Classification of Elements

Classification of Elements

Classification Of Elements

Dividing the elements in different groups according to their similarities and dissimilarities is called classification of elements.

Mendeleev’s periodic table

Propound by: Dmitri Mendeleev (Russian Scientist)

Mendeleev’s Periodic Law states that “the physical and chemical properties of all the elements are periodic function of their atomic weights.”

Mendeleev’s Periodic table is the table which is made after arranging the elements on the basis of increasing atomic weights.

Characteristics of Mendeleev’s Periodic Table

  1. Elements are classified on the basis of increasing atomic weights.
  2. It consists of seven horizontal rows(periods) and eight vertical columns(groups).
  3. Presence of some gaps for the undiscovered elements, so that it can be filled later without disturbing the position of other elements.

Merits of Mendeleev’s Periodic Table

  1. It was the first scientific and systematic classification which made study of all discovered elements easier and practicable.
  2. Presence of gaps for undiscovered elements knowingly.
  3. This table inspired chemist to discover new elements as some gaps were left to filled.

Demerits of Mendeleev’s Periodic Table

  1. Position of hydrogen is not properly defined as hydrogen shows dual nature i.e. sometimes loses electron like Group I elements and sometimes gains electron like Group VII elements.
  2. Some chemically similar elements are kept in different group such as copper and mercury, gold and platinum, silver and thallium, barium and lead.
  3. Absence of place for isotopes.
  4. Mendeleev’s didn’t obey his own law. Elements with higher atomic weight were placed before those with less atomic weight.
    Argon (Atomic weight:39.9) was placed before Potassium (Atomic weight:39.1).
  5. Position of lanthanides and actinides were not arranged properly in fixed position.
  6. It was not able to explain the atomic properties of elements like valency, metallic character, reactivity, ionization potential etc.

Modern Periodic Table

Propound by: Henery Moseley (England Scientist)

Modern Periodic Law states that “the physical and chemical properties of all the elements are periodic function of their atomic number.”

Modern Periodic table is the table which is made after arranging the elements on the basis of increasing atomic number.

Characteristic of Modern Periodic Table

  1. Elements are classified on the basis of increasing atomic number.
  2. It consists of seven periods and eighteen groups.
  3. Metals are placed on the left side of transition elements, on metals at the right side and metalloids between metal and non-metals.
  4. Inert gases are kept in the zero group at the extreme right side of the table.
  5. Lanthanides and Actinides series are kept below the main table separately.
  6. Division of groups into two subgroups A and B except zero group and VIII group.

Merits of Modern Periodic Table

  1. Isotopes of same elements have same atomic number. So, they can be found in the same position.
  2. Groups are divided into subgroups A and B and treated as separate group A and group B as elements of group A differ in properties from the elements of group B which made study even easier.
  3. It separated metals from non-metals.
  4. Position of hydrogen is still not solved completely. However, it is placed in the group IA along with alkali metal due to its least atomic number one.

Periods:

The modern periodic table consist of horizontal rows called periods.

Periods

Named as

1

Very short period (1 element)

2 and 3

Short periods (8 elements each)

4 and 5

Long periods (28 elements each)

6

Very Long period (32 elements)

7

Incomplete Very Long period (30 elements till now)

 

Groups:

The elements with similar characteristics are arranged in the vertical columns of periodic table which is known as groups.

Group IA

Group IA elements have one valence electron in the valence shell. They form monovalent cations like Li+, Na+, K+ etc. They are also called alkali metals because their hydroxides are strongly basic. These elements are soft and very reactive.

Group IIA

Group IIA elements have two valence electrons in the valence shell. They form bi-valent cations like Mg++, Be++, Ca++ etc. They are also called alkaline earth metals because they form basic hydroxides which are less soluble in water.

Group VIIA

Group VIIA elements have seven valence electrons in the valence shell. They form mono-valent anions like F-, Cl- etc. They are also called halogens because they form salt with group IA elements. For example: Na (IA element) and Cl (VIIA elements) elements gets reacted to form NaCl(salt). These elements are highly toxic in high concentration.

Group 0 or 18

Except Helium, all the elements belonging to groups has 8 electrons in their valence shell. They all are inactive. They are also called inert or noble gas because they don’t take part in chemical reaction.

Characteristics of Groups:

  1. Valence electron
    Atoms of all elements in a group have same valence electron.
  2. Valency
    Elements in a group usually have same valency.
  3. Atomic Size
    The distance between nucleus and the valence shell of an isolated gaseous atom is atomic size. As new shell is being added on going down the group, the size of atoms increases.
  4. Ionization energy
    The amount of energy required to remove the most loosely bound electron from the valence shell of an isolated gaseous atom to form an ion. Ionization energy decrease while moving down the group as less energy is required to remove electron when atomic size is larger.
  5. Electro-negativity
    The amount of energy required for an atom to attract foreign electrons towards itself is called electronegativity.
    Electronegativity decreases when moving down the group as atomic size increases.
  6. Electro-positivity
    The ability of loosing electrons from the valence shell of an atom of elements for getting stability is called electro-positivity. Electro positivity increases when moving down the group as atomic size increases.
  7. Chemical reactivity
    The chemical reactivity of metals in group increases on moving down while the chemical reactivity of non-metal in group decreases on moving down.

