Identifying Most Electropositive and Electronegative Elements in the Third Period

The question asks us to identify the most electropositive and the most electronegative elements within the third period of the modern periodic table.

First, let's list the elements belonging to the third period:

• Sodium (Na)
• Magnesium (Mg)
• Aluminium (Al)
• Silicon (Si)
• Phosphorus (P)
• Sulfur (S)
• Chlorine (Cl)
• Argon (Ar)

Now, let's understand the trends of electropositivity and electronegativity across a period in the periodic table.

Understanding Electropositivity Across a Period

Electropositivity is the tendency of an atom to lose electrons and form a positive ion (cation). It is a measure of metallic character. Across a period from left to right, the atomic size decreases, and the nuclear effective charge increases. This makes it harder for atoms to lose electrons. Therefore, electropositivity generally decreases across a period from left to right.

In the third period, electropositivity decreases in the order:

Na > Mg > Al > Si > P > S > Cl

Thus, the most electropositive element in the third period is Sodium (Na).

Understanding Electronegativity Across a Period

Electronegativity is the tendency of an atom to attract a shared pair of electrons towards itself in a covalent bond. It is a measure of non-metallic character. Across a period from left to right, the nuclear effective charge increases, and the atomic size decreases. This increased attraction for electrons makes atoms more likely to attract shared electrons. Therefore, electronegativity generally increases across a period from left to right.

Electronegativity generally increases from Na to Cl. Noble gases like Argon (Ar) are typically excluded from electronegativity considerations as they are chemically inert and do not readily form bonds.

In the third period (excluding Argon), electronegativity increases in the order:

Na < Mg < Al < Si < P < S < Cl

Thus, the most electronegative element in the third period is Chlorine (Cl).

Combining the findings, the most electropositive element is Sodium, and the most electronegative element is Chlorine.

Revision Table: Periodic Trends in the Third Period

Let's summarize the key trends for elements in the third period (from left to right, Na to Ar):

• Atomic radius generally decreases.
• Ionization energy generally increases.
• Electron affinity generally increases (becomes more negative).
• Metallic character decreases.
• Non-metallic character increases.
• Electropositivity decreases.
• Electronegativity increases (excluding noble gases).

Additional Information: Why Noble Gases are Excluded

Noble gases like Argon (Ar) are located in Group 18. They have a stable electron configuration (a full valence shell), making them chemically inert. Because they do not readily form chemical bonds, their electronegativity values are typically not defined or are considered to be zero or very low, and they are not considered in the context of electropositivity either.

Therefore, when discussing trends in electropositivity and electronegativity across a period, especially the highest values, the noble gas at the end of the period is usually excluded from consideration in standard definitions related to bond formation.