Periodic Properties - Valency

1. Valency: Valency is the combining capacity of an element and is determined by the number of electrons in the outermost shell (valence electrons) of an atom.

The electronic configuration of an element determines its valency. Let's take potassium (K) as an example:

• Symbol: K
• Electronic Configuration: 2, 8, 8, 1
• Valence Electron: 1 (in the outermost shell)
• Valency: 1 (since it loses 1 electron to attain stability like noble gas Argon)

2. Constant Valency: The valency of an element usually stays the same in different compounds. For example, hydrogen always has a valency of one.

3. Molecular Formulas and Valency: By analysing a compound’s molecular formula, we can determine the valencies of the elements involved. The univalency of hydrogen serves as a reference for calculating valencies.

4. Cross Multiplication: This method involves balancing the valencies of elements to create a stable compound formula, ensuring that the total number of bonds formed by each element is satisfied. Cross Multiplication Method for Writing the Molecular Formula of Simple Compounds:

Step 1: Write symbols of constituent elements.
          C          O

Step 2: Write the valency below the respective element.
          C          O
          4           2

Step 3: Cross-multiply to obtain the number of atoms of the constituent elements in the molecule of the compound.
         

Step 4: To write the formula of the compound obtained by cross-multiplication.
C₂O₄

Step 5: To write the final molecular formula of the compound. The number of constituent atoms in the final molecular formula should be the smallest possible whole numbers.

To get this, divide the formula obtained in Step 4 by a suitable number.
Formula obtained by cross multiplication: C₂O₄
Final molecular formula obtained by dividing by ‘2’: CO₂

1. Valence Electrons: The valency of an atom is related to the number of electrons in its outermost shell, called valence electrons. The outermost shell is known as the valence shell.

2. Formation of Chemical Bonds: Atoms form chemical bonds by using their valence electrons. The valency depends on how many valence electrons an atom has and how many it needs to achieve a stable configuration.

3. Inert Gases and Stability

• Helium has two electrons in its K shell, completely filling it. This configuration is called a duplet, and helium’s valency is zero because it is stable and does not form bonds.
• Neon has a complete octet with eight electrons in its L shell, making it stable and chemically inert with a valency of zero.
• Argon has eight electrons in its M shell, also forming a complete octet, resulting in a valency of zero.

4. Non-Inert Elements and Incomplete Shells

• Atoms with Incomplete Shells: Elements that are not inert gases do not have complete octets or duplets. This incompleteness makes them reactive, with a non-zero valency.
• Hydrogen has one electron in its K shell, which is one electron short of a complete duplet. Thus, hydrogen has a valency of one.
• Sodium’s electronic configuration is 2, 8, 1, with one electron in its valence shell (M shell). Sodium has a valency of one because it tends to lose this electron to achieve a stable octet, as seen in compounds like NaCl and NaH.

The valency of the same group of the element present in the periodic table is the same. If we consider group 8 in the periodic table, all the elements of group 8 have completely filled the outermost orbit and have attained octet arrangement. So, the elements of group 8 have zero valencies.

Skeleton of Electronic configuration 

1. Electrovalency (Ionic Valency)

• Found in ionic compounds (formed by transfer of electrons).
• Metals lose electrons → form positive ions (cations) (e.g., Na⁺, K⁺, Ca²⁺).
• Nonmetals gain electrons → form negative ions (anions) (e.g., Cl⁻, O²⁻).

2. Covalency (Shared Valency)

• Found in covalent compounds (formed by sharing electrons).
• Example: Carbon (C) shares 4 electrons → Covalency = 4 (e.g., CH₄, CO₂).

Determining Valency Based on Group in the Periodic Table:

• Group 1 (Alkali Metals) – Valency: 1 (e.g., Li, Na, K)
• Group 2 (Alkaline Earth Metals) – Valency: 2 (e.g., Be, Mg, Ca)
• Group 13 – Valency: 3 (e.g., B, Al)
• Group 14 – Valency: 4 (e.g., C, Si)
• Group 15 – Valency: 3 (e.g., N, P)
• Group 16 – Valency: 2 (e.g., O, S)
• Group 17 (Halogens) – Valency: 1 (e.g., F, Cl, Br)
• Group 18 (Noble Gases) – Valency: 0 (inert gases, stable electronic configuration).

Special Cases in Valency:

• Variable Valency: Some elements exhibit more than one valency due to the involvement of d-orbitals (e.g., Iron Fe²⁺ & Fe³⁺, Copper Cu¹⁺ & Cu²⁺).
• Transition Metals: Show multiple oxidation states and valencies (e.g., Mn has valencies +2, +4, and +7).

The valency of any element can be determined primarily by 3 different methods:

1) The Octet Rule

If the periodic table cannot be used to determine valency, the octet rule is applied. This rule states that atoms tend to achieve 8 electrons in their outermost shell to become stable, either by gaining or losing electrons in chemical reactions.

• Losing Electrons: Atoms with 1 to 4 electrons in their outer shell tend to lose electrons, resulting in a positive valency.
• Gaining Electrons: Atoms with 4 to 7 electrons in their outer shell tend to gain electrons, as it is easier to accept additional electrons rather than losing them. In this case, valency is calculated by subtracting the number of outer electrons from 8.
• Stability of Noble Gases: Noble gases are stable because they have 8 electrons in their outer shell, except for helium, which is stable with 2 electrons.

2) Using the Periodic Table

Valency can be determined using the periodic table:

• Column 1 Elements: Metals like hydrogen, lithium, and sodium have a valency of +1.
• Column 17 Elements: Nonmetals like fluorine and chlorine have a valency of -1.
• Column 18 Elements: Noble gases are inert and have a valency of 0.

Transitional metals (columns 3-10) and heavier elements (columns 11-14, along with lanthanides and actinides) have variable valencies because they have multiple active electron shells. Examples include copper, iron, and gold.

3) On the Basis of the Chemical Formulae

This method is based on the octet rule. The valencies of many transitional elements or radicals can be determined in a particular compound by observing how it chemically unites with elements of known valency. In this case, the octet rule is followed, where the elements and radicals combine and try to attain eight electrons in the outermost shell in order to become stable.

For example, in the compound NaCl:

• Sodium (Na) has a valency of +1, meaning it needs to lose one electron to achieve a stable octet.
• Chlorine (Cl) has a valency of -1, meaning it needs to gain one electron to complete its octet.
• Sodium donates one electron to chlorine, forming an ionic bond, and both elements achieve stable outermost electron configurations.