Understand why atoms form bonds, how Lewis symbols represent valence electrons, why the octet rule has exceptions, and how the electron-sea model explains the properties of metals.
Atoms form chemical bonds when the bonded arrangement is more stable than the separate atoms. Many atoms become more stable when their outermost shell resembles that of a noble gas. For many elements, this means having eight electrons in the valence shell.
Octet rule: Atoms tend to gain, lose or share electrons during bond formation so that they are surrounded by eight valence electrons. It is a useful guiding principle for simple molecules, not a universal law.
Answer: The octet rule states that atoms commonly gain, lose or share electrons to attain eight electrons in their outermost shell, reaching a stable noble-gas-like arrangement.
Answer: Hydrogen has only the first electron shell, which can hold a maximum of two electrons. It becomes stable by attaining a duplet configuration, like helium, rather than an octet.
A Lewis symbol shows an element's chemical symbol surrounded by dots representing its valence electrons. For example, fluorine has the electronic configuration 2,7 and therefore has seven valence electrons.
Bonding electrons are shared between atoms to form a covalent bond. Non-bonding electrons, also called lone pairs, remain associated with an atom and do not form that bond.
Hydrogen has one valence electron and fluorine has seven. They share one pair of electrons, forming a single covalent bond. Hydrogen then has two electrons around it, while fluorine has eight (three lone pairs and one shared pair).
The octet rule is not followed by every stable molecule. Three important exceptions are incomplete octets, expanded octets and molecules with an odd number of valence electrons.
Answer: (a) BF₃ has an incomplete octet around boron. (b) SF₆ is a standard textbook example of an expanded octet around sulphur. (c) NO has an odd number of valence electrons and one unpaired electron.
Answer: Boron has three valence electrons. In BF₃ it forms three covalent bonds with three fluorine atoms. These bonds place six electrons around boron; it has no additional valence electron available to form a fourth bond in this structure.
Answer: A duplet configuration is a stable arrangement of two electrons in the first shell. Hydrogen attains it by sharing one electron pair in a covalent bond or by gaining one electron in suitable compounds.
Answer: Nitrogen contributes five valence electrons and oxygen contributes six, giving NO a total of 11 valence electrons. Since the total is odd, the molecule has an unpaired electron and cannot give both atoms a complete octet in a simple Lewis structure.
Answer: Place B in the centre and connect it to three F atoms with three single B–F bonds. Give each fluorine three lone pairs. Each F has an octet, but only six electrons surround boron.
Schematic structure:
F
|
F — B — F
(Each F has three lone pairs of electrons.)
The lines represent shared electron pairs. This schematic shows connectivity; in the actual molecule the three B–F bonds are arranged in a trigonal planar shape.
Answer: A. In the textbook model, both statements are treated as correct and the reason explains the expanded valence shell shown for sulphur in SF₆.
Remember: The octet rule is a useful starting point, but the stability and bonding of a molecule cannot always be predicted from this rule alone.
In the electron-sea model, metal atoms contribute outer electrons that are delocalised throughout the solid. Positive metal ions occupy a regular arrangement, while the mobile electrons move around and between them. The electrostatic attraction between these positive ions and the mobile electrons is called metallic bonding.
Answer: It is the collection of delocalised outer electrons that can move throughout the metal rather than remaining attached to one particular atom.
Answer: Positive metal ions are arranged in a regular structure. The electrons are mobile and move through the structure.
Answer: Metallic bonding is the electrostatic attraction between positive metal ions and the sea of delocalised electrons in a metal.
Answer: The delocalised electrons are shared collectively throughout the metal rather than being confined between one specific pair of atoms. The attraction therefore acts throughout the structure and is described as non-directional.
Answer: Electrical conductivity and malleability are two such properties. The model also helps explain thermal conductivity and ductility.
Answer: Mobile electrons can drift through the metal when an electric field is applied, carrying electric charge. This is why metals such as copper are good electrical conductors.
Answer: Mobile electrons transfer energy through the metal, helping heat spread quickly from a hotter region to a cooler region.
Answer: This property is called malleability. Layers of metal ions can shift past one another while the mobile electrons continue to attract and hold the ions together, so the metal changes shape instead of immediately shattering.
Answer: Ductility is the ability of a material to be drawn into wires. In a metal, layers of ions can rearrange while the delocalised electrons continue to provide cohesive attraction.
Answer: In a typical covalent bond, a pair of electrons is shared between particular atoms. In metallic bonding, delocalised electrons are shared collectively throughout the metal. Covalent bonds are generally localised and directional; metallic bonding is non-localised and non-directional.
Answer: A metal consists of positive ions arranged in a regular structure, surrounded by mobile delocalised electrons. The attraction between the ions and electrons holds the structure together.
Answer: Electrical conductivity would be strongly affected because mobile electrons are the main charge carriers in a metal. Thermal conductivity would also be reduced.
Answer: The layers of ions can slide relative to one another while the delocalised electrons continue to hold the structure together. This gives metals malleability.
Answer: Copper contains mobile electrons that carry charge through the metal. Rubber does not have comparable freely moving charge carriers under ordinary conditions, so it acts as an electrical insulator.
Answer: Mobile electrons in metals transfer energy rapidly through the material. Heat is also transferred by lattice vibrations, but the mobile electrons make an important contribution in metals.
Answer: A. Both statements are correct, and the reason correctly explains the assertion.
Answer: C. The assertion is correct, but the reason is incorrect. Metallic electrons are delocalised rather than localised between a specific pair of atoms.
Answer: A. Both statements are correct, and the reason explains why metals can be hammered into sheets.