S3.1 The periodic table: Classification of elements

The table shows atomic radius decreasing from Li (152 pm) to C (77 pm) as we move left to right across Period 2.

Across a period, the number of protons increases (from $Z=3$ for Li to $Z=6$ for C), but the electrons are added to the same principal energy level ($n=2$), providing almost no additional shielding.

Because nuclear charge increases while shielding stays approximately constant, the effective nuclear charge ($Z_{\text{eff}}$) felt by valence electrons increases. This stronger pull draws the electron cloud closer to the nucleus, reducing atomic radius.

The statement that nuclear charge increases while shielding remains approximately constant directly explains the trend using $Z_{\text{eff}}$.

We need the factor that causes atomic radius to decrease across Period 2.

All Period 2 elements have their outermost electrons in $n=2$; no new shell is added across the period. This option is incorrect.

Across a period, electrons are added to the same shell, so inner-shell shielding does not increase significantly. This option is incorrect.

While there is a shift from 2s to 2p, this does not explain a consistent decrease in radius across the period and is not the primary explanation.

The remaining option — nuclear charge increases while shielding remains approximately constant — correctly explains the decrease in atomic radius via increasing $Z_{\text{eff}}$.

According to the IB definition, a transition metal must form at least one stable ion with a partially filled d-subshell (i.e., between $d^1$ and $d^9$).

This configuration contains $3d^7$. When this element forms ions (e.g., $2+$ ion: remove 4s electrons first → $[\text{Ar}]3d^7$), the d-subshell remains partially filled. This satisfies the IB definition.

Removing the 4s electrons gives $[\text{Ar}]3d^{10}$, which is a completely filled d-subshell. This element (zinc) does not qualify as a transition metal under the IB definition.

$[\text{Ar}]4s^{2}3d^7$ corresponds to cobalt (Co), which forms ${\text{Co}}^{2+}$ with $3d^7$ — a partially filled d-subshell. This is the correct answer.

We need a configuration whose element forms at least one ion with a partially filled d-subshell.

This configuration has no d-electrons at all — it belongs to an s- or p-block element (argon). Eliminated.

This is a p-block element (gallium). It has a full $3d^{10}$ subshell that remains full upon ionisation. Eliminated.

This is zinc; its ion ${\text{Zn}}^{2+}$ has $3d^{10}$ — a completely filled d-subshell. Under the IB definition, zinc is not a transition metal. Eliminated.

$[\text{Ar}]4s^{2}3d^7$ is cobalt; ${\text{Co}}^{2+}$ has $3d^7$, which is partially filled. This is the correct answer.