Given his arrangement of the periodic table, Mendeleev was able to predict three elements that had not yet been discovered. He predicted their atomic weights and chemical properties. How was this possible?

Look in explanation.

Explanation:

Mendeleev has ordered the elements based on increasing atomic mass, he has skipped some elements but assumed that they existed and managed to find the atomic mass of the "unknown" elements.

away from the curb

explanation:

the wheels would then hit the curb if it started to rolling, decreasing the chance of an accident.

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