Q.The elements $Z = 117$ and $120$ have not yet been discovered. In which family/group would you place these elements and also give the electronic configuration in each case.

Concept: Element Nomenclature and Periodic Placement

To locate undiscovered elements, we extend the periodic table using the Aufbau principle and recognize that period 7 ends at $Z = 118$ (oganesson, a noble gas). The long form of the Periodic Table (Fig. 3.2) shows each group's own recurring outer-electron pattern, which is what lets us assign a not-yet-discovered element to a family just from its electronic configuration.

Step 1: Element $Z = 117$ (tennessine, now discovered) lies in period 7, group 17 (halogens). Its configuration follows radon ($Z = 86$):

$[\text{Rn}] \, 5f^{14} \, 6d^{10} \, 7s^2 \, 7p^5$ …

Why Group Placement Follows Electronic Configuration

The periodic table is organized so that elements in the same group share the same valence electron configuration—the arrangement of electrons in their outermost shell. This similarity drives their chemical behavior. To predict where an undiscovered element belongs, we build its electronic configuration by filling orbitals in order of increasing energy, then examine which subshell receives the last electron.

The group number for main-group elements directly reflects the number of valence electrons. For elements beyond the 7th period, we continue the same filling sequence that governs the entire table. Cross-checking against the long form of the Periodic Table (Fig. 3.2) confirms this: reading down any group column shows every member sharing the same outer $ns^x np^y$ pattern, which is exactly how Fig. 3.2 lets us extend the table's own group structure to elements not yet discovered.

Element 117: The Missing Halogen

1. Identify the preceding noble gas

Element 118 (Oganesson, Og) is the noble gas that closes the 7th period. Its configuration ends at $7p^6$, giving a complete octet in the outermost shell. Element 117 sits one electron short of this.

2. Write the configuration for $Z = 117$

Starting from Oganesson and removing one electron (or building up to 117 from Radon), we get:

Breaking this down:

• $[Rn]$ accounts for the first 86 electrons
• The 5f subshell fills across the actinides: 14 electrons
• The 6d subshell completes: 10 electrons
• The 7s orbital: 2 electrons
• The 7p subshell holds: 5 electrons (one short of the full 6)

Running total: $86 + 14 + 10 + 2 + 5 = 117$ ✓

3. Determine the group

The outermost configuration is $7s^2\,7p^5$, giving 7 valence electrons. This matches the halogen family pattern ($ns^2\,np^5$).

Group assignment: Group 17 (Halogens)

Element 120: Extending the Alkaline Earths

1. Build beyond the 7th period

Element 120 lies two electrons past Oganesson. The next available orbital after $7p^6$ is $8s$, which can hold 2 electrons.

2. Write the configuration for $Z = 120$

$[Og]\,8s^2 \quad \text{or fully} \quad [Rn]\,5f^{14}\,6d^{10}\,7s^2\,7p^6\,8s^2$ …