Chemical element

A substance which cannot be separated into its constituent parts and still retains its chemical identity. For example, sodium (Na) is an element.

Source: US EPA Drinking Water Glossary 

For more information on the table of chemical elements see:

WebElements Periodical table of elements 

Elemental mercury

Deutsch: Chemisches Element
Español: Elemento químico
Français: Elément chimique

Boron

Arsenic

Fluoride

Climate Change

Fisheries