CU is the element symbol for Copper.
A. Air expands, becomes less dense and rises when the temperature is high due to the increased kinetic energy. This causes the air molecules to move around much more so they are further apart.
X is the answer that we do not know yet!!:)
Rows of elements are called periods. The period number<span> of an element signifies the highest unexcited energy level for an electron in that element.
</span>Columns of elements help define element groups<span>. </span>Elements within a group share<span> several common properties. Groups are elements have the same outer electron arrangement.</span>
