Concept:Elements in a period are arranged horizontally, and their properties change progressively across the period.
Explanation:Moving from left to right across a period, the atomic number increases by one for each successive element.
However, the number of electron shells used by the elements remains the same.
For example, elements in period 3 all have three electron shells.
Therefore, the common feature of elements in the same period is the same number of occupied shells.
Chemical and physical properties are similar for elements in the same group, not the same period.
The atomic number differs for every element in a period.
Answer:A. number of shells