日本白嫩丰满老师xxxxhd,国产成人综合伊人av,欧美日韩在线视频一区二区

periodic table of elements

A periodic table is a tabular display of the chemical elements. The standard form of table comprises an 18 × 7 grid or main body of elements. The table is deconstructed into four rectangular blocks: the s-block to the left, the p-block to the right, the d-block in the middle, and the f-block below that. The rows of the table are called periods; the columns of the s-, d-, and p-blocks are called groups.