Trends in the Modern Periodic Table

Trends in the Modern Periodic Table

The word Trends refers to the direction in which a property is changing.

It may be an increase or decrease in tendency to display the property.  

Why do we need to know the trends?

• There may be elements that have not been discovered yet.
• On the other hand, the elements in the periodic table are placed on the basis of certain similarities in properties discussed earlier.
• Knowing the trends helps us to predict the properties of elements that may be discovered in the future and helps us to place them correctly in the periodic table.
  
  

Valency

• It is the combining capacity of an element.

Example:

• Valency of Hydrogen is 1.
• Valency of Oxygen is 2.
• Hence 2 atoms of Hydrogen are needed to combine with 1 atom of Oxygen to form 1 Atom of Water (H 2 O).

Now, how do we determine the valency of an atom?

• The maximum number of electrons in a valence shell (outermost shell) can be 8.
• Hence, the number of electrons in a valence shell can vary from 1 to 8 .

1. Now, if an atom has 1, 2 or 3 number of valence electrons

It is easier for them to lose 1, 2 or 3 electrons respectively from their valence to achieve an  octet. 

Valency of atoms losing 1 electron = 1

Valency of atoms losing 2 electrons = 2

Valency of atoms losing 3 electrons = 3

Example

• Sodium has a total 11 Electrons.
• Electronic configuration: 2,8,1
• Sodium needs to lose 1 electron to achieve octet  
• Hence, Valency of Sodium is 1.

2. If an atom has 5, 6 or 7 number of valence electrons.

It is easier for them to gain 3, 2 or 1 electron respectively in their valence to achieve an octet.

Valency of atoms gaining 1 electron = 1

Valency of atoms gaining 2 electron = 2

Valency of atoms gaining 3 electron = 3

Example 

• Oxygen has a total 8 Electrons.
• Electronic configuration: 2,6
• Oxygen needs to gain 2 electrons to achieve an octet
• Hence Valency of Oxygen is 2.

3. If an atom has 4 electrons in its valence shell , then it has an equal chance of gaining, losing or sharing it's valence electrons in order to achieve octet.

           In any case, the valency of such atoms is 4

Example 

• Silicon has a total 14 Electrons.
• Electronic configuration: 2,8,4
• Thus silicon needs to gain 4 electrons or lose 4 electrons to achieve octet (8 Electrons in the outermost shell).
• Hence, the Valency of silicon is 4.

Note: - If an atom has 8 electrons in its valence shell

  Then it has already achieved octet and is stable

  Hence, it won't lose, gain or share electrons.

  Their valency will be 0.

Variable Valency

• Some elements like Copper (Cu), Iron (Fe) and Mercury (Hg) show variable valencies (more than one valency).
• The different valencies of some elements are mentioned below.

Trends in Valence Electrons in a Group

Valence electrons of one group of a periodic table is same

Example

1. All element of Group 1 of Periodic Table have 1 valence electron
   Lithium, Sodium, Potassium all have 1 Valence Electrons
   
   
2. All element of Group 2 of Periodic Table have 2 valence electrons
   Beryllium, Magnesium and Calcium have 2 Valence electrons each

In this chapter, we will be focussing on Groups 1, 2 and Groups 13 to 18. The valency of elements in these groups are as follows:

Note: In group 18, Helium has only 2 valence electrons in the K shell. This is an exception as all others have 8 valence electrons.

NUMBER OF VALENCE ELECTRONS IN 8 GROUPS OF THE MODERN PERIODIC TABLE   

Trends in Valence Electrons in a Period

As we move form left to right,

Valence Electrons increases from 1 to 8

So valency also increases from 1 to 4 and then decreases to 0

• First element in every period has 1 valence electron
• Second Element in every period has 2 valence electrons and so on...
• Last element in every period has 8 valence electrons
  (except helium which has 2 valence electrons)
  
  

Atomic Size

• Atomic Size is the radius of the atom
• It is the distance between center of nucleus and outermost shell of an isolated atom
• It is measured in picometer (pm)

1 pm = 10 -12 meter

Example

Atomic size of hydrogen atom is 37 pm

What happens to atomic size as we go across the period?

Atomic radius decreases as we move left to right across the period.

