I assume that you have read and understand chapter 2.
Life involves primarily organic molecules that are based on carbon.
That means there are two electrons in the 1s orbital, and four in the second set of orbitals. However, there is room for eight electrons in the second level, so carbon forms four stable covalent bonds by sharing electrons with other atoms. The most simple of these organic molecules is methane , where four hydrogens are covalently bonded to carbon. Methane burns with oxygen releasing carbon dioxide and water, and we commonly use it to warm our homes as natural gas. Two carbons like this would be propane.