2 Periodic Trends Free Online FlashCards

What does effective nuclear charge represent?

Effective nuclear charge is the net nuclear attraction an electron experiences after shielding; it is approximated by $Z_{\mathrm{eff}} \approx Z - S$, where $Z$ is proton number and $S$ is shielding.

Why does effective nuclear charge generally rise across a period?

Across a period, proton number increases while shielding usually does not increase enough to cancel the added nuclear charge, so effective nuclear charge generally rises.

How does atomic radius generally change across a period and down a group?

Atomic radius generally decreases across a period as effective nuclear charge pulls electrons closer, and increases down a group as valence electrons occupy more distant shells.

What is a covalent radius?

A covalent radius is half the distance between the nuclei of two identical atoms joined by a covalent bond.

Why is a cation usually smaller than its parent atom?

A cation is usually smaller than its parent atom because removing an electron reduces electron–electron repulsion; losing an outer shell can make it much smaller.

Why is an anion usually larger than its parent atom?

An anion is usually larger than its parent atom because the added electron increases repulsion among the electrons.

How does radius change across an isoelectronic series as proton number rises?

In an isoelectronic series, radius decreases as proton number increases. For example, $\mathrm{O^{2-} > F^- > Na^+ > Mg^{2+}}$; each species has 10 electrons.

What is first ionization energy?

First ionization energy is the energy required to remove an electron from a ground-state gaseous atom: $\mathrm{X(g) \rightarrow X^+(g) + e^-}$. The removal always requires energy.

How does first ionization energy generally vary across periods and down groups?

First ionization energy generally increases across a period and decreases down a group. Subshell energies and electron pairing cause exceptions to the across-period trend.

What does a large jump between successive ionization energies indicate?

Successive ionization energies increase. A particularly large jump occurs after all valence electrons have been removed, because the next electron comes from a more tightly bound inner shell.

What does electron affinity describe, and what does a negative energy change mean?

Electron affinity concerns the energy change when a gaseous atom gains an electron: $\mathrm{X(g) + e^- \rightarrow X^-(g)}$. With the energy-change convention, a negative value means energy is released.

What broad periodic patterns describe electron affinity?

Electron addition generally becomes more favorable across a period, but there are important exceptions, including groups 2, 15, and 18. The pattern down a group is irregular.