Please help giving out brainlist!

"The higher the atomic number of group 1a elements, the lower the melting point"

This is what can be concluded from this table, but a more conceptual explanation would be that since we're going down the group of Alkali metals, the atomic radii increases, which makes the bond between the nuclei and the outermost shell weaker, thus less energy is required to break them, so their melting points will decrease.

c-14 is carbon-14, or radiocarbon.   it is a radioactive isotope of carbon with an atomic nucleus containing 6 protons and 8 neutrons.