Which is true of a period in the periodic table?
A.
Atomic numbers increase from left to right across each period.
B.
Elements in each period have the same number of valence electrons.
C.
Each period contains elements with the same number of protons.
D.
Elements in each period have similar properties.
To determine which statement is true of a period in the periodic table, we need to understand the periodic table's structure and properties of elements within it.
The periodic table is organized into periods, which are horizontal rows. Each period corresponds to a principal energy level or shell, and as we move from left to right across each period, the atomic number of the elements increases. Therefore, statement A is correct. Atomic numbers represent the number of protons in an atom's nucleus.
Statement B is incorrect. Elements in each period may have different numbers of valence electrons, which are the electrons in the outermost energy level of an atom. Valence electrons determine the chemical properties of an element.
Statement C is also incorrect. While all elements in a period share the same principal energy level, their atomic numbers (number of protons) vary. Therefore, each period contains elements with a different number of protons.
Statement D is not universally true. While elements within the same group (vertical columns) often exhibit similar properties, elements within a period may have diverse properties. The similarity of properties depends more on the elements' group membership than their period.
In conclusion, the correct statement regarding a period in the periodic table is that atomic numbers increase from left to right across each period (Statement A).