Question

a. How does the octet rule explain the formation of a sodium ion? b. What noble gas has the same electron arrangement as the sodium ion? c. Why are Group \(1 \mathrm{~A}\) (1) and Group \(2 \mathrm{~A}\) (2) elements found in many compounds, but not Group \(8 \mathrm{~A}(18)\) elements?

Short answer

The octet rule explains the formation of a sodium ion by it losing one electron to achieve a stable electron configuration like neon. Group 1A and 2A elements react to complete their octet, unlike Group 8A elements, which are already stable.

Step-by-step solution

Understand the Octet Rule

The octet rule states that atoms tend to gain, lose, or share electrons to have a full outer shell of eight electrons, achieving a stable electron arrangement similar to that of noble gases.

Sodium's Electron Configuration

Determine the electron configuration of a neutral sodium (Na) atom: 1s² 2s² 2p⁶ 3s¹. Sodium has one electron more than a full second shell.

Formation of a Sodium Ion

To achieve a stable electron configuration, sodium loses its one valence electron in the 3s orbital. This results in a sodium ion (Na⁺) with an electron configuration of 1s² 2s² 2p⁶, matching a full octet.

Identify the Noble Gas

The electron configuration of the sodium ion (Na⁺) is identical to that of neon (Ne), which is 1s² 2s² 2p⁶.

Compare Group 1A, 2A with Group 8A Elements

Group 1A (1) and 2A (2) elements are highly reactive because they tend to lose one or two electrons, respectively, to achieve a full outer shell. Group 8A (18) elements, also known as noble gases, already have a complete octet, making them chemically inert and less likely to form compounds.

Key concepts

Electron Configuration

Every atom has a specific arrangement of electrons around its nucleus. This arrangement is called the electron configuration. Electrons travel in regions known as orbitals, and these orbitals are associated with different energy levels. Each energy level can hold only a certain number of electrons. For example:

• The first energy level holds up to 2 electrons.
• The second energy level holds up to 8 electrons.
• The third energy level starts filling with 8 electrons and can hold more in more complex atoms.

Sodium (Na) has an atomic number of 11, meaning it has 11 electrons. Its electron configuration is 1s² 2s² 2p⁶ 3s¹. This shows that sodium has one excess electron in the third shell.

Sodium Ion Formation

The octet rule explains the stability that atoms achieve when their outermost electron shell is complete with 8 electrons. Sodium (Na) has only one electron in its outermost shell, which makes it unstable. To achieve stability, sodium loses this one outer electron. This process converts the neutral sodium atom into a positively charged sodium ion (Na⁺). When sodium loses one electron, it achieves the electron configuration of neon. The new configuration for the sodium ion is 1s² 2s² 2p⁶, a complete octet, hence more stable.

Noble Gases

Noble gases are a special group of elements found in Group 8A (18) of the periodic table. These gases include helium (He), neon (Ne), argon (Ar), krypton (Kr), xenon (Xe), and radon (Rn). Noble gases are characterized by their full outer electron shells, making them very stable and inert. This stability is due to their electron configuration, which has a complete octet, except for helium which has a full first shell with 2 electrons. For example:

• Neon's electron configuration: 1s² 2s² 2p⁶.
• Argon's electron configuration: 1s² 2s² 2p⁶ 3s² 3p⁶.

Because their outermost shells are complete, noble gases rarely participate in chemical reactions.

Periodic Table Groups

The periodic table is divided into groups (columns) and periods (rows). Elements in the same group share similar chemical properties because they have the same number of electrons in their outer shells.

• Group 1A (1) elements, like sodium, have 1 electron in their outermost shell. They are highly reactive and are known as alkali metals.
• Group 2A (2) elements have 2 electrons in their outer shell and are also reactive. These are the alkaline earth metals.
• Group 8A (18) elements, or noble gases, have full outer shells. They do not react easily with other elements.

The reactivity of Groups 1A and 2A is due to their tendency to lose electrons to achieve a stable electron configuration, while noble gases already have stable configurations and thus rarely form compounds.