chem (中3)

an atom becomes most stable when its outermost electron shell is fully filled.

in an lead atom, there are 2 electrons in the outermost electron shell, it tends to lose 2 electrons so that the next electron shell (which is fully filled) become the outermost, hence most stable. by means of combining with othe anion, lead loses 2 electrons and formed lead(II) ions. Final charge of the ion is 0 - ( -2) = +2, therefore the charge is 2+.

as for bromine atom, there are 7 electrons in the outermost electron shell. it tends to gain one electron to fill the outermost shell fully, forming a bromide ion.
the charge of bromide ion is 0 + (-1) = -1, therefore there bromide ion has a negative sign.

in electrolysis of lead(II) bromide, there are Pb2+ ions and Br- ions in the solution.
Pb2+, being positive, are attracted to cathode. there are electrons in the cathode, so Pb2+ combine with the electrons to become lead element
Pb2+ (aq) + 2e- ------> Pb (s)
we observe grey solid near the negative electrode.

on the anode side, there is a lack of electrons, bromide ions being negative, are attracted to there. they lose their electrons to become bromiNe.
2Br- (aq) --------> Br2 (g) + 2e-
we observe brown colour change around the positive electrode.

you have to note the difference between element and ion.

一d metal就要loss electron去變成令到佢地既outmostshell有8粒。
其實electron本身就係負電。咁唔見咁幾粒負數，咪多左d proton既
d proton係正架嘛，咁呢果個得出黎既野就係+數喇。
Pb係 METAL，SO要去LOSS ELECTRONS。佢既outermost shell完本係2粒。
咁loss晒佢。咪2+囉。
另外Br就係non-metal,佢就要gain electrons。咁咪正數囉。

2007-01-04 20:33:41 補充：
明唔明=)?

2007-01-04 20:34:14 補充：
另外Br就係non-metal,佢就要gain electrons。咁咪負數囉先岩`