Which of the following is true about the element with atomic number 18?

Options

A. its has a very low ionisation potential
B. it has a very high electron affinity
C. its molecules are monoatomic
D. its electronegativity is very high.

Explanation

To determine which statement is true about the element with atomic number 18, we need to follow these steps:

### Step 1: Identify the Element
The element with atomic number 18 is Argon (Ar).

### Step 2: Determine the Electron Configuration
Argon has the electron configuration of \( 1s^2 2s^2 2p^6 3s^2 3p^6 \). This shows that Argon has a complete outer shell with 8 electrons in its outermost energy level.

### Step 3: Analyze Ionization Energy
Ionization energy is the energy required to remove an electron from an atom. Since Argon has a complete octet, it is stable and has a high ionization energy. Therefore, the statement claiming Argon has very low ionization energy is **false**.

### Step 4: Analyze Electron Affinity
Electron affinity is the energy change when an electron is added to an atom. For Argon, since it already has a complete octet, it does not tend to gain electrons, and thus its electron affinity is negligible or close to zero. Therefore, the statement claiming Argon has very high electron affinity is **false**.

### Step 5: Analyze Molecular Structure
Argon is a noble gas and exists as a monoatomic gas under standard conditions. This means that Argon atoms do not bond with each other or with other elements to form molecules. Therefore, the statement that Argon’s molecules are monoatomic is **true**.

### Step 6: Analyze Electronegativity
Electronegativity is the tendency of an atom to attract electrons in a bond. Since Argon has a complete octet, it does not have a tendency to attract additional electrons, making the statement about Argon having very high electronegativity **false**.

### Conclusion
The only correct statement about the element with atomic number 18 (Argon) is that its molecules are monoatomic.

### Summary of the Correct Statement:
- Argon is a monoatomic gas.

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