Characteristics of Period

  1. Valence Electron
    The number of valence electron increases on moving right along the period.
  2. Valency
    Valency increases from 1 to 4 then decreases to 0 on moving right along the period.
  3. Atomic size
    Atomic size decreases on moving left to right along the periods because more electrons are added to the same shell and the nuclear charge also increases with the atomic number.
  4. Ionization energy
    Ionization energy increases on moving left to right along the periods because as more energy is required to remove electron when atomic size is smaller.
  5. Electro-negativity
    Electronegativity increases on moving left to right as atomic size decreases.
  6. Electro-positivity
    Electro positivity decreases on moving left to right as atomic size decreases.

Sub-shells

The shells of atoms are further consisting of a number of sub-shells. There are 4 sub-shells: s, p, d and f.

Sub-shell

Maximum electrons

s

2

p

6

d

10

f

14

 

Main Shell

Sub shells

Total electrons

K

s

2

L

s, p

2+6=8

M

s, p, d

2+6+10=18

N

s, p, d, f

2+6+10+14=32

 

Electronic Configuration on sub-shell level

Chlorine

Atomic Number=17
No. of proton=17
No. of electron=17
No. of neutron=18

Electronic Configuration on Shell Level

K

L

M

N

2

8

7

-

 

Electronic Configuration on Subshell Level

K

L

M

N

S

S

P

S

P

D

S

P

D

F

2

2

6

2

5

-

-

-

-

-

 

Thus, Cl=1s2,2s2 2p6,3s2 3p5

Aufbau Principle

Propound: Wolfgang Pauli (German Scientist)

Aufbau Principle states that “electrons in an atom are so arranged that they occupy sub-shells in the order of their

Increasing energy level.

Note: Half-fill or completely filled in d-subshell is more stable than d9 or d4. So electronic configuration of chromium

and copper differs slightly.

Chromium:

No. of electron=24

1s2,2s2 2p6,3s2 3p6,4s2,3d4=> Unstable so,

1s2,2s2 2p6,3s2 3p6,4s1,3d5=> Stable (Half-filled d-sub shell)

Some Important Terms:

  1. Lanthanides
    The 14 rare earth elements from atomic number 58(Cerium-Ce) to 71(Lutetium-Lu) which have similar properties with that of lanthanum are called lanthanides.
  2. Actinides
    The 14 trans-uranium elements from atomic number 90(Thorium-Th) to 103(Lawrencium-Lr) which have similar properties with that of actinium are called lanthanides.
  3. Isotopes
    Isotopes are the elements having same atomic number and similar chemical properties but different atomic mass. Example: 35Cl and 37Cl

Give reasons.

  1. Elements of group IA are known as reactive metals.
    Elements of group IA are known as reactive metals because they all have a single valence electron. So, they can easily lose single electron to other to become stable.
  2. Elements of group IA are known as alkali metals.
    Elements of group IA are known as alkali metals because their hydroxide are highly basic.
  3. Elements of group IIA are known as alkaline earth metals.
    Elements of group IIA are known as alkaline earth metals because they form basic hydroxide which are less soluble in water.
  4. Elements of group VIIA are known as reactive non-metals.
    Elements of group VIIA are known as reactive non-metals because they all have seven valence electron. So, they can easily gain one electron from other to become stable.
  5. Elements of group VIIA are known as halogens.
    Elements of group VIIA are known as halogens because they form salts with group IA elements.
  6. Elements of Group 0 or 18 are known as inert/noble gas.
    Elements of Group 0 or 18 are known as inert/noble gas because the valence shell of them are completely filled with electrons. So, they don’t react with other.
  7. Potassium is more reactive than sodium.
    Potassium and sodium both are metals. They have tendency to lose electrons. The atomic size of potassium is larger than sodium. So, it will be easier for potassium to lose one electron than sodium. Hence, potassium is more reactive than sodium.
  8. Fluorine is more reactive than chlorine.
    Fluorine and chlorine both are non-metals. They have tendency to gain electrons. The atomic size of fluorine is smaller than chlorine. So, it will be easier for fluorine to gain one electron and hold on valence shell than chlorine. Hence, fluorine is more reactive than chlorine.
  9. Atomic size decrease from left to right in a period.
    On moving from left to right, the number of valence electron increases, thus increase in the force of attraction between the nucleus and valence electron. Hence, it makes the last orbit compressed towards the nucleus and the atomic size decreases.
  10. d-block elements are called transitional elements.
    d- block elements are called transitional elements because they exhibit transitional behaviour between s-block and p-block elements.
  11. f-block elements are called inner transition elements.
    f-block elements are called inner-transition elements because while moving from the transitional elements, we have to transit through these elements.