-IIA

s-block

Valency (2)

Period (2)

D (Carbon)

Group IVA

p-block

Valency (4)

Period (2)

E (Oxygen)

Group-VIA

s-block

Valency (2)

Period (2)

B (Sodium)

Group-IA

s-block

Valency (1)

Period (3)

F (Cholrine)

Group-VIIA

p-block

Valency (1)

Period (3)

H (Argon)

Group-0

p-block

Valency (0)

Period (3)

Q7. Study and answer

a. Name A and C

A – Sodium

C – Potassium

b. Name the compounds formed from B and C. 

The compound is Potassium Chloride

c. Sketch atomic structure of C.

d. Which is more reactive between A and C? Why?

C is more active than A because among both metals, C is bigger in size, So C has more chance of losing an electron to be stable since electrostatic force of the atom is less due to larger area.

e. Which one is more reactive, B or D? why?

B is more active than D because D is already stable so it is not reactive.

f. Which groups of periodic table do A and B belongs to?

A belongs to Group IA of modern periodic table and B to group VIIA.

g. Which element doesn’t take part in combination? Why?

D doesn’t take part in combination as it is configured to be stable and it doesn’t need to take part in reactions.

Q8. How are elements grouped in modern periodic table in accordance to electronic configuration of elements.

In accordance to electronic configuration of elements, elements are grouped into s, p, d and f block element.

Q9. In which group do the element with electronic configuration 1s², 2s², 2p⁶, 3s², 3p⁴ lies?

It lies in Group VIA.

Q10. What is the block and group number of element with atomic no. 14?

It’s block is p-block and group number is IVA.

Q11. What are the factors on which reactivity of an element depend.

Reactivity of on element depend on valence electrons and its size.

Q12. Why are elements like sodium and potassium called s-block elements?

Elements like sodium and potassium are called s-block elements because their last electron has entered into s-subshell of valence shell.

Q13. Which properties of metals determine their reactivity?

Reactivity of metals increase while moving down a group and reactivity of metals decrease while moving right of a period.

Q14. What is halogen? Why are they called so?

Non-metals from Group VIIA which are very reactive are called halogens. They are called so because they form salt while combining with metals.