GoalKnow the types of crystal lattice and how they affect properties.
New words
crystal lattice · kristall panjaralattice site · panjara tuguniionic and atomic lattice · ionli va atom panjaramolecular lattice · molekulyar panjara
Explanation
In most solids the particles are arranged in order and form a crystal lattice; the points where particles sit are called lattice sites. By the kind of particle at the sites there are four types. In an ionic lattice cations and anions alternate (NaCl); such substances are hard, brittle, high-melting, and their solution or melt conducts current. In an atomic lattice atoms at the sites are joined by covalent bonds (diamond, silicon); such substances are very hard and melt at very high temperatures. In a molecular lattice molecules are held by weak attraction (iodine I₂, ice, dry ice CO₂); they are soft and melt or evaporate easily. In a metallic lattice there are metal ions with free-moving electrons between them, so metals conduct current and heat well.
Worked examples
Table salt NaCl has an ionic lattice and melts at about 801 °C. Iodine I₂ has a molecular lattice: when heated it can turn straight into vapour (sublimation), so even as crystals it evaporates.
Diamond and graphite are two forms of carbon. In diamond every atom is bonded to four others (very hard); in graphite the layers slide over each other (soft, used in pencils).
Class activity
“Lattice model”: with plasticine balls and sticks make one layer of NaCl, with two colours alternating. Then make a second model of I₂ molecules (two balls each) in order. Only plasticine and sticks; no substance is tested or tasted.