Chemistry XI Chapter 4

Question 4.46: Diamond is soft and electrical conductor of electrical insulator. Why?
Answer: In graphite each carbon is SP² hybridized. Each carbon has three SP² hybrid orbitals making an angle of 120^o with each other in the same plane and one unchanged p orbital to perpendicular this plane. Each carbon is bonded with other three carbon atoms by sp²-sp² overlapping in the plane. Different layers of graphite are held together by weak Van der Waals’ forces of attraction, so they are soft. The unchanged p-orbital electron of carbon is delocalized over entire graphite sheet as in metallic sheet. So they are good conductor of electricity.

Figure: Crytstal structure of Graphi

Question 4.47: Why most of the solids are
soft, have low melting and boiling points?

Answer: Molecular solids contain polar or non-polar molecule. In molecular solids these molecules are held together by either dipole–dipole force of attraction (in case polar molecules) or London dispersion forces (in case of non-polar molecules). As these forces are very weak so low energy is required to separate these molecules. So molecular solids are very soft, volatile and show low melting and boiling points.
Question 4.48: Why does most of freshly cut metal show metallic luster? Answer: Whenever metal is freshly cut, most of them possess metallic luster. This is due to fact that when light falls freshly cut surface, mobile electrons are excited by absorbing the light. These electrons when deexcited give off some energy in the form of light. This light appears as a metallic luster.

Question 4.49: Ionic crystals are brittle in nature while metals are ductile and malleable. Explain the difference.

Answer: Ionic crystals are highly brittle. Ionic crystals are highly brittle because ionic solids are composed of parallel layers which contain cations and anions in alternate positions. So opposite ions in the various parallel layers lie over each other. When an external force is applied, one layer of the ions slides a bit over the layer. In this way the like ions come in front of each other and begin to repel so causing brittleness.

Figure: Brittleness of Ionic crystal

Metals are malleable and ductile

Figure: Metals as malleable and ductile

Metals are composed of cations and free

electrons. These cations are held in the together by free electrons. when stress is applied on them, their layers slip pass each other. Due to this reason metals are malleable and ductile.

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