Understanding Valency of Elements

The question asks us to determine the correct order of valencies for four specific elements: Neon (Ne), Silicon (Si), Nitrogen (N), and Magnesium (Mg).

Valency is defined as the combining capacity of an element. It is typically equal to the number of electrons an atom needs to gain, lose, or share to achieve a stable electron configuration, like that of a noble gas. This stable configuration usually involves having eight electrons in the outermost shell (octet rule), except for elements like Hydrogen and Helium which aim for two electrons (duplet rule).

Determining Valency for Each Element

Let's look at each element and determine its typical valency:

Valency of Neon (Ne)

• Neon (Ne) is in Group 18 (Noble Gases) of the periodic table.
• Its atomic number is 10, and its electron configuration is $1s^2 2s^2 2p^6$.
• The outermost shell (n=2) has $2 + 6 = 8$ electrons. This is a stable octet configuration.
• Noble gases like Neon are generally very unreactive because they already have a stable electron configuration.
• Therefore, the typical valency of Neon is 0, as it does not need to gain, lose, or share electrons.

Valency of Magnesium (Mg)

• Magnesium (Mg) is in Group 2 (Alkaline Earth Metals) of the periodic table.
• Its atomic number is 12, and its electron configuration is $1s^2 2s^2 2p^6 3s^2$.
• The outermost shell (n=3) has 2 electrons.
• To achieve a stable octet configuration, Magnesium tends to lose these 2 valence electrons, forming a $\text{Mg}^{2+}$ ion.
• The number of electrons lost or gained determines the valency.
• Therefore, the typical valency of Magnesium is 2.

Valency of Nitrogen (N)

• Nitrogen (N) is in Group 15 of the periodic table.
• Its atomic number is 7, and its electron configuration is $1s^2 2s^2 2p^3$.
• The outermost shell (n=2) has $2 + 3 = 5$ electrons.
• To achieve a stable octet, Nitrogen needs to gain 3 electrons or share 3 electrons.
• In many compounds, Nitrogen forms 3 covalent bonds (e.g., $\text{NH}_3$) or gains 3 electrons to form the nitride ion ($\text{N}^{3-}$).
• Therefore, the typical valency of Nitrogen is 3.

Valency of Silicon (Si)

• Silicon (Si) is in Group 14 of the periodic table.
• Its atomic number is 14, and its electron configuration is $1s^2 2s^2 2p^6 3s^2 3p^2$.
• The outermost shell (n=3) has $2 + 2 = 4$ electrons.
• Silicon typically achieves a stable configuration by sharing its 4 valence electrons, forming 4 covalent bonds (e.g., $\text{SiO}_2$, $\text{SiH}_4$).
• Therefore, the typical valency of Silicon is 4.

Summary of Valencies

Let's list the valencies we found:

Ordering the Valencies

Now we arrange the elements based on their valencies in increasing order:

• Valency 0: Neon (Ne)
• Valency 2: Magnesium (Mg)
• Valency 3: Nitrogen (N)
• Valency 4: Silicon (Si)

So, the correct order of valencies from smallest to largest is: 0 < 2 < 3 < 4.

Corresponding to the elements, this order is:

Ne < Mg < N < Si

Final Order of Valencies

The order of the valencies of the elements Ne, Si, N, and Mg is Ne < Mg < N < Si.

Revision Table: Valency Concepts

Additional Information: Periodic Trends in Valency

While the valency can sometimes vary for certain elements, the typical valency often follows patterns related to the element's position in the periodic table. For main group elements:

• Elements in Group 1 typically have a valency of 1 (lose 1 electron).
• Elements in Group 2 typically have a valency of 2 (lose 2 electrons).
• Elements in Group 13 typically have a valency of 3 (lose 3 electrons).
• Elements in Group 14 typically have a valency of 4 (share 4 electrons).
• Elements in Group 15 typically have a valency of 3 (gain 3 electrons) or can form compounds with valencies like 5.
• Elements in Group 16 typically have a valency of 2 (gain 2 electrons).
• Elements in Group 17 (Halogens) typically have a valency of 1 (gain 1 electron).
• Elements in Group 18 (Noble Gases) typically have a valency of 0.

These trends help predict the common combining capacity of elements, although exceptions and variable valencies exist, especially for transition metals and heavier elements.