The atomic mass of an element listed in the Periodic Table represents the average mass of a single atom of that element and is measured in atomic mass units (amu). This number is an average as some elements have isotopes with atoms that vary in their number of neturons and, therefore, differ in weight.

Animal cells are surrounded by a semipermeable membrane which allows for the transfer of water and oxygen to and from the cell. In plant cells, the cell membrane is surrounded by a somewhat rigid cell wall which provides the cell structure and support.

The cerebellum is a large cluster of nerves at the base of the brain that's responsible for balance, movement, and muscle coordination.

An element is matter than cannot be separated into different types of matter by ordinary chemical methods.

The rows of the Periodic Table are called periods and contain elements that have the same number of electron shells ordered from lower to higher atomic number.