1. Inspite of the fact that the third energy level in an atom contains a d- -orbital. why does the "d-- -block" start in the fourth row of the periodic table? 2. For the elements of the s---block, how is the row number related to the principal energy level or principal quantum number (n) in their electron configuration?
3. Explain why the "f-- -block" does not appear until the 6^th row, though the fourth energy level in an atom contains a "f---orbital"?
4. For the elements of the f-- -block, how is the row number related to the principal energy level or principal quantum number (n) in the electron configuration?

explanation:

the largest database of organic compounds lists about 10 million substances, which include compounds originating from living organisms and those synthesized by chemists. the number of potential organic compounds has been estimated at 1060—an astronomically high number. the existence of so many organic molecules is a consequence of the ability of carbon atoms to form up to four strong bonds to other carbon atoms, resulting in chains and rings of many different sizes, shapes, and complexities. the simplest organic compounds contain only the elements carbon and hydrogen, and are called hydrocarbons. even though they are composed of only two types of atoms, there is a wide variety of hydrocarbons because they may consist of varying lengths of chains, branched chains, and rings of carbon atoms, or combinations of these structures. in addition, hydrocarbons may differ in the types of carbon-carbon bonds present in their molecules. many hydrocarbons are found in plants, animals, and their fossils; other hydrocarbons have been prepared in the laboratory. we use hydrocarbons every day, mainly as fuels, such as natural gas, acetylene, propane, butane, and the principal components of gasoline, diesel fuel, and heating oil. the familiar plastics polyethylene, polypropylene, and polystyrene are also hydrocarbons. we can distinguish several types of hydrocarbons by differences in the bonding between carbon atoms. this leads to differences in geometries and in the hybridization of the carbon orbitals.

elements make up everything, and they combine to make compounds.

the answer is c