Both the elements combine together due to incomplete outermost shell.
Two atoms of element Q mixed with element E in order to achieve stability because both the elements have incomplete outermost shell which make them very reactive. In element E we can see that it has 7 electrons while on the other hand, element Q has 5 number of electrons.
For gaining stability, each element has to reach 8 electrons so that's why both the elements react with each other and get stability so we can conclude that both the elements combine together due to incomplete outermost shell.
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