covalent bonds同ionic bonds有咩分別

covalent bond = attraction between nuclei and electron
ionic bond = attraction between cation(positively charged) and anion(negatively charged)

To determine when to use covalent and when to use ionc, you need to cosider the electronegativity between the atom

Generally speak, If the difference is smaller than 2, the atoms are covalently bonded (further divided to polar and non-polar)
while if the difference is larger than 2, the atoms are ionically bonded

But for your level, you may understand
ionic bond as metal ion/ammonium ion(NH4+)/hydronium ion(H3O+) bonded with non-metal ion; and
covalent bond as the bond formed between two or more non-metal atoms.


2011-06-23 18:38:53 補充：
for the drawings,

ionic bond, for example MgCl2
the diagram will be like
[Mg]^2+ 2[Cl]-
(note that you need to draw the valence electrons of the anion)

2011-06-23 18:40:55 補充：
covalent bond for example, O2

You need to draw the O atoms together, that is their outermost electron shell overlap each other

then, draw the bonded electrons and unbonded electrons in the outermost shell of each atom

2011-06-24 13:11:58 補充：
To Knowledge enhance Life:

As I remember, when drawing the diagram of ionic compound, it is not necessary to draw the valence electron of the cation.

2011-06-28 14:39:32 補充：
Oh I see, thanks a lot!!!

Ionic bond
－is formed by electron transfer電子轉移 from a metallic atom
　to a non-metallic atom.
－the electrostatic attractive force between the positively
　charged cation and the negatively charged anion
－Non-directional because the attraction is in all directions
－Example: NaCl

Covalent bond
－formed by electron sharing電子分享 between two non-metallic atoms.
－The shared electrons are located in between the nuclei
　of the bonding atoms.
－Directional because the attraction only occurs on the direction joining
the 2 atoms
－Example: Cl2

How to draw?
I have made two examples for you.

Na(2,8,1) transfer one electron from its outermost shell to the outermost shell of Cl (2,8,7)
Thus, they obtain the octet structure, Na become positively charged
and Cl become negatively charged.

圖片參考：http://imgcld.yimg.com/8/n/HA00524824/o/701106220074413873462470.jpg

By the sharing of electron, both Cl(2,7) can obtain the Octet structure.

Remarks:
in drawing covalent bond,
You are required to show the electron of outermost shell .
Dot and crisscross must be used together in order to distinguish electrons between two atoms.

2011-06-28 00:03:34 補充：
I make an amendment here!!!
If no any special requirement of the question, it is not necessary to draw the valence electron of the cation.

Thank you!!

Please notice that:
Covalent bond can be applied to both metals and non-metals
(e.g. polyatomic ions)

一個化合物中
由金屬及非金屬組成 如 NaCl （氯化鈉）
就系ionic

covalent就系由非金屬組成 如 CO2（二氧化碳）
咁你明啦- -
畫圖的話呢
書上就有ga啦。