Question

Explain, in terms of their electrons, how an atom of sodium and an atom of chlorine form a molecule of sodium chloride. (p. 26)

Short answer

Answer: A sodium atom loses one electron to form a positively charged sodium ion (Na⁺), while a chlorine atom gains one electron to form a negatively charged chloride ion (Cl⁻). The opposite charges attract, forming an ionic bond between them and resulting in a sodium chloride (NaCl) molecule.

Step-by-step solution

Understand the electron configurations of sodium and chlorine atoms

Sodium has an atomic number of 11, which means it has 11 electrons in its neutral state. The electron configuration of a sodium atom is 1s² 2s² 2p⁶ 3s¹, which means it has 1 electron in its outermost shell (3s¹). Chlorine has an atomic number of 17, which means it has 17 electrons in its neutral state. The electron configuration of a chlorine atom is 1s² 2s² 2p⁶ 3s² 3p⁵, which means it has 7 electrons in its outermost shell (3s² 3p⁵).

Understand stability and ionic bonding between sodium and chlorine atoms

Atoms achieve stability when they have a full outer electron shell. For sodium and chlorine, the most stable electron configuration is to have a full outer shell of 8 electrons (octet rule). Sodium, with only 1 electron in its outermost shell, can achieve this stable configuration by losing that electron, forming a sodium ion (Na⁺) with a positive charge. The electron configuration of Na⁺ would be 1s² 2s² 2p⁶, which resembles the noble gas neon. Chlorine, with 7 electrons in its outermost shell, can achieve a stable configuration by gaining an electron, forming a chloride ion (Cl⁻) with a negative charge. The electron configuration of Cl⁻ would be 1s² 2s² 2p⁶ 3s² 3p⁶, which resembles the noble gas argon.

Formation of sodium chloride molecule through ionic bonding

When a sodium atom loses one electron, it forms a positively charged sodium ion, Na⁺. When a chlorine atom gains one electron, it forms a negatively charged chloride ion, Cl⁻. Because opposite charges attract, the Na⁺ and Cl⁻ ions are attracted to each other. This attraction forms an ionic bond between the sodium and chloride ions, resulting in the formation of a sodium chloride (NaCl) molecule.

Key concepts

Electron Configuration

Understanding the electron configuration of atoms is crucial when addressing how ionic compounds, such as sodium chloride, are formed. An atom's electron configuration is a representation of the arrangement of its electrons within different energy levels and sublevels. For example, sodium has an atomic number of 11, which corresponds to its number of electrons in a neutral state. Its configuration is therefore denoted as 1s² 2s² 2p⁶ 3s¹, highlighting that the highest energy level (the third shell) contains just one electron in the s sublevel.

Similarly, chlorine, with an atomic number of 17, has the electron configuration 1s² 2s² 2p⁶ 3s² 3p⁵, indicating seven electrons in the third shell, which is its outermost level. This electron arrangement determines an atom's reactivity and plays a pivotal role in chemical bonding.

Octet Rule

The octet rule is a principle that atoms tend to bond in such a way that they each have eight electrons in their valence shell, achieving the electron configuration of a noble gas. This is believed to be a driving force behind the chemical stability in compounds. Sodium, with one lone electron in the 3s sublevel, deviates from this rule. Chlorine, on the other hand, falls short by one electron, possessing seven valence electrons.

The rule predicts that sodium can achieve a stable octet by losing its single valence electron, and chlorine can fill its valence shell by accepting one electron. The exchange of electrons between atoms like these underpins the formation of ionic bonds and leads to the creation of compounds with significantly increased stability.

Sodium Chloride Formation

The formation of sodium chloride (NaCl) is a classic example of how the octet rule governs ionic bonding. When a sodium atom encounters a chlorine atom, sodium's one valence electron is transferred to chlorine. In the process, sodium becomes a sodium ion (Na⁺), with a positively charged electron configuration that now resembles the noble gas neon, while chlorine becomes a chloride ion (Cl⁻), bearing a negative charge akin to the noble gas argon.

This electron transfer completes the valence shell for both ions, resulting in their chemical stability. The resulting cation (Na⁺) and anion (Cl⁻) are held together by the strong electrostatic forces indicative of ionic bonding, leading to the crystalline structure characteristic of table salt.

Chemical Stability

Chemical stability refers to the state of an atom when it is least reactive or at a lower energy level, typically accompanied by a full valence electron shell, as per the noble gases. The desire of sodium and chlorine atoms to reach this state drives the formation of ionic bonds. In the case of sodium chloride, each sodium ion achieves stability by having a complete octet, matching neon's electron configuration, and each chloride ion does the same, mirroring argon's configuration.

These stable ions form a lattice structure, held together by their ionic bonds, which gives NaCl its high melting and boiling points, solubility in water, and electrical conductivity when dissolved or molten - all indicators of its chemical stability.