Atomic size and its effect
An atom does not have rigid boundaries because the density of electron clouds decreases gradually. The atomic radius is determined experimentally by the distance between nuclei in molecules or crystals.
When moving to a new period, the radius increases sharply due to the appearance of electrons on a new energy level. An increase in radius weakens the attraction between the outer electrons and the nucleus, which directly affects the properties of the elements. For instance, in alkali metals, as the radius increases from lithium to cesium, the melting point decreases predictably:
| Element | Li | Na | K | Rb | Cs |
|---|---|---|---|---|---|
| Radius, pm | 159 | 171 | 216 | 229 | 282 |
| Melting point, °C | 180 | 98 | 63 | 39 | 28 |
Ionization energy and cations
The ability of an atom to turn into a positively charged ion (cation) is characterized by the ionization energy ($E_i$). This is the minimum energy required to remove an outer electron from a gaseous atom.
- In periods, $E_i$ increases from left to right.
- In groups, $E_i$ decreases from top to bottom.
Important: as each successive electron is removed from a multivalent ion, the energy expenditure increases sharply.
Electron affinity
Some atoms are capable of capturing electrons, turning into negatively charged ions (anions). This process is accompanied by the release of energy, which is called electron affinity ($E_{ea}$). Like ionization energy, this parameter is typically calculated per 1 mole of substance and expressed in kJ/mol.
Briefly: answers to topical questions
How does electron affinity change across periods and groups?
Electron affinity is the energy released when an electron attaches to an atom in the gaseous state. In periods, this value increases up to group VII inclusive, while in groups it decreases from top to bottom. Important exceptions include elements of groups IIA (alkaline earth metals and analogs) and VIIIA (noble gases), which lack electron affinity.
What is electronegativity and what are the patterns of its variation in the periodic table?
Electronegativity is a property of atoms determined by their ionization energy and electron affinity. The variation of electronegativity correlates with the changes in ionization energy and electron affinity across periods and groups. In each period, the halogen is the most electronegative element. In group VI, electronegativity decreases from top to bottom. Fluorine (F) has the highest electronegativity value ($\chi = 4$), while francium (Fr) has the lowest. For most non-metals $\chi > 2$, and for most metals $\chi < 2$.
The radius increases "down and to the left" (like an arrow in the table), while the ionization energy increases "up and to the right" (inverse to the radius).
Class and exam questions
Why is the atomic radius of lithium significantly larger than that of helium?
A new energy level begins at lithium, leading to a sharp increase in radius compared to the previous period.
How does ionization energy change down a group?
It decreases because as the radius increases, the attraction between the outer electrons and the nucleus weakens, making them easier to remove.
What happens to energy during the formation of multivalent ions?
The ionization energy for each successive electron increases significantly compared to the previous one.
What else to study on this topic
- Specifics of 4f sublevel filling in lanthanides.
- Influence of electron configuration on the chemical similarity of elements.
- Quantum-mechanical methods for calculating atomic radii.
- Causes of instability in heavy nuclides of the 7th period.
- Differences in the properties of anions and cations.
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