What are the two types of elements that make covalent bonds

What is important to study for nios grade 12 chemistry? i have only one month left.

What three things determine the shape and size of a puddle when water is poured out onto a surface

Great amounts of electromagnetic energy from our sun and other bodies in space travel through space. which is a logical conclusion about these electromagnetic waves? their energy must be very their frequency must be very low these waves can travel without a medium they only travel through a vacuum of space