I first encountered the periodic table when I was a 7th grader. I was taught that the periodic table is a systematic arrangement of elements that exist (or discovered and recorded) on planet Earth.
There are generally 3 kinds of information found in a cell for any element:
1) Elemental symbol (e.g. H for hydrogen, Au for gold)
2) Atomic number (the whole number found above the elemental symbol)
3) Mass number (the number found below the elemental symbol)
A look at any periodic table, it would not be difficult to notice that the table is arranged from left to right in increasing atomic number, i.e. the number of protons in the atom, which is found above the notation for each element.
The periodic table is used in several aspects:
The periodic table is obviously an important tool in the study of Chemistry. I was taught by my 9th grade chemistry tutor to memorise the atomic and mass numbers of the first 20 elements of the periodic table. It was certainly useful, as these 20 elements are the more commonly-encountered elements in the study of Chemistry, particularly in organic chemistry.
The table also provides a systematic approach in introducing chemical elements to students, it also standardized the notations used in chemistry, which is certainly very helpful because it aids in avoiding confusions and misunderstandings when communicating with people with chemical backgrounds.
Very often, the periodic table is used for calculations in chemistry. Calculations of the concentration of solutions refer to the periodic table frequently. For example, the mass numbers for sodium and chlorine are 23 and 35.5 respectively. So that would make a molecule of sodium chloride (commonly known as table salt) has a total mass number of 58.5 (total molecular mass number is termed as molecular mass or weight).
It is also known that the number of atoms in 23 grams of sodium and number of molecules in 58.5 grams of sodium chloride are the same. To ease calculations in concentrations, the mass (in grams) which corresponds to the elemental mass number gives a mole of the atoms or molecules.
If we were to dissolve 58.5 grams of table salt (assuming it consists of only sodium chloride molecules) in 1 liter of water, it would give us a salt solution of 1 mole per liter concentration.
Elements are arranged in rows of increasing atomic numbers running from left to right and in columns of elements with similar properties, with the row of elements on the far left as alkaline metals and the one on the far right as noble gases.
The elements in the same column have similar properties because they have the same number of valence electrons. They are expected to go through the same reaction with different intensities: for example, the reaction of the halogens (group VII elements) with hydrogen is similar, and products are similar. The halogens are usually denoted by X and they often exist as diatomic molecules, like F2, Cl2, Br2.
They react with hydrogen in the same manner: X2 + H2 = 2HX, where X = F, Cl, Br.
The reactivity differs down the column, a very popular example is the youTube video on the reactivity of the alkaline metals lithium (Li), sodium (Na) and potassium (K) with water, the reaction becomes more vigorous as the element used go down the column. So one may predict the reaction between rubidium (Rb) and water would be even more vigorous compared to the reaction between water and potassium.
This is very helpful to predict behaviors of elements to avoid any accidents from dangerous experiments with inadequate protection.
The above are the 3 general uses of the periodic table. There are also different versions of the periodic tables that may give more information, such as radioactivity and safety numbers or code that are useful for chemists to understand the hazards involved in handling certain elemental chemicals. There are definitely more uses of the period table than I have described, but that may be referring to more professional periodic tables and may not be applicable to all.