The periodic table of elements is a visual depiction of all the known chemical elements. Each element is listed in order according to its atomic number, which is a measure of the number of protons found in the nucleus of an atom of that element.
The rows and columns of the periodic table, called periods and groups, are organized in a way that helps scientists understand and find patterns in the similarities and differences between the chemical properties of the elements.
Out of the 118 known elements in the periodic table, only 94 have been found naturally on Earth. The other 24—including the four newly-discovered elements nihonium, moscovium, tennessine, and oganesson—were created in labs by bombarding atoms with additional protons!
The only letter that doesn't appear on the periodic table is the letter 'J.' This is likely because most of the elements' names are based off of words in old Latin or Greek, both languages whose alphabets lack the letter.