What does it mean to say that the atoms in a sample of an element are
isotopes of each other?

They have the same number of protons, but a different number of neutrons.

Explanation:

All atoms of the same element have the same number of protons, however, they don't necessarily have the same number of neutrons. If atoms of the same element have different numbers of neutrons, they are isotopes of one another. An example is carbon-12 and carbon-13. Carbon-12 has 6 protons and 6 neutrons. Carbon-13 has 6 protons and 7 neutrons. They are isotopes.

alkaline metals are lithium, sodium, rubidium, potassium, cesium and francium.

alkali metals have electronegativity from 0.7 to 1, lowest in periodic table of elements, which means that alkali metals (i group in periodic table) has positive oxidation number in compounds.

electronegativity is a chemical property that describes the tendency of an atom to attract a shared pair of electrons towards itself.

alkaline metals (far left in main group) have lowest ionizations energy and easy remove valence electrons,

for example, potassium is the alkali metal and has a single valence electron in the outer electron shell.

electron configuration of potassium is: ₁₉k 1s²2s²2p⁶3s²3p⁶4s¹.

they can not exist alone in nature, because they are very reactive.

"record the time to complete a chemical reaction" would most likely be part of a lab procedure.