Smallest among these species is

Options

A. Hydrogen
B. Helium
C. Lithium
D. Lithion ion.

Explanation

To determine the smallest species among hydrogen, helium, lithium, and lithium ions, we can follow these steps:

### Step 1: Understand the Species
We are comparing four species: hydrogen (H), helium (He), lithium (Li), and lithium ions (Li⁺). It's important to note that hydrogen and helium are neutral atoms, while lithium ions are positively charged.

### Step 2: Compare Atomic Sizes
- **Hydrogen (H)**: The atomic radius of hydrogen is about 53 pm (picometers).
- **Helium (He)**: The atomic radius of helium is about 31 pm. Helium is smaller than hydrogen because it has a higher nuclear charge, which pulls the electrons closer to the nucleus.
- **Lithium (Li)**: The atomic radius of lithium is about 152 pm. Lithium is larger than both hydrogen and helium due to having more electron shells.
- **Lithium Ion (Li⁺)**: When lithium loses an electron to become Li⁺, it has a smaller radius than neutral lithium. The radius of Li⁺ is about 76 pm.

### Step 3: Analyze the Sizes
From the sizes we have:
- H: 53 pm
- He: 31 pm
- Li: 152 pm
- Li⁺: 76 pm

### Step 4: Determine the Smallest Species
Among these values, helium (31 pm) is smaller than hydrogen (53 pm), and lithium ion (76 pm) is larger than both hydrogen and helium. Therefore, the smallest species among the given options is **helium (He)**.

### Final Answer
The smallest among these species is **helium (He)**.

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