All the elements in a group in the long form of periodic table have the same Number of shellsNumber of valence electronsChemical propertiesAll of these

To determine the correct answer, let's analyze each option related to the properties of elements in a group of the periodic table.

1. Number of Shells: Elements in the same group do not necessarily have the same number of shells. For example, Group 1 elements (alkali metals) contain lithium (Li) with 2 shells and sodium (Na) with 3 shells. Thus, this statement is not true for all elements in a group.

2. Number of Valence Electrons: Elements in the same group of the periodic table do have the same number of valence electrons. For example, all elements in Group 1 have one valence electron, and those in Group 2 have two valence electrons. This characteristic is essential for determining the chemical reactivity and bonding behavior of the elements.

3. Chemical Properties: The chemical properties of elements in the same group are similar due to their identical valence electron configurations. For instance, alkali metals all react in a similar manner with water.

4. All of These: Since not all elements in a group share the same number of shells, this option can also be dismissed.

Final Answer

The correct answer is Number of valence electrons. Elements in a group of the periodic table have the same number of valence electrons, which gives them similar chemical properties.