In the modern long form periodic table with 18 groups and 7 periods, what is the atomic number of the element that is placed in the 1st (Group 1) group and the 4th period?

Introduction / Context:
The modern long form periodic table arranges elements in 18 vertical groups and 7 horizontal periods according to increasing atomic number. Each element's position tells us about its electronic configuration and chemical behaviour. This question checks whether you can correctly identify the element that lies in Group 1 and Period 4, and then recall or deduce its atomic number, which is a very common type of periodic table question in exams.

Given Data / Assumptions:

• The periodic table being used is the modern long form with 18 groups and 7 periods.
• We are asked about the element in Group 1 (the alkali metal group) and Period 4.
• Atomic number is the number of protons in the nucleus of an element.
• We assume the standard order of elements based on increasing atomic number.

Concept / Approach:
Group 1 elements (alkali metals) in order of increasing atomic number are: hydrogen (H), lithium (Li), sodium (Na), potassium (K), rubidium (Rb), caesium (Cs), and francium (Fr). Period 1 contains hydrogen, Period 2 contains lithium, Period 3 contains sodium, and Period 4 contains potassium. Once we identify that the Group 1, Period 4 element is potassium, we simply recall or look up its atomic number, which is 19. This combines understanding of the periodic layout with knowledge of individual elements.

Step-by-Step Solution:
Step 1: List Group 1 elements in order: H, Li, Na, K, Rb, Cs, Fr. Step 2: Match each with its period: H in Period 1, Li in Period 2, Na in Period 3, K in Period 4, and so on. Step 3: Identify the Group 1 element in Period 4: this is potassium (K). Step 4: Recall the atomic number of potassium: potassium has 19 protons, so its atomic number is 19. Step 5: Match 19 with the given options; it corresponds to option 19.

Verification / Alternative check:
We can verify using the sequence of atomic numbers. Sodium (Na) in Period 3, Group 1, has atomic number 11. The next alkali metal down the group is potassium (K), followed by rubidium (Rb) at 37. Standard periodic tables always show potassium at atomic number 19 in Period 4, Group 1. Therefore, any periodic table reference will confirm that the Group 1, Period 4 element is potassium with atomic number 19, validating our answer.

Why Other Options Are Wrong:
Option A (21): Atomic number 21 is scandium (Sc), which lies in Group 3, not Group 1. Option B (17): Atomic number 17 is chlorine (Cl), a Group 17 halogen, not in Group 1. Option D (9): Atomic number 9 is fluorine (F), a Period 2 halogen, not the Period 4 Group 1 element.

Common Pitfalls:
A frequent mistake is to confuse period numbers or to miscount down the group, especially if you do not visualise the table clearly. Another error is mixing up atomic numbers of nearby elements such as calcium (20) and potassium (19). To avoid this, remember the clear sequence: Na (11), K (19), Rb (37), and that potassium is the first alkali metal in the fourth period. Practising with a blank periodic table and filling in Group 1 elements helps fix this pattern in memory.

Final Answer:
The element in Group 1 and Period 4 is potassium, which has atomic number 19.