Why are metals called electropositive elements whereas nonmetals are called electronegative elements?

Hint:Elements which have tendency to lose or donate electrons are termed as electropositive elements whereas the elements that have a tendency to accept or gain electrons are known as electronegative elements.

Complete answer:
As we know, on moving left to right in a period, the atomic number increases i.e., the number of electrons in the valence shell increases. Due to the increase in the number of electrons, the force of attraction between nucleus and electrons becomes stronger which leads to an increase in effective nuclear charge. Therefore, we can say that effective nuclear charge increases on moving left to right in a period.
When there is an increase in effective nuclear charge, the electrons present in the valence shell are more strongly attracted towards the nucleus, so the atom will require more energy to release electrons. Hence, the ionization energy increases on moving left to right in a period.
We also know that the metallic character decreases along the period. So, metals have relatively less ionization energy as compared to nonmetals and therefore can easily donate electrons to form cations. Hence metals tend to show electropositive character.
On the other hand, the non metallic character increases along the period. So, nonmetals have relatively high ionization energy as compared to metals and therefore cannot easily donate electrons to form cations. But due to increase in electronegativity, nonmetals can easily accept electrons to form anions. Hence nonmetals tend to show electronegative character.

Note:
It is important to know that in a period, the number of valence shells for all elements is the same i.e., valence electrons will be filled in the same shell in a period.