The rows of the periodic table are called periods.



Periods:
- In the periodic table, the horizontal rows are known as periods.
- There are a total of 7 periods in the modern periodic table, numbered from 1 to 7.
- Each period represents a different energy level or electron shell that the elements in that period possess.
- As we move from left to right across a period, the atomic number of elements increases, indicating the addition of one more proton to the nucleus.
- Elements in the same period do not have similar chemical properties, but they do have similar electronic configurations due to being in the same energy level.

Group:
- Groups, also known as families, are the vertical columns in the periodic table.
- There are 18 groups in the modern periodic table, numbered from 1 to 18.
- Elements within the same group tend to have similar chemical properties because they have the same number of valence electrons.
- Valence electrons are the electrons in the outermost energy level of an atom, and they play a significant role in determining the chemical behavior of an element.
- For example, elements in Group 1 (alkali metals) all have one valence electron, which makes them highly reactive and prone to losing that electron to achieve a stable electron configuration.

Differences:
- Periods are horizontal rows, while groups are vertical columns in the periodic table.
- Elements in the same period do not have similar chemical properties but have similar electronic configurations.
- Elements in the same group have similar chemical properties due to the same number of valence electrons.

Therefore, the correct answer is option B) periods.