Mendeleev classified elements inSix periods and seven groupsEight periods and eight groupsSeven periods and eight groupsEight periods and seven groups

The question pertains to the classification of elements by Mendeleev, which is a fundamental aspect of the periodic table. Mendeleev organized elements based on atomic mass and properties, leading to his formulation of periodic law.

Mendeleev classified elements in seven periods and eight groups. This classification allowed him to predict the properties of undiscovered elements, demonstrating the periodicity observed in the atomic characteristics of elements.

1. Periods: Mendeleev's table consisted of seven horizontal rows, known as periods. The periods represent the energy levels of electrons in an atom.

2. Groups: He organized elements into vertical columns, known as groups. The elements in each group often exhibit similar chemical behaviors due to having similar valence electron configurations.

Thus, the correct answer to the classification of elements according to Mendeleev's system is "Seven periods and eight groups". This framework laid the foundation for the modern periodic table, which has been further refined with discoveries of new elements and the development of atomic theory.