Mass numbers of Li, Be and B are 7,9 and 10 respectively. Which of the following has two electrons, three protons and four neutrons ?

Options

A. `B^(+)`
B. `Be^(2+)`
C. `Li^(+)`
D. Be

Answer

To solve the question regarding which element has two electrons, three protons, and four neutrons, we can follow these steps:

### Step 1: Understand the Definitions
- **Protons**: Positively charged particles found in the nucleus of an atom. The number of protons defines the atomic number of an element.
- **Electrons**: Negatively charged particles that orbit the nucleus. In a neutral atom, the number of electrons equals the number of protons.
- **Neutrons**: Neutral particles found in the nucleus. The mass number (A) of an atom is the sum of protons and neutrons.

### Step 2: Identify the Given Information
- We have the following mass numbers:
- Lithium (Li): Mass number = 7, Atomic number = 3 (3 protons)
- Beryllium (Be): Mass number = 9, Atomic number = 4 (4 protons)
- Boron (B): Mass number = 10, Atomic number = 5 (5 protons)

### Step 3: Analyze the Required Characteristics
- We need to find an atom with:
- 2 electrons
- 3 protons
- 4 neutrons

### Step 4: Determine the Element with 3 Protons
- The element with 3 protons is Lithium (Li), which has an atomic number of 3.

### Step 5: Calculate the Number of Neutrons
- The mass number (A) of Lithium is 7.
- The number of neutrons can be calculated using the formula:
\[ \text{Number of Neutrons} = \text{Mass Number} - \text{Atomic Number} \]
\[ \text{Number of Neutrons} = 7 - 3 = 4 \]

### Step 6: Determine the Electron Configuration
- Lithium in its neutral state has 3 electrons. However, if it loses one electron, it becomes Li⁺ (Lithium ion).
- Therefore, Li⁺ has:
- Protons = 3
- Electrons = 2 (since it lost one)
- Neutrons = 4 (as calculated)

### Conclusion
- The species that has 2 electrons, 3 protons, and 4 neutrons is the Lithium ion (Li⁺).

### Final Answer
The correct answer is Lithium ion (Li⁺).

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