Jawapan

2014-07-09T18:46:53+08:00
Covalent bonds are formed when one or more electron pairs are shared between non-metallic atoms to form a molecule in order to achieve a stable duplet or octet electron arrangements of noble gas.

How covalent bond is formed in carbon dioxide:Atom oxygen has 6 valence electrons and 2.6 electron arrangement.Each atom oxygen needs 2 more electrons to fill the valence shell in order to achieve a stable octet electron arrangement. Hence, each atom oxygen contributes 2 electrons for sharing.Atom carbon has 4 valence electrons and 2.4 electron arrangement.Each atom carbon needs 4 more electrons to fill the valence shell in order to achieve a stable octet electron arrangement. Hence, each atom carbon contributes 4 electrons for sharing.One atom carbon shares 4 electrons with two atom oxygen to achieve a stable octet electron arrangement. 
2014-07-10T14:16:16+08:00
There's 2 types of bonds, Ionic Bond & Covalent Bonds! :)

Ionic bonds are usually between metals and non-metals, like the compound NaCl. Both atoms will give/take electrons from each other to achieve the stable arrangement. The metal will usually give electrons and the non-metal will take electrons.

Covalent bonds are different, and it's usually between non-metals, like oxygen compound (O2). They won't take or give elctrons, but they will share valence electrons to achieve stable octet arrangement! Sometimes they can share up to 2 or 3 pairs of electrons and have more bonds too.