How many valence electrons does thallium (Tl) have?

• A. 3
• B. 2
• C. 5
• D. 1

Answer: (D)

Verified step by step guidance

1. Identify the group number of thallium (Tl) in the periodic table. Thallium is in group 13, which means it has 3 electrons in its outermost shell according to the group number.
2. Recall that valence electrons are the electrons in the outermost shell that participate in chemical bonding. For main group elements, this usually corresponds to the group number.
3. Consider the electron configuration of thallium: its outermost electrons are in the 6s and 6p orbitals. The configuration ends with 6s^2 6p^1, indicating 3 electrons in the outermost shell.
4. Understand that although thallium has 3 valence electrons, it often exhibits a +1 oxidation state because the 6s electrons are more tightly held (inert pair effect), so it commonly uses only 1 valence electron in bonding.
5. Therefore, while the group number suggests 3 valence electrons, the chemically active valence electrons for thallium in many compounds is 1, which explains the correct answer.