In the modern periodic table, what is the common characteristic of all elements present in the same vertical group (family)?

Introduction / Context:
The periodic table is organised in such a way that elements with similar properties fall into the same vertical columns, called groups or families. Students are often asked what basic feature all elements in a given group share, because this feature explains why their chemical behaviour is similar. Understanding this concept is central to predicting valency, typical reactions, and the formation of compounds for main group elements.

Given Data / Assumptions:

• Elements are arranged in periods (rows) and groups (columns) in the periodic table.
• Atomic number equals the number of protons in the nucleus, and also the number of electrons in a neutral atom.
• Electrons are arranged in shells and subshells around the nucleus.
• Elements in the same group show similar valency and chemical properties.

Concept / Approach:
The key to group similarity lies in valence electrons, the electrons in the outermost shell. These electrons participate in bonding and determine the typical valency and reactivity of an element. All elements of a given group have the same number of valence electrons, even though they may differ in total electrons, total protons, atomic mass, and number of shells. For example, all alkali metals in group 1 (Li, Na, K, and so on) have one electron in their outermost shell, and all halogens in group 17 have seven outer electrons. This shared valence electron count produces similar chemical behaviour within a group.

Step-by-Step Solution:
Step 1: Recall that in the periodic table, groups are vertical columns labelled 1 to 18 in the modern system. Step 2: Remember that electronic configuration of elements in a group follows the same pattern in the outermost shell, even though the inner shells differ. Step 3: Check examples: group 2 elements (Be, Mg, Ca, etc.) each have two electrons in their outermost shell. Step 4: Recognise that this shared valence electron number leads to similar valency and similar types of compounds. Step 5: Conclude that the most important common feature of elements in the same group is the same number of electrons in the outermost shell.

Verification / Alternative check:
Compare sodium (Na, atomic number 11) and potassium (K, atomic number 19). Their electronic configurations end with 3s1 and 4s1 respectively, both having one valence electron, so they behave similarly as alkali metals. Compare chlorine (Cl, atomic number 17) and bromine (Br, atomic number 35). Their configurations end with 3s2 3p5 and 4s2 4p5, both having seven valence electrons, which explains their similar halogen chemistry. These examples confirm that a common number of outer shell electrons is the key group feature.

Why Other Options Are Wrong:
The total number of electrons (and protons) increases steadily from top to bottom in a group, so it is not the same for all members. Atomic weight also increases down a group and is not constant. The number of occupied shells increases from top to bottom, so that also differs between elements in the same group. Therefore, those alternatives cannot explain the similarity in chemical properties within a group.

Common Pitfalls:
Learners sometimes assume that elements in the same group must have the same total number of electrons or similar atomic masses, which is not correct. Another confusion is between periods and groups, leading students to associate the same number of shells with the same group, whereas shell number is actually common to a period. To avoid mistakes, memorise that groups share valence electrons, while periods share the number of occupied shells.

Final Answer:
Elements in the same group have in common that they possess the same number of electrons in their outermost shell.