When Mendeleev created the periodic table he arranged the elements he knew of into groups based on their properties and similarities. He then put the groups next to each other and he found that as he went across each of the groups he worked out the mass of them increased each time. The periodic table today is arranged with two different parts, the groups and the periods.
The groups are the vertical columns. Groups are given a number to show where they are in the periodic table and also to identify the group of elements in them. Mendeleev put elements with similar properties and that react in similar ways into the same groups. For example group 1 contains, lithium, sodium and potassium and when they react with water they all react in the same way. You can see as we go down the group the mass of the atoms also increases and they get bigger!
The periods of the table are the rows that go from left to right. As you move from left to right there are no similarities in these elements unlike in the groups but the number of protons and neutrons increases as the masses of the elements get larger. Mendeleev also arranged his periodic table using this idea, as you go across the atoms get larger and as you go down they get larger.
The periodic table is the collection of elements arranged by atomic number
Dmitri Mendeleev was a Russian scientist who in 1869 proposed the first structure of the periodic table
Groups are the columns of the periodic table with elements that are grouped together with similar properties
Periods are the rows of the periodic table that from left to right increase in order of mass
Group 1 elements are in the first group of the periodic table with similar properties of being soft and reacting violently with water
Mass is the amount of substance that can be measured in grams or kilograms