4.3.2 Atomic Size

This topic explains atomic size (atomic radius) — how the size of an atom changes across a period and down a group in the periodic table.

Definition

Atomic size is the average distance between the atomic nucleus and the electronic outer shell.

Simple explanation: Atomic size is the distance from the center of the atom (nucleus) to the outermost shell of electrons.

Main Idea

The size of an atom depends on its electronic configuration. Atomic radius shows two general trends in the periodic table:

• Atomic radius decreases across a period (left to right).
• Atomic radius increases down a group (top to bottom).

Simple explanation: Atoms get smaller as you go right, and bigger as you go down.

Core Concept

Trend 1: Atomic Radius Decreases Across a Period

The atomic radius decreases in each period as you move across the period.

• As you move from one element to the next (to the right), another electron is added to the same valence shell.
• At the same time, the positive charge of the core also increases by one.
• The attraction of the nucleus to the electron in the valence shell increases.
• Therefore, the size of the shell decreases.

Simple explanation: More protons in the nucleus pull the electrons closer, so the atom gets smaller.

Trend 2: Atomic Radius Increases Down a Group

Atomic radius increases in each main group as you move down the group.

• The size of an atom is determined by the size of its valence shell.
• As you move down the group to the next lower element, the atom has an additional shell of electrons.
• This increases the radius of the atom.

• Going from Li to Na, the atomic radius increases.
• Consider the electron configuration of:
• Li (1s² 2s¹)
• Na (1s² 2s² 2p⁶ 3s¹)
• A new electron shell is added, increasing the size of the atom.

Simple explanation: Adding a whole new shell of electrons makes the atom bigger.

Detailed Explanation

Atomic Size and Electronic Configuration

The size of an atom depends on its electronic configuration:

Across a Period

Across a period, the number of shells stays the same, but the nuclear charge increases. This pulls the electrons closer, so the atom shrinks.

• Li (Group 1) is the largest.
• F (Group 17) is the smallest.

Down a Group

Down a group, new shells are added. Even though the nuclear charge also increases, the additional shell makes the atom larger.

• Li is the smallest.
• Na is larger.
• K is larger still.
• Rb, Cs are even larger.

Book Example: Identifying the Element with Greater Atomic Radius

Question: Choose the element whose atom you expect to have larger atomic radius in each of the following pairs. (a) Mg, Al (b) C, Si

Problem Solving Strategy: Remember that the larger atom in any: (a) Period lies further to the left in the periodic table. (b) Group lies closer to the bottom in the periodic table.

Solution: (a) The larger atom is Mg (Mg is to the left of Al in Period 3). (b) The larger atom is Si (Si is below C in Group 14).

Concept Assessment Exercise 4.6

Using the periodic table but without looking at the figure, choose the element whose atom you expect to have smaller atomic radius in each of the following pairs. (a) O or S (b) O or F

Answers: (a) O (O is above S in Group 16 — smaller down a group means the top one is smaller). (b) F (F is to the right of O in Period 2 — smaller across a period means the right one is smaller).

• (a) O or S: O is above S. Atomic radius increases down a group. So O is smaller.
• (b) O or F: F is to the right of O in the same period. Atomic radius decreases across a period. So F is smaller.

Intermediate Level

Why Atomic Radius Decreases Across a Period

• Protons increase → nuclear charge increases.
• Electrons are added to the same shell → shielding stays constant.
• The effective nuclear charge increases.
• The nucleus pulls the electrons closer.
• So the atomic radius decreases.

Why Atomic Radius Increases Down a Group

• New shells are added.
• The valence electrons are in a higher shell (farther from nucleus).
• Although nuclear charge increases, the shielding effect also increases.
• The net effect is that the atom gets larger.

Atomic Radius and Other Properties

Atomic size affects several other properties:

Simple explanation: Bigger atoms hold their outer electrons more loosely, so they lose them more easily.

Comparison Table

Important Points

• - Atomic size = average distance between nucleus and outer shell.
• - Across a period: atomic radius decreases (nuclear charge increases, shielding constant).
• - Down a group: atomic radius increases (new shells added).
• - Larger atom lies to the left in a period and lower in a group.
• - Smaller atom lies to the right in a period and higher in a group.
• - Examples:
• - Mg > Al in size (Mg is to the left).
• - Si > C in size (Si is below C).
• - Na > Li in size (Na is below Li).
• - O < S in size (O is above S).
• - F < O in size (F is to the right of O).

Remember

• - Atomic size = distance from nucleus to outer shell.
• - Across a period → decreases.
• - Down a group → increases.
• - Larger atom → left in period, lower in group.
• - Smaller atom → right in period, higher in group.
• - Examples: Mg > Al, Si > C, Na > Li, O < S, F < O.

Common Mistakes

• 1. Thinking atomic size increases across a period. It decreases.
• 2. Thinking atomic size decreases down a group. It increases.
• 3. Forgetting that nuclear charge increases across a period. This is why the atom shrinks.
• 4. Forgetting that new shells are added down a group. This is why the atom grows.
• 5. Confusing atomic size with mass. They are different — mass increases down a group, but size does not always follow mass exactly.
• 6. Thinking the largest atom is in the top-right corner. The largest atoms are in the bottom-left corner.

Quick Revision

• Atomic size = average distance between the atomic nucleus and the electronic outer shell.
• Across a period: atomic radius decreases (nuclear charge increases, shielding remains constant, electrons pulled closer).
• Down a group: atomic radius increases (new electron shells are added).
• Examples:
• Mg is larger than Al (Mg is to the left).
• Si is larger than C (Si is below C).
• Na is larger than Li (Na is below Li).
• O is smaller than S (O is above S).
• F is smaller than O (F is to the right of O).

60-Second Revision

• Atomic size = distance from nucleus to outer shell.
• Across a period → decreases.
• Down a group → increases.
• Larger atom → left in period, lower in group.
• Smaller atom → right in period, higher in group.
• Warning: don't confuse atomic size with mass.