A duplet is a stable outer shell of two electrons, the arrangement of helium; small atoms such as hydrogen reach stability with a duplet rather than an octet.
A duplet is a stable outer shell of two electrons, the electron arrangement of helium. Small atoms near the beginning of the periodic table reach stability with a duplet rather than an octet. Malay: duplet · Chinese: 双电子稳定.
For most atoms, stability means a full outer shell of eight electrons, the octet. But the very first shell can only hold two electrons, so an atom whose only occupied shell is this first shell is already full and stable once it holds two. Helium, with the arrangement 2, is the noble gas that this idea copies: it has a complete outer shell of just two electrons and is completely unreactive. Hydrogen, with a single electron, needs only one more to reach the same stable duplet.
Example. In a hydrogen molecule, H2, each hydrogen atom shares one electron with the other. Each atom then has two electrons around it, a duplet, so both are stable, which is why hydrogen exists as a molecule rather than as single atoms. The same happens in water and methane: each hydrogen achieves a duplet by sharing, while the central oxygen or carbon achieves an octet.
Confusion to avoid. Do not write that hydrogen “obeys the octet rule” or draw it surrounded by eight electrons. Hydrogen and helium are the exceptions: they need a duplet, not an octet. A common error in dot-and-cross diagrams is to give hydrogen too many electrons.
The duplet sits beside the octet as the two targets of stable bonding in this chapter. Knowing which atoms aim for which, a duplet for hydrogen and helium, an octet for the rest, is exactly what a clean, correct dot-and-cross diagram in SPM Chemistry requires, and it is a distinction the marking scheme checks closely.
Want a teacher to make this click?
We teach SPM Chemistry one to one, so your child understands it and scores it.
from RM50/hr · One-hour paid trial · Same-day reply