The modern periodic table has 18 groups and 7 periods. What is the atomic number of the element placed in the 3rd group and the 4th period of this table?

Introduction / Context:
The modern periodic table arranges elements by increasing atomic number in rows called periods and columns called groups. Each location in this grid corresponds to a specific element. This question asks for the atomic number of the element that lies at the intersection of group 3 and period 4, requiring familiarity with the layout of the periodic table and the order of elements in the fourth period.

Given Data / Assumptions:

• The periodic table has 7 horizontal periods and 18 vertical groups.
• We are asked about the element in group 3 and period 4.
• Elements in a given period are arranged in order of increasing atomic number from left to right.
• We assume a standard long form periodic table.

Concept / Approach:
Period 4 of the periodic table begins with potassium (K, atomic number 19) in group 1 and continues with calcium (Ca, atomic number 20) in group 2. The next element as we move to the right is scandium (Sc), which belongs to group 3. This pattern reflects the filling of the 4s and then 3d subshells in electron configuration. Therefore, the element in group 3, period 4 is scandium, and we simply need to recall its atomic number, which is 21.

Step-by-Step Solution:
Step 1: List the first elements of period 4: potassium (K, atomic number 19) in group 1 and calcium (Ca, atomic number 20) in group 2. Step 2: Move one more place to the right in period 4 to reach group 3, which contains scandium (Sc). Step 3: Recall or look up that scandium has atomic number 21. Step 4: Confirm that scandium indeed lies in group 3 and period 4 in the standard table. Step 5: Select 21 as the correct atomic number from the options.

Verification / Alternative check:
You can verify by writing the sequence of atomic numbers for the fourth period: 19 (K), 20 (Ca), 21 (Sc), 22 (Ti), 23 (V), and so on. Scandium is the first of the transition metals in this period and is commonly taught as the group 3 element. Many periodic table diagrams label scandium explicitly at the intersection of period 4 and group 3. These confirmations reinforce that the atomic number you need is 21, not one of the neighbouring values 19, 20, 22, or 23.

Why Other Options Are Wrong:
- 23: This is the atomic number of vanadium (V), which is in group 5 of period 4, not group 3.
- 19: This is the atomic number of potassium (K), which is in group 1 of period 4.
- 11: This is the atomic number of sodium (Na), which is in period 3, not period 4.
- 20: This is the atomic number of calcium (Ca), which belongs to group 2 of period 4.

Common Pitfalls:
Students often forget the exact group numbering of transition metals or mix up group 3 elements with group 13 or group 2. Another pitfall is to rely on approximate memory of positions without remembering that potassium and calcium start period 4. Visualising or sketching the beginning of period 4 and tracking the sequence K (19), Ca (20), Sc (21) helps fix this in memory and avoids confusion during exams.

Final Answer:
The element in group 3 and period 4 is scandium, which has atomic number 21.