Which of the following atoms has the lowest ionisation potential?
(a) 14_7 N
(b) 133_55 Cs
(c) 40_18

Q: Why does Argon have a high ionization potential if it's in the third period?

Argon is a noble gas with a completely filled valence shell (a stable octet). This extreme quantum stability means it strongly resists losing an electron, resulting in a very high ionization potential despite being larger than Nitrogen or Oxygen.

Q: Doesn't Oxygen have a lower ionization potential than Nitrogen?

Yes, Oxygen actually has a slightly lower ionization potential than Nitrogen because Nitrogen has a stable half-filled p-orbital ($2p^3$). However, both are much smaller than Cesium, so their IPs are still vastly higher than Cesium's.

Q: Why does atomic size increase down a group?

As you move down a group in the periodic table, a new principal quantum shell is added for each element. These additional shells shield the outer electrons from the nucleus and physically extend the spatial boundary of the atom.

Q: How does effective nuclear charge affect ionization potential?

Effective nuclear charge ($Z_{eff}$) is the net positive charge experienced by an electron. Across a period, $Z_{eff}$ increases, pulling electrons closer and increasing IP. Down a group, the increase in distance and shielding outweighs the increase in nuclear charge, lowering the IP.

Q: Is Cesium the element with the absolute lowest ionization potential?

Among stable, naturally occurring elements, yes! Francium technically has a lower ionization potential, but it is highly radioactive, extremely rare, and unstable.

Question

Which of the following atoms has the lowest ionisation potential?

Options:

(A) $^{14}_{7}	ext{N}$

(B) $^{133}_{55}	ext{Cs}$

(C) $^{40}_{18}	ext{Ar}$

(D) $^{16}_{8}	ext{O}$

Sigma Percentile · Accepted Answer

### The Secret to Plucking Electrons: Understanding Ionization Potential

Have you ever wondered how hard it is to steal an electron from an atom? This cosmic game of tug-of-war is governed by a concept known as **Ionization Potential (IP)**. Simply put, ionization potential is the minimum amount of energy you need to supply to an isolated gaseous atom to completely remove its most loosely bound electron.

Imagine the nucleus as a strict parent and the electrons as children playing in the yard. The closer the children are to the parent, the harder it is to pull them away.

### The Tug of War: Electrostatic Force vs. Distance

The force keeping the electron tethered to the atom is the electrostatic force of attraction between the positively charged nucleus and the negatively charged electron. According to Coulomb's Law, this force is inversely proportional to the square of the distance between them:

$$F_e \propto \frac{1}{r^2}$$

This means that as the atomic radius ($r$) increases, the electrostatic grip of the nucleus on the outermost electron weakens dramatically. Consequently, less energy is required to overcome this attraction. Therefore, we can establish a golden rule: **Ionization potential is inversely proportional to atomic radius.**

$$IP \propto \frac{1}{	ext{Atomic Radius}}$$

### Analyzing the Contenders: N, O, Ar, and Cs

Let's evaluate the atoms given in our problem: Nitrogen ($^{14}_{7}	ext{N}$), Oxygen ($^{16}_{8}	ext{O}$), Argon ($^{40}_{18}	ext{Ar}$), and Cesium ($^{133}_{55}	ext{Cs}$).

To find the lowest ionization potential, we are essentially looking for the largest atom.

Nitrogen and Oxygen are small non-metals located in the second period of the periodic table. Argon is a noble gas in the third period, slightly larger but incredibly stable.

Then we have **Cesium**. Cesium is an alkali metal residing all the way down in the sixth period. As you move down a group in the periodic table, new electron shells are added, making the atom significantly larger. Cesium is an absolute giant compared to the other three contenders.

### The Grand Conclusion

Because Cesium has such a massive atomic radius, its outermost valence electron is located very far from the nucleus. The electrostatic force holding it is incredibly weak.

Therefore, it takes very little energy to pluck that electron away. This makes Cesium the atom with the **lowest ionization potential** among the given choices. In fact, Cesium has one of the lowest ionization potentials of all stable elements in the entire periodic table!

Animated Solution for Physics - Atoms and Nuclei: Which of the following atoms has the lowest ionisation potential?

Select Answer:

Visualized Solution

$I P = Energy to remove e^{-}$

$I P \propto \frac{1}{r}$

$Comparing Atomic Sizes$

$Final Conclusion$

The Sigma Insight: Bohr's Atomic Model and Energy Levels

The Secret to Plucking Electrons

The Tug of War

Analyzing the Contenders

The Grand Conclusion

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