Look at the two question marks between zinc (Zn) and arsenic (As). At the time, no elements were known with atomic weights between 65.2 and 75. But Mendeleev predicted that two elements must exist with
atomic weights in this range. What led Mendeleev to predict that two undiscovered elements existed in
that range?

Agroup of students is studying convection current. they fill two identical balloons with the same amount of helium. one balloon is placed in a freezer and the other is in an area with warm air. after 10 minutes, the balloon are released from a height of 1 meter. which of the following to the students most likely observe? a) the warm balloon expands and rises. the cold balloon shrinks and sinks b) the balloon both rise. the cold balloon is larger than the warm balloon c) the cold balloon expands and rises. the warm balloon shrinks and sinks d) the balloon rise at the same rate. both balloons are the same size