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Dot and cross diagrams

The sharing of electrons in covalent bonds can be illustrated using dot and cross diagrams.

The dots and crosses in the diagrams indicate valence electrons. Electrons represented by dots originate from different elements from those represented by crosses.

In reality, electrons are indistinguishable. The dots and crosses are for illustration purposes only.

It is not possible to tell which electron comes from hydrogen or carbon in the $$\ce{C-H}$$ bond of methane.

An atom usually shares as many valence electrons as needed to achieve a stable electron configuration.

Carbon bonds with four hydrogen atoms, allowing it to fill its valence shell. Each hydrogen also gains a full shell (2 electrons) by sharing electrons from carbon.
Carbon bonds with four hydrogen atoms, allowing it to fill its valence shell. Each hydrogen also gains a full shell (2 electrons) by sharing electrons from carbon.