How many grams of water can be from 8.000 grams of hydrogen gas?

Q!)
An element is matter that is composed of one type of atom.
Atoms make up elements. If its made of one type of atom, its called an element.
eg: Na is an element and made up of Na atoms only. If its a compound its made of different types of atoms, e.g.: H₂O is a compound made of H atoms and O atoms.
 
Q2) the unit of measurement used for atomic particles is the atomic mass unit.
this is the unit used to measure size of an atom. One atomic mass unit is roughly equivalent to the mass of a nucleon, nucleons could be either protons or neutrons as they both have the same mass. Atomic mass unit can also be defined as 1/12th the mass of a 12-C atom.

Q3)
 atoms of the same element that have different numbers of neutrons are called isotopes.
isotopes are when when atoms have the same number of protons and electrons but different numbers of neutrons. Since these atoms have the same number of protons they are of the same element, but neutron number changes therefore atomic mass changes too.

Q4) in the periodic table, elements are arranged by increasing atomic number.
the atomic number is characteristic of the element. atomic number is the number of protons the atoms has. While the number of electrons and neutrons may vary within atoms of the same element, the protons usually dont change. Therefore its characteristic for the element. Periodic table, elements are arranged in order of increasing atomic number, by doing so elements with similar characteristics fall into the same column/ groups.