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So, the electrons in the outermost orbit are very important in deciding how elements interact with one another. The electrons in the outermost shell are called valence electrons.

The valency of an element is the number of electrons that the element's atom either gives up or gets so that it can have $$8$$ electrons in its outermost orbit.

For example, the valency of Oxygen is $$2$$, as it receives $$2$$ electrons to have $$8$$ electrons in its outermost orbit. Similarly, the valency of Potassium is $$1$$ as it gives up $$1$$ electron to have $$8$$ electrons in its outermost orbit.