When element $_{20}$Y combines with element $_8$Z, it forms

Analyze the question: We need to find the type of bond and the formula when element Y (atomic number 20) combines with element Z (atomic number 8).

Explain the concept:

• Metals (few valence electrons) lose electrons and non-metals (nearly full outer shell) gain electrons. Transfer of electrons gives an ionic (electrovalent) bond.
• Sharing of electrons between non-metals gives a covalent bond.

Solution:

1. Electronic configuration of Y (Z = 20): 2, 8, 8, 2 • It has 2 valence electrons, so it is a metal (calcium). It loses 2 electrons to form Y²⁺.

2. Electronic configuration of Z (Z = 8): 2, 6 • It has 6 valence electrons, so it is a non-metal (oxygen). It gains 2 electrons to form Z²⁻.

3. Y transfers 2 electrons to Z: • Y²⁺ + Z²⁻ → YZ • The charges are equal and opposite (2+ and 2-), so they combine in a 1:1 ratio.

4. Bond type: a metal and a non-metal give an ionic compound. This is CaO (calcium oxide).

Naming the formula: By convention, the metal (positive ion) is written first, so the formula is YZ, not ZY.

Answer: An ionic compound, YZ, is formed, which is option C.

Tip: Quickly find the valence electrons from the atomic number. Group 2 metal + Group 6 non-metal means an ionic compound with a 1:1 ratio (like MgO, CaO). Always write the metal first in the formula.