Which of the following atoms has the highest first ionisation energy ?

Options

A. Na
B. K
C. Sc
D. Rb

Answer

To determine which of the following atoms has the highest first ionization energy, we can follow these steps:

### Step 1: Understand Ionization Energy
Ionization energy is the energy required to remove the outermost electron from an atom in its gaseous state. Generally, the first ionization energy increases across a period (from left to right) and decreases down a group (from top to bottom) in the periodic table.

**Hint:** Remember that ionization energy is influenced by the atomic structure, including the number of protons and the distance of the outermost electron from the nucleus.

### Step 2: Identify the Atoms
List the atoms provided in the question. For example, if the options are Sodium (Na), Potassium (K), Rubidium (Rb), and Scandium (Sc), we will analyze these elements.

**Hint:** Make sure to note the group and period of each element in the periodic table.

### Step 3: Analyze the Group Trend
As we move down a group in the periodic table, the ionization energy decreases. This is because the outermost electron is farther from the nucleus, and there is increased electron shielding from inner electrons.

- Sodium (Na) is in Group 1.
- Potassium (K) is below Sodium in Group 1.
- Rubidium (Rb) is below Potassium in Group 1.

Since K and Rb are below Na, they will have lower ionization energies than Na.

**Hint:** Recall that alkali metals (Group 1) have low ionization energies compared to other groups.

### Step 4: Analyze the Period Trend
As we move across a period from left to right, the ionization energy increases due to the increased nuclear charge attracting the electrons more strongly.

- Scandium (Sc) is in Group 3 and is further to the right than Na, K, and Rb.

Since Sc is in a different group and period, it will have a higher ionization energy compared to the alkali metals.

**Hint:** Elements in the same period generally have increasing ionization energy as you move from left to right.

### Step 5: Conclusion
Based on the analysis:
- Sodium has a higher ionization energy than Potassium and Rubidium.
- Scandium, being further to the right in the periodic table, will have a higher ionization energy than Sodium.

Thus, the atom with the highest first ionization energy among the options is **Scandium (Sc)**.

**Final Answer:** Scandium (Sc) has the highest first ionization energy.