Midterm Practice Sheet

Valence Electrons

  • Definition: Electrons in the outermost energy level of an atom.

  • How to Identify: Use the periodic table to determine the number of valence electrons for a given element.

  • Practice Questions:

  1. How many valence electrons do the following elements have?

  • Carbon (C) - 4

  • Sodium (Na) - 1

  • Oxygen (O) - 6

  • Chlorine (Cl) - 7

  • Aluminum (Al) - 3

  1. Explain why noble gases are generally unreactive in terms of their valence electrons.

Their outermost shell is already holding 8 electrons.

Electron Configurations

  • Concept: Understand how electrons are arranged in atoms and ions.

  • Practice Questions:

  1. Write the electron configurations for the following elements:

  • Hydrogen (H) - 1s¹

  • Magnesium (Mg) - 1s² 2s² 2p⁶3s²

  • Phosphorus (P) - 1s² 2s² 2p⁶3s² 3p³

  • Sulfur (S) - 1s² 2s² 2p⁶3s² 3p⁴

  1. Write the electron configurations for the following ions:

  • Na+ / 1s² 2s² 2p⁶

  • Cl- / 1s² 2s² 2p⁶ 3s² 3p⁶

  • O2- / 1s² 2s² 2p⁶

  • Ca2+ / 1s² 2s² 2p⁶ 3s² 3p⁶

  1. Which of the above ions have the same electron configuration as a noble gas? Explain your reasoning.

Na+ and O2- have the same electron configuration as Neon and Cl- and Ca2+ have the same electron configuration as Argon because they have all either lost or gained electrons to achieve a stable electron configuration.

The Octet Rule

  • Concept: Atoms bond to achieve a full outer shell of 8 electrons.

  • Practice Questions:

  1. Explain how the octet rule applies to the formation of:

  • NaCl (ionic bond) - Sodium (Na) loses one electron to form Na⁺, achieving an octet.

  • Chlorine (Cl) gains one electron to form Cl⁻, achieving an octet.

  • H2O (covalent bond) - Oxygen (O) shares two electrons with two hydrogen atoms, completing its octet. Each hydrogen atom shares one electron with oxygen, completing its duet (like Helium).

  1. Predict the type of bond that forms in the following pairs and explain why based on the octet rule:

  • Lithium (Li) and Fluorine (F) - Ionic bond (electron transfer, forming Li⁺ and F⁻ ions)

  • Two Oxygen (O) atoms - Covalent bond (electron sharing, forming a double bond)

  • Magnesium (Mg) and Sulfur (S) - Ionic bond (electron transfer, forming Mg²⁺ and S²⁻ ions)

  1. Identify if the following compounds satisfy the octet rule:

  • CO2 - Yes, CO₂ satisfies the octet rule. Each atom (C and O) achieves a full octet.

  • NH3 - Yes, NH₃ satisfies the octet rule for nitrogen, and hydrogen satisfies the duet rule.

  • BeCl2 - No, BeCl₂ does not satisfy the octet rule for beryllium, but it does for chlorine. Beryllium is stable with only 4 electrons, which is a common exception for Group 2 elements.