Among the following groups given below, the one where elements can have positive as well as negative oxidation states is:

Options

A. H, F, O
B. Na, Mg, Al
C. He, Li, Be
D. H, Cl, Br

Answer

To solve the question regarding which group of elements can have both positive and negative oxidation states, we will analyze each option step by step.

### Step 1: Understand Oxidation States
Oxidation state (or oxidation number) refers to the total number of electrons that an atom gains or loses when it forms a chemical bond. Elements can exhibit different oxidation states based on their electron configurations and tendencies to gain or lose electrons.

### Step 2: Analyze Each Option

**Option 1: Hydrogen, Fluorine, and Oxygen**
- **Hydrogen (H)**: Can either gain one electron (to become H⁻) or lose one electron (to become H⁺). Therefore, it can have both positive and negative oxidation states.
- **Fluorine (F)**: The most electronegative element, typically gains one electron to achieve a stable configuration, resulting in an oxidation state of -1. It does not exhibit positive oxidation states.
- **Oxygen (O)**: Generally has a -2 oxidation state when it gains two electrons, but can also exhibit a +2 state in some compounds (though this is less common).

Conclusion: While hydrogen can have both oxidation states, fluorine cannot. Thus, this option is not correct.

**Option 2: Sodium, Magnesium, and Aluminum**
- **Sodium (Na)**: Has one electron in its outer shell and tends to lose it, resulting in a +1 oxidation state.
- **Magnesium (Mg)**: Has two outer electrons and typically loses both, resulting in a +2 oxidation state.
- **Aluminum (Al)**: Has three outer electrons and usually loses them, resulting in a +3 oxidation state.

Conclusion: All these elements primarily exhibit positive oxidation states and do not have negative oxidation states. Thus, this option is not correct.

**Option 3: Helium, Lithium, and Beryllium**
- **Helium (He)**: A noble gas with a complete outer shell (1s²) and has an oxidation state of 0.
- **Lithium (Li)**: Has one electron in its outer shell and tends to lose it, resulting in a +1 oxidation state.
- **Beryllium (Be)**: Has two outer electrons and typically loses both, resulting in a +2 oxidation state.

Conclusion: Helium does not have a positive or negative oxidation state, and lithium and beryllium only exhibit positive oxidation states. Thus, this option is not correct.

**Option 4: Hydrogen, Chlorine, and Bromine**
- **Hydrogen (H)**: Can gain one electron (H⁻) or lose one electron (H⁺), so it has both positive and negative oxidation states.
- **Chlorine (Cl)**: Typically has a -1 oxidation state but can also exhibit positive oxidation states (+1, +3, +5, +7) in compounds.
- **Bromine (Br)**: Similar to chlorine, it usually has a -1 oxidation state but can also show positive oxidation states.

Conclusion: All three elements can exhibit both positive and negative oxidation states. Thus, this option is correct.

### Final Answer
The group where elements can have positive as well as negative oxidation states is **Option 4: Hydrogen, Chlorine, and Bromine**.

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