Example, In the third period, atomic size of Sodium > Magnesium > Aluminium

It is because as we move across the period,

There is increase in nuclear charge (force of nucleus exerted on atoms)

Due to this nuclear charge, electrons move closer to nucleus, hence atomic size decreases

What happens to atomic size on going down the periodic table?

As we go down the group, atomic size decreases

Example, In the first group atomic size of Potassium > Sodium > Lithium

It is because as we go down the group, 

Number of electrons increases and so the number of shells increases .

Since there are more shells, distance from the center of the nucleus to the outermost shell increases in spite of an increase in the nuclear charge .

Metallic and Nonmetallic Properties

In the periodic table,

• Metals are placed towards the left hand side of the periodic table
• Nonmetals are placed towards the right hand side of the periodic table
• Metalloids or semimetals (which show behaviour of both metals and nonmetals) are placed in between separated by a zigzag line.

Electropositive Nature of Metals

As discussed in Chapter 3, 

Metals lose electrons to form positively charged ions while forming bonds

So, Metals are Electropositive in Nature

Electropositivity is the tendency of an atom to donate electrons and form positively charged cations.

NOTE: Here Sodium (Na) loses an electron from its outermost shell to form a positively charged ion Na + on reacting with Chlorine. This tendency is termed as electropositivity.

PERIODIC TREND - ELECTROPOSITIVITY

Electropositivity of Metals increases down the group in the Periodic Table

Example: The order of electropositivity of Lithium, Sodium, Potassium is,

Lithium < Sodium < Potassium

Reason : 

• Number of shells increases down the group
• Hence valence electrons are farther away from the nucleus .
• Hence these valence electrons are free to move away from nucleus as nuclear charge experienced by valence electron decreases
• Atoms can lose more and more of electrons to become positively charged

Electropositivity of Metals Decreases across the period from left to right.

Example: The order of electropositivity of Sodium, Magnesium, Aluminium is,

Sodium > Magnesium > Aluminium

Reason:

• Size of atom decreases  across the period
• So, nuclear charge of an atom increases on going across the period
• Hence nucleus is able to pull its valence electrons closer, so their ability to move away from the nucleus decreases
• Hence the tendency of atoms to become positively charged is less.

Chemical reactivity:

Chemical reactivity is the tendency of chemical substances to form products either by itself or with other materials.

What happens to chemical reactivity of metals on going down?

When metals react with other substances, the metal atoms lose electrons to form positive ions.

As we go down the group, chemical reactivity of metal increases

Example, The order of reactivity of Lithium, Sodium, Potassium is,

Lithium < Sodium < Potassium

Reason:

As we go down the periodic table, ability of metals to lose electrons increases

Hence they becomes more chemically reactive

Nature of Oxides formed by Metals

We have already learnt in last chapter that Oxides of metals are basic

Metals react with Oxygen to form Basic Oxides

There is no change in trends in formation of basic oxides on going down in periodic table

Electronegative Nature of Non Metals

As discussed in Chapter 3, 

Nonmetals gain electrons to form negatively charged ions while forming bonds

So, Nonmetals are Electronegative in Nature

Electronegativity is the measure of tendency of an atom to attract a shared pair of electrons.

NOTE: Here Chlorine(Cl) gains an electron in its outermost shell to form a negatively charged ion Cl - on reacting with Sodium. This tendency is termed as electronegativity.

Electronegativity of Nonmetals Decreases down the group

Example- Electronegativity of Fluorine > Chlorine

Reason:

Size of the atom keeps on increasing down the group and the number of shells increases.

Hence, effect of nuclear charge of atom on incoming electrons keeps on decreasing

Due to this, atoms cannot form negative ions easily and electronegative character decreases

What happens to chemical reactivity of nonmetals on going down the group?

When nonmetals react with other substances, the nonmetal atoms gain electrons to form negative ions.

It decreases on going down

Example: Chemical reactivity of Fluorine > Iodine

Reason:

It is because ability of atoms to gain negative electrons decreases as we go down

The number of shells increases down the group 

Hence, effect of nuclear charge of atom on incoming electrons keeps on decreasing Thus they become less reactive down the group.

Nature of Oxides formed by Nonmetals

We have already learnt in last chapter that Oxides of nonmetals are acidic

Nonmetals react with Oxygen to form Acidic Oxides

There is no change in trends in formation of acidic oxides on going down in periodic table

Summary of Metallic and Nonmetallic properties