The Modern Periodic Table

As you know everything on Earth is made out of specific combinations of atoms on the periodic table, you read it from left to right and top to bottom, the order of these atoms is not random, as the elements are arranged in increasing atomic number or the number of protons the element has. 

The periodic table was made in the 19th century by Dimitri Mendeleev

The periodic table has symbols each explaining what the element is, the two-letter symbol is the Nuclear or Elemental symbol like Na for sodium or Ca for calcium. And on the bottom left is the Atomic number which is how many protons the element has, and finally is the mass number which is the total amount of protons and electrons an element has. 

When Dimitri was plotting the table, he found a pattern, so instead of arranging the elements in a straight line.

He instead made verticle columns which he named periods every time the pattern repeated, each element in each period has similar properties and is called a group,  going from 1 at the leftmost, to 7, but the last group is dubbed group 0, named the noble gasses these are the most stable elements on the table. 
For example, Helium which has one shell, has two electrons and since they don't need to gain or lose electrons to be stable, so that makes them very stable.

Also, another thing to remember is that the block of metals between groups 2 and 3 doesn't have.

The reason why elements are grouped this way is because they have the same amount of elections in their outermost shell, and depending on the group and how many elections it has affects how the element reacts with other elements.

Using the group numbers you can figure out how many electrons they have in their outer shell, while every element has a different amount of electrons.
All group 1s have one electron in the outer shell, and group 2 has two in their outer shells.

Let's start with the Alkali metals, which are in group 1 since they have the same amount of electrons in their nutshells.
They share similar properties, for example, they all react violently with water.

Group 7, named Halogens, have seven electrons in their outer shells, and they get less reactive the lower you go.