Elements, Isotopes and Relative Atomic Mass

The amount of protons tells us how heavy and what number it is assigned to in the periodic table, so Hydrogen is the lightest and has a single proton and electron.

Next on the line is Helium because it has 2 protons and 2 electrons. As you look at any periodic table, there are over 100 different elements some that act similar to each other but they are different. Copper and Tin are different elements, but they are both metals. 

The number at the bottom of most elements is the Atomic symbol, which is unique to every element, as it is the number of protons in the element, and elements cannot overlap in atomic numbers. 

If we take another element...Carbon its atomic number is 6, so every carbon atom should have 6 protons, if we take an atom with 3 protons it cannot be carbon but instead, it is Lithium.
Also the alphabetic letter on the element says its name so Li is Lithium, C for Carbon, but not all elements follow this naming scheme, a good example is Fe or Iron and Na which is Sodium.

Well if the number of protons determines the element, what about Neutrons, well neutrons vary between elements and even in the same element we call these kinds of elements Isotopes. The definition of an isotope is that
"They are different forms of each element that have the same number of protons but not neutrons."

If we take carbon, the most common version of carbon is Carbon 12 which has 6 protons, electrons and neutrons, while another form is Carbon 13, which has the normal 6 protons but has 7 neutrons and 6 electrons.

Something to keep in mind is that all isotopes alike elements have different masses, let's take copper for example.
Copper has two stable isotopes, copper 63 taking around 69.2% of all copper elements, and copper 65 has an abundance of 30.8%, when we are talking about the abundance of an isotope it means how common these isotopes are.
In a quiz or exam, you might see a question like: "Calculate the relative atomic mass to 1 decimal place".
For this equation, you will do "(the sum of all isotope abundance * mass) divided by the sum of abundance of all isotopes".
So lets take copper-63 which is 69.2% * 63 (copper 63), but for copper-65 it would be 30.8% * 65. Then we add both equasions (copper-63 + copper-65) then we add the isotope abundance of 69.2 + 30.8.

So the 69.2 * 63 is equal to 4359.6 and the 30.8 * 65 is equal to 2002, then you add them together to get 6361.69,  then we divide them by the 69.2 + 30.8 which is equal to 100. so we divide 63.6169 / 100. And we get 63.6169.
But the question wants it to be "Calculate the relative atomic mass to 1 decimal place", we will instead put 63.6 instead.