In the periodic table, which one of the following elements is classified as a transition element (d-block metal)?

Introduction / Context:
Transition elements, also called transition metals, occupy the central block of the periodic table and are characterised by partially filled d subshells in one or more of their common oxidation states. They often show variable oxidation states, form coloured compounds, and can act as good catalysts. This question asks you to identify which among the given symbols represents a transition element.

Given Data / Assumptions:
- The options are hydrogen (H), arsenic (As), iron (Fe), and strontium (Sr). - Transition elements are typically found in groups 3 to 12 of the periodic table. - Elements outside the d block are not considered transition elements in the standard sense.

Concept / Approach:
The definition of a transition element usually requires that the element has an incomplete d subshell in either its neutral atom or in one of its common ions. Elements in the central d block of the periodic table, such as iron, copper, and nickel, meet this criterion. Hydrogen is a nonmetal located at the top of Group 1, arsenic is a metalloid in the p block, and strontium is an alkaline earth metal in Group 2. Iron, with atomic number 26, is a typical d block metal and a classic example of a transition element.

Step-by-Step Solution:
Step 1: Locate hydrogen (H) in the periodic table. It is placed in Group 1 but is a nonmetal and belongs to the s block, not the d block. Step 2: Locate arsenic (As). It is in Group 15 in the p block and is classified as a metalloid, not a transition metal. Step 3: Locate iron (Fe). It is found in the central region of the periodic table in the d block, typically classified as a transition metal. Step 4: Locate strontium (Sr). It is in Group 2 as an alkaline earth metal, in the s block, not in the transition metal block. Step 5: Check the electron configuration of iron. It has partially filled 3d orbitals in many oxidation states and clearly satisfies the transition element definition. Step 6: Conclude that Fe (iron) is the only transition element among the options.

Verification / Alternative check:
Textbooks and periodic table charts list iron as a transition metal and often use it as a key example when discussing transition metal properties such as magnetism, formation of coloured compounds, and catalytic activity. Hydrogen is discussed separately as a unique nonmetal, arsenic is grouped with p block elements, and strontium is treated as an alkaline earth metal. This classification consistently supports the conclusion that iron is the transition element in the given list.

Why Other Options Are Wrong:
Option A (H) is wrong because hydrogen is a nonmetal and does not have d electrons in the ground state. Option B (As) is incorrect because arsenic is a p block metalloid, not a d block transition metal. Option D (Sr) is incorrect because strontium is an alkaline earth metal in Group 2 and belongs to the s block.

Common Pitfalls:
Some students confuse the term metal with transition metal and may think that any metallic element is a transition metal. Others may misread the periodic table layout and not clearly see the separation between s, p, d, and f blocks. To avoid these mistakes, remember that transition elements are specifically the d block metals in groups 3 to 12, and that classic examples are iron, copper, and nickel, not hydrogen or alkaline earth metals.

Final Answer:
The correct answer is: Fe (iron).