lesson2.ppt

Lesson 2
THE NONMETAL ELEMENTS
1
The halogens usually form covalent rather than ionic bonds.
Melting points of the alkali metals decrease with increasing atomic number because metallic bond between the atoms es weaker with increasing atomic size.
1
Boron has a tendency to form covalent bonds.
Tungsten and chromium have rather high melting points.
Silicon and germanium are semiconductors.
1
These weaker bonds result in lower melting points for silicon and greater chemical reactivity.
2
The two most familiar oxides of carbon,CO and CO2,are colorless,odorless and tasteless gases.
3
Silicon and germanium have weaker covalent bonds than carbon as a consequence of less efficient orbital overlap.
The hydrides of these elements are also much less stable than carbon.
4
In minerals,metals bined chemically with nonmetals such as oxygen,sulfur,nitrogen,and phosphorus.
Nonmetals bine with one another to pounds such as carbon dioxide,carbon monoxide,sulfur dioxide.
4
In ammonia gas,each molecule posed of one nitrogen atom and three hydrogen atoms.
On the other hand,tin has two allotropes,a diamond form more stable below 130C and a metallic form more stable at higher temperatures.
4
The pure element has a wide diversity of allotropes.
Bromine is obstained by reacting chlorine with MgBr2.
6
Beryllium,the lightest of the alkaline earth metals is widely used in alloys with copper,nickel ,and other metals.
Magnesium,with its alloys,is used widely as a strong,but very light,construction material.
Compounds of boron have have been used since ancient times in the prepatation of silicate glasses.