Matter and measurements
Matter is everything. Matter constructs every object in the known universe, from individual atoms to the largest galaxies. Matter can be defined as anything that takes up space and has mass. Mass is very similar to a property of matter called Inertia (resistance to a change in motion), but for our purposes, it is a measurement of matter.
The vast majority of known matter comes in the form of atoms. Atoms are thus called the building block of the universe. An atom is the smallest unit of an element that still maintains the properties of that element. An element is pure substance made of only one type of atom. For example, the air around you is largely nitrogen gas. Nitrogen is an atom, and no two nitrogen atoms are distinguishable from one another if they are the same isotope. They have identical properties. They are thus the same element.
Avogadro's Number: 6.022x1023. This constitutes the amount in one mole of something. Eg. 12 eggs in one dozen. One mole of Magnesium (Mg) contains 6.022x1023 atoms.