Elements X and Y have electronic configurations 1s ^2 2s ^2 2p ^4 and 1s ^2 2s ^2 2p ^6 3s ^2 3p ^1…

1. Analyze the Question

We are given the electronic configurations of two elements, X and Y. We need to determine their valencies (combining powers) based on their valence electrons to find the chemical formula of the compound they form.

2. Explain the Concept

• Element X: The configuration is $1s^2 2s^2 2p^4$. The outermost shell is the second shell ($n=2$), which contains $2 + 4 = 6$ electrons. To achieve a stable octet (8 electrons), it needs to gain 2 electrons. Therefore, its valency is -2.
• Element Y: The configuration is $1s^2 2s^2 2p^6 3s^2 3p^1$. The outermost shell is the third shell ($n=3$), which contains $2 + 1 = 3$ electrons. To achieve stability, it will lose these 3 electrons. Therefore, its valency is +3.

3. Show the Solution

To find the formula of the compound, we use the "criss-cross" method:

1. Identify the charges (valencies): • X has a charge of -2 (it acts as an anion). • Y has a charge of +3 (it acts as a cation).

2. Write the symbols with their charges: • $Y^{3+}$ and $X^{2-}$

3. Criss-cross the charges to become subscripts: • The charge of Y (3) becomes the subscript of X. • The charge of X (2) becomes the subscript of Y.

4. Resulting formula: • $Y_2X_3$

4. Identify the Answer

Comparing our result ($Y_2X_3$) with the given options: A. XY B. X$_2$Y C. Y$_2$X$_3$ D. X$_2$Y$_3$

The correct option is C.

5. Exam Tip

When determining the formula of a compound, remember the "Octet Rule": Elements with 1, 2, or 3 valence electrons generally lose them to form positive ions (cations), while elements with 5, 6, or 7 valence electrons generally gain electrons to form negative ions (anions). Always write the metal (cation) first, followed by the non-metal (anion).