There are two naturally occurring isotopes of thallium: ²⁰³Tl and ²⁰⁵Tl. Use the atomic mass of thallium listed on the periodic table to identify the more prevalent isotope.

you need to take a look at the periodic system in order to sort these elements into pairs that would most likely exhibit similar chemical properties. first, you need to find elements that are found in the same group (groups are vertical in the periodic system), and then to see whether they have the same valence number. if they meet these criteria, they will have similar properties.

the first pair: ne (neon) and kr (krypton), are noble gases found in group 8

the second pair: li (lithium) and k (potassium), found in group 1

the third pair: s (sulfur) and te (tellurium), found in group 6

explanation:

0.07 g/cm³

1.37 g/cm³