What are the main features of the modern periodic table?

Groups and Periods:

The modern periodic table is arranged in rows and columns. The rows are called periods and the columns are called groups.

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  Periods: The periods (rows) of the periodic table contain elements with the same number of electron shells. A number is usually written to the left of each period. This number is called the period number and is an indication of the number of electron shells in each period's elements.

Sections of periods six and seven are often pulled out of the main periodic table and placed at the bottom. Elements in these sections are collectively called the Inner Transition Elements. The section of elements that are pulled from period six are classified as the Lanthanide Series elements. The section of elements that are pulled from period 7 are classified as the Actinide Series elements.

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  Groups: The groups (columns) of the periodic table contain elements with similiar physical and chemical properties. The groups of the periodic table are often labeled with a Roman numeral followed by the letter "A" or the letter "B". The elements in the groups that are labeled with the letter "A", are called the Main Group elements. The elements in the groups that are labeled with the letter "B, are called the Transition Elements.

Four groups have been given special family names: Group IA elements are called the Alkali Metals. Group IIA elements are called Alkaline Earth Metals. Group VIIA elements are called the Halogens. Group VIIIA elements are called the Noble Gases.

Element Squares:

Each row and column is composed of small squares. Each square contains information about a different element. The information contained in each square includes:

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  Element Symbol: The element symbol for each element is composed of one or two letters. When the element symbol is composed of one letter, the letter is capitalized. When the element symbol is composed of two letters, the first letter is capitalized and the second letter is written in lower case. 

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  Element Name: The name of each element is usually written in each box.

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  Atomic Number: Each element has a different atomic number. The atomic number is equal to the number of protons in each element. If an atom is neutral (not charged), the atomic number is also equal to the number of electrons in each element. The elements on the periodic table are arranged by increasing atomic number.

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  Atomic Mass: The atomic mass is the average weighted mass of all of the isotopes of an element. Isotopes are different forms of an element which contain different numbers of neutrons. The term "weighted mass" indicates that the mass has been calculated using both the mass of each isotope and its percent abundance. The atomic mass can be used to approximate the sum of the protons and neutrons in a particular type of atom.

Metals, Metalloids, and Nonmetals:

The periodic table can also be used to determine if an element is a metal, metalloid, or nonmetal. Most periodic tables show a zigzag line on the right side of the periodic table. Elements that are located along the zigzag line are usually classified as metalloids. Metalloids have properties of both metals and nonmetals. The elements located to the right of the metalloids are classified as nonmetals. The elements located to the left of the metalloids (excluding hydrogen) are classified as metals. Hydrogen is classified as a nonmetal despite being located with the metals.