A group in the periodic table has the same A. number of protons.
B. number of neutrons.
C. number of outer shell electrons.
D. shell number.

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C. Number of outer shell electrons.

Explanation:

Groups consist of the vertical columns of the periodic table, such as 1A, 2A, etc. (Not including transition metal groups) which depict the number of valence electrons, or the number of electrons in the atom's outer shell.

For example, Oxygen, which is in group 6A, contains 6 valence (outer shell) electrons.

the mantle

explanation:

earth is divided into three layers: the crust or the outermost layer on which we live; the underlying, middle layer - the mantle - and the core (the innermost layer of earth).

it's c. elements are placed in rows based on how may valencies electrons have.