The chemical element with the symbol 'Na' is sodium. Sodium is an alkali metal and is highly reactive, especially in water, where it forms sodium hydroxide and hydrogen gas. It plays an essential role in biology, particularly in cellular function, nerve transmission, and fluid regulation within the human body. Sodium naturally occurs in various compounds, most notably in table salt (sodium chloride), which is crucial for both human and animal life. Due to its reactivity, pure sodium is usually stored under oil to prevent it from reacting with moisture in the air.