Question

The correct order of decreasing first ionisation potential is (1) \(C>B>B c>L i\) (2) \(\mathrm{C}>\mathrm{Bc}>\mathrm{B}>\mathrm{Li}\) (3) \(\mathrm{B}>\mathrm{C}>\mathrm{Bc}>\mathrm{Li}\) (4) \(\mathrm{Bc}>\mathrm{Li}>\mathrm{B}>\mathrm{C}\)

Short answer

Option 2: \(\text{C > Be > B > Li}\) is the correct order.

Step-by-step solution

Understand Ionization Potential

Ionization potential or energy is the energy required to remove an electron from an atom in its gaseous state. It generally increases across a period from left to right and decreases down a group in the periodic table.

Analyze Element Positions in the Periodic Table

The elements given are Carbon (C), Boron (B), Beryllium (Bc or Be), and Lithium (Li). These are all elements in the second period of the periodic table. Their respective positions are: Li (Group 1), Be (Group 2), B (Group 13), and C (Group 14).

Determine General Trends in Ionization Energy

In the second period, ionization energy generally increases from left to right: Li < Be < B < C.

Compare Given Options to Trends

Using the trend from Step 3, compare each option to see which matches the order: Option 1: C > B > Be > LiOption 2: C > Be > B > LiOption 3: B > C > Be > LiOption 4: Be > Li > B > CFrom these, Option 2 follows the general trend.

Key concepts

Ionization Energy

Ionization energy is the energy required to remove the outermost electron from an atom in its gaseous form. Think of it like the effort needed to pull a magnet off a fridge. The stronger the magnet's pull, the more energy you need.
For atoms, this pull is the attraction between negatively charged electrons and the positively charged nucleus. The more strongly an atom holds its outer electrons, the higher its ionization energy.
Factors influencing ionization energy include:

• Atomic size: Smaller atoms hold their electrons more tightly because their electrons are closer to the nucleus.
• Electron shielding: With more inner electrons, outer electrons are less attracted to the nucleus, reducing ionization energy.

By understanding these concepts, you can predict and compare the ionization energy of different elements.

Periodic Table Trends

Periodic table trends help us predict the properties of elements. When discussing ionization energy, two main trends are crucial:

Across a Period:
As you move from left to right across a period (horizontal row) in the periodic table, ionization energy generally increases. This increase is due to the rising number of protons in the nucleus, which pulls electrons closer and tighter.
For example, in the second period, we see the following sequence:

• Lithium (Li)
• Beryllium (Be)
• Boron (B)
• Carbon (C)

Going from Li to C, the ionization energy increases.

Down a Group:
As you move down a group (vertical column), ionization energy decreases. This decrease happens because more electron shells are added, increasing the distance between outer electrons and the nucleus. Additionally, inner-shell electrons shield outer ones from the full nuclear charge.

Element Positions

Knowing an element's position in the periodic table tells us a lot about its ionization energy. Let's look at the given elements:

• Lithium (Li): Found in Group 1, the first element of the second period. It has a low ionization energy compared to others in its period.
• Beryllium (Be): Located in Group 2, right after Li in the second period. It has higher ionization energy than Li but less than B and C.
• Boron (B): This element is in Group 13. Its ionization energy is higher than Be's.
• Carbon (C): Situated in Group 14, it has the highest ionization energy among these four elements.

With this information, we can confirm that the sequence for decreasing first ionization potential is: C > Be > B > Li. Option 2 from the exercise is correct because it aligns with these positions and trends.