In the modern periodic table, which element with atomic number 20 is placed in the 2nd group and the 4th period?

Introduction / Context:
The modern periodic table arranges elements in periods (horizontal rows) and groups (vertical columns) based on their atomic numbers and electronic configurations. Group numbers often correspond to the number of valence electrons in the outer shell for main group elements, and period numbers indicate the principal energy level of the valence shell. This question asks you to identify the atomic number of the element that lies in the 2nd group and the 4th period of the modern periodic table, a typical exam style question that checks your familiarity with element positions.

Given Data / Assumptions:

• The table has 18 groups and 7 periods in the long form.
• We focus on group 2, which contains alkaline earth metals.
• We focus on period 4, which includes elements from potassium to krypton.
• The options given are atomic numbers 20, 22, 18, and 10.

Concept / Approach:
Group 2 elements are the alkaline earth metals: beryllium (atomic number 4), magnesium (12), calcium (20), strontium (38), barium (56), and radium (88). Period 4 begins with potassium (atomic number 19) and ends with krypton (36). Therefore, the group 2 element in period 4 must be calcium. Calcium has the electronic configuration ending in 4s^2, showing two valence electrons in the fourth shell, placing it in group 2 and period 4. This corresponds to atomic number 20, which is the correct choice among the options.

Step-by-Step Solution:
Step 1: Recall that group 2 contains the alkaline earth metals, starting with beryllium and followed by magnesium and calcium. Step 2: Remember that period 4 begins with potassium (atomic number 19) and then comes calcium (atomic number 20). Step 3: Identify that calcium has an outer electron configuration of 4s^2, indicating it belongs to group 2 and period 4. Step 4: Match this information to the list of atomic numbers in the options and note that 20 corresponds to calcium. Step 5: Check the other atomic numbers: 22 (titanium) lies in group 4, 18 (argon) is a noble gas in group 18 and period 3, and 10 (neon) is in group 18 and period 2. Step 6: Conclude that the only atomic number corresponding to a group 2 and period 4 element is 20, so that is the correct answer.

Verification / Alternative check:
A quick way to verify is to write the electron configuration of calcium: 1s^2 2s^2 2p^6 3s^2 3p^6 4s^2. The presence of 4s electrons shows that the valence shell is the fourth shell, placing the element in period 4. The two electrons in the s sub shell of the valence shell indicate that calcium belongs to group 2. This matches the definition of the position given in the question and confirms that the atomic number is 20. Checking a standard periodic table chart will show calcium in group 2, period 4, reinforcing the conclusion.

Why Other Options Are Wrong:
Atomic number 22 corresponds to titanium, which is a transition metal found in group 4, not group 2. Atomic number 18 corresponds to argon, a noble gas located in group 18 and period 3. Atomic number 10 corresponds to neon, also a noble gas but in period 2. None of these elements is in group 2 and period 4, so they do not satisfy the conditions stated in the question and are therefore incorrect choices.

Common Pitfalls:
Students sometimes confuse period numbers with group numbers or misremember where periods start and end. Another common mistake is to forget that transition metals occupy groups between the main group elements, leading to misplacement of atomic numbers. To avoid these issues, practice locating elements by both atomic number and electron configuration, and remember the sequence of alkaline earth metals: beryllium, magnesium, calcium, strontium, barium, and radium. Knowing that calcium is the period 4 member of this family makes the question straightforward.

Final Answer:
The element in group 2 and period 4 has atomic number 20, corresponding to calcium (Ca).