What is a "group" on the Periodic Table? Explain both systems for numbering the groups. List the elements in each of the following groups: alkali metals, alkaline earth metals, halogens, and noble gases.

Elements are typically classified as either a metal or nonmetal, but the dividing line between the two is fuzzy. Metal elements are usually good conductors of electricity and heat. The subgroups within the metals are based on the similar characteristics and chemical properties of these collections.

The alkali metals make up most of Group 1, the table's first column.

The alkaline-earth metals make up Group 2 of the periodic table.

The third group is much too long to fit into the third column, so it is broken out and flipped sideways to become the top row of the island that floats at the bottom of the table. This is the lanthanides.

The actinides line the bottom row of the island.

Returning to the main body of the table, the remainder of Groups 3 through 12 represent the rest of the transition metals.

Ahead of the jump into the nonmetal world, shared characteristics aren't neatly divided along vertical group lines. The post-transition metals are aluminum (Al), gallium (Ga), indium (In), thallium (Tl), tin (Sn), lead (Pb) and bismuth (Bi), and they span Group 13 to Group 17. These elements have some of the classic characteristics of the transition metals, but they tend to be softer and conduct more poorly than other transition metals. Many periodic tables will feature a bolded "staircase" line below the diagonal connecting boron with astatine. The post-transition metals cluster to the lower left of this line.

The metalloids are boron (B), silicon (Si), germanium (Ge), arsenic (As), antimony (Sb), tellurium (Te) and polonium (Po). They form the staircase that represents the gradual transition from metals to nonmetals.

Everything else to the upper right of the staircase — plus hydrogen (H), stranded way back in Group 1 — is a nonmetal.

The top four elements of Group 17, from fluorine (F) through astatine (At), represent one of two subsets of the nonmetals. The halogens are quite chemically reactive and tend to pair up with alkali metals to produce various types of salt.

Colorless, odorless and almost completely nonreactive, the inert, or noble gases round out the table in Group 18.

Explanation:

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a electronegativity decreases beacuse the size of nucleus is bigger.