Periodic Trends

Outline

• Periodic Table Position and Electron Configuration
  • Atomic Properties
  • Ionization Energy and Electron Affinity
• Practice with Concepts
• Discussion Questions
• Optional Website Reading

Periodic Table Position and Electron Configuration

Last time, we looked at how the periodic table can give us clues to the electron configuration of different elements in their ground state. Now we look at other "trends" in the table for atomic size, ionization energy, electron affinity, ion sizes, and chemical reaction properties.

Atomic properties

Atomic properties include size and mass. We've discussed mass at length by doing molar mass conversions. We know that mass increases as the atomic number increases, and depends primarily on the number of protons and neutrons in the atoms. So mass should increase across periodic table rows from left to right and down periodic table columns; experimental evidence supports this prediction.

Atomic size does not follow such simple rules. Sizes can be estimated by finding the distance between centers of atoms in pure substances of the element (using electron microscopes to examine the surface of the sample). For example, in diamond, the carbon nuclei are 154 picometers apart, so the diameter of each carbon atom is estimated to be half that, or 77 pm. Notice that we assume no vacuum between the atoms.

Compare the size of neutral atoms (red) and ions (green) in this periodic table arrangement of radius information. How does the radius of the atom when potassium K loses an electron? When carbon loses an electron? Is there a trend in the radius change?

These are estimates--the electron shells have no solid surfaces. If you want to compare them to something, they are more like the gas planets Jupiter and Saturn, where gas density gradually increases as we move to the center of the planet, but where there is no planetary surface as there is on earth or Venus.

In general, atomic radii decrease from left to right across the periodic table. This may seem counter intuitive--after all, each new element to the right has more electrons, which should take up more room. But we have to remember that as we move from left to right, the electrons are added to the same energy level or to lower energy levels (as the d and f shells fill up), closer to the nucleus. The increasing postive charge in the nucleus results in an increasing effective nuclear charge Z which holds the electrons closer and closer.

Moving down the table is a different story. As we move from sodium (Na) to potassium (K) for example, we are moving from a 3s to a 4s orbital. The increase in energy level betokens an increase in distance from the nucleus--so potassium atoms are bigger than sodium atoms.

Ionization Energy and Electron Affinity

Ionization energy is the energy required to remove an electron from the atom in the gas phase (in order to make comparisons, we need to use the same physical conditions). Electron affinity is the ability to attach an electron. As with ionization energy, we actually measure electron affinity as the energy required to remove the extra electron.

Again, these are derived from experiment but trends can be predicted by looking at the periodic table. As we move down a single column on the table, the electron configuration of the outer or valence electrons remains the same, so the electron affinity and ionization energies are nearly level for lithium and sodium, for berillium and magnesium, and so on. The electron affinity for an element with a full s shell is low, since the new electron would need to be in the higher-energy p orbital. Adding an electron to an element with 3 electrons in p orbitals means the new electron must form a pair in one of the subshells —a high-energy proposition. So the graph of electron affinity against atomic number is non-linear, rising as we move left across a row but dropping as orbitals fill, or pairs must be formed.

A study of atomic characteristics shows us that certain ionic compounds are not possible if the energy required to switch the electron from one atom and add it to the other is greater than that returned by the formation of the compound. We don't see NaCl₂ molecules, for instance, because removing the second electron in Na⁺ to make room for the second Cl atom requires more energy than is released in the rearrangement of the electrons to form the molecule. Any situation that moves toward the formation of NaCl2 will "correct" itself by losing the electron back to the Na⁺, and jettisoning the second Cl to achieve a lower energy state.

For the same reason, compounds of noble gases are also very rare. Electron configurations for the noble gases are already complete and stable, which means the atoms share electrons very reluctantly, and losing them to form ionic bonds is almost unheard of. Because of its high electron affinity, fluorine is able to form covalent bonds with xenon. XeF₆ is a viable molecule under certain circumstances, forming an association similar to hydration molecules.

Practice with the Concepts

Atomic mass trends

Do you expect oxygen to be more or less massive than selenium?

Atomic radii trends

Rank in order of radius, smallest to largest: potassium, kryptonium, lead.

Electron affinity/ionization energy trends

Which has the higher electron affinity, oxygen or selenium?

Discussion Questions

• Why is it important to look for patterns of behavior in natural objects?
• What underlying factors account for deviations from a simple linear increase or decrease in electron affinity as we move across a row or down a column in the periodic table?

Optional Readings

Study these different versions of the periodic table which include electron configuration information.

• The Electron Configrations in the Periodi Table at Chemwiki emphasizes the levels (quantum number n) and shapes (quantum number l) of ranges of electron configurations.
• So does this periodic table of electron configurations, which includes the lactantium and actinium series in their right place.
• This periodic table shows the pattern without the actual elements labeled -- which is sometimes a useful way to get the configuration pattern straight.
• Here is the detailed table of electron configurations using noble gas notation.

The point here is to find a representation that makes sense to you. The periodic table was laid out based on chemical reaction patterns. The electron configuration patterns tell us why the chemical reaction patterns occur.

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