In an atom, the first energy level can hold electrons. the second energy level can hold electrons. the third energy level can hold electrons.

2, 8, 18

Explanation:

The number of orbitals for any level of interest n is calculated by a formula . Since each orbital contains a maximum of 2 electrons, we can calculate the total number of electrons in an nth orbital by .

1 a

2 d

3 a

4 c

explanation:

chemical changes occur when a substance combines with another to form a new substance, called chemical synthesis or, alternatively, chemical decomposition into two or more different substances. these processes are called chemical reactions and, in general, are not reversible except by further chemical reactions. some reactions produce heat and are called exothermic reactions and others may require heat to enable the reaction to occur, which are called endothermic reactions. understanding chemical changes is a major part of the science of chemistry.