The smallest unit of an element is a blank

The correct answer would be an atom.

If a hydrogen atom is neutral, it must also have  one electron. an oxygen atom (o) has an atomic number of eight (8). this means a neutral oxygen atom has eight protons and eight electrons. the element actium (ac) has an atomic number of 89, so it has 89 protons and 89 electrons in a neutral atom

Adialysis unit requires 78.5 l of water.the estimated digit is 5.