Among the elements F, O, H and Li, which one has the largest atomic radius?

Introduction / Context:
This question focuses on periodic trends, specifically the variation of atomic radius across periods and down groups in the periodic table. Understanding atomic radius is important because it influences many other properties such as ionisation energy, electronegativity and chemical reactivity. Here, you are asked to compare the sizes of four simple elements: fluorine, oxygen, hydrogen and lithium.

Given Data / Assumptions:

• Elements to compare: fluorine (F), oxygen (O), hydrogen (H) and lithium (Li).
• We need to determine which has the largest atomic radius.
• We assume neutral isolated atoms in their ground state.
• Periodic positions: H and Li are in period 1 and 2 respectively; O and F are in period 2.

Concept / Approach:
Atomic radius increases down a group and decreases from left to right across a period. This is because additional shells are added when moving down a group, making atoms larger, while increasing nuclear charge across a period pulls electrons closer, making atoms smaller. In period 2, lithium is on the far left, oxygen and fluorine are on the right side and are nonmetals. Hydrogen is in period 1 with only one shell. For elements in the same period, the leftmost element has the largest atomic radius. When comparing period 1 and period 2, period 2 elements usually have a larger radius because they have two shells instead of one.

Step-by-Step Solution:
Step 1: Note that hydrogen is in period 1 with electron configuration 1s1, so it has only one shell. Step 2: Lithium is in group 1 and period 2 with configuration 2,1, so it has two shells. Step 3: Oxygen is in period 2, group 16 and fluorine is in period 2, group 17, with higher nuclear charge than lithium. Step 4: Across a period from left to right, atomic radius decreases due to increasing nuclear charge pulling electrons closer. Step 5: Therefore, in period 2, lithium has the largest radius, followed by beryllium, while oxygen and fluorine are much smaller. Step 6: When comparing hydrogen and lithium, lithium has an extra shell, so its atomic radius is larger than that of hydrogen. Step 7: Thus, among F, O, H and Li, lithium has the largest atomic radius.

Verification / Alternative check:
Standard periodic tables often provide approximate atomic radii. If you check these values you will see that lithium has a radius larger than that of hydrogen and much larger than that of oxygen and fluorine. Conceptually, the key point is that additional shells always increase radius significantly, while moving right across a period gradually decreases radius. Lithium being a period 2 alkali metal combines both effects to give a relatively large atomic size among the given options.

Why Other Options Are Wrong:
Fluorine and oxygen are both to the right of lithium in period 2, so they are smaller because their larger nuclear charge pulls the same shell of electrons closer. Hydrogen, although it is in the first period, has only one shell and therefore a smaller radius than lithium, which has two shells. The idea that all have the same radius is incorrect, since periodic trends clearly show variation in atomic sizes across periods and down groups.

Common Pitfalls:
Students sometimes assume that hydrogen, being very light, must also be very small in atomic radius, leading them to ignore the role of additional shells in lithium. Others incorrectly think that elements with higher atomic number are always larger, without considering where they lie in the periodic table. The correct approach is always to use trends: radius increases down a group and decreases from left to right across a period.

Final Answer:
Among F, O, H and Li, the element with the largest atomic radius is lithium (Li).