Jawapan

2016-04-17T15:05:15+08:00
Group 18: noble gases, mono-atoms,chemically unreactive ,very stable (duplet electron arrangement or octet electron arrangement) Helium: noble gas, mono-atoms ,chemically unreactive,very stable,duplet electron arrangement,has 2valance electron
2016-04-17T15:10:11+08:00
Kerana Helium mempunyai 2 elektron valens dan telah mencapai susunan elektron duplet yang stabil.