KEY2CHEM

Predicting Periodic Trends based on Periodic Table Position

The structure of the periodic table is a consequence of the pattern of electron configurations and the presence of shells (and subshells) of electrons in atoms. This organization results in periodic trends, such as atomic size, first ionization energy, electronegativity, and typical ion charges. These trends help to better understand the properties of atoms, ions, and molecules. Note: these trends will be discussed for main group elements; d-block elements do not necessarily follow these trends.

Atomic size

Across a period, atomic size decreases.

Down a group, atomic size increases.

First ionization energy

The first ionization energy describes the process of losing an electron from a neutral atom (for example, $$\require{mhchem}\ce{X(g) -> X+(g) + e-}$$). The trend opposes atomic size.

Electronegativity

Electronegativity is a bonded atom’s desire to pull electron density toward itself. Note that electronegativity is a property of an atom in a bond while atomic size and first ionization energy describe an atom by itself. The trend for electronegativity mirrors first ionization energy.

Typical ion charges

Atoms generally gain or lose enough electrons to attain a full subshell (sometimes called a noble gas configuration). For example, alkali metals (Group 1A) such as $$\require{mhchem}\ce{Na}$$ will lose one electron to attain a full subshell and form a $$1+$$ion. Halogens (Group 7A) will gain one electron to attain a full subshell and form a $$1-$$ ion. The other groups in the periodic table follow suit. There are sometimes multiple ions formed in the $$\require{mhchem}\ce{N}$$ and $$\require{mhchem}\ce{C}$$ groups. This will be discussed more fully in the context of covalent bonding.

Example 1.

Place the following atoms in order of increasing atomic size: $$\require{mhchem}\ce{K}, \;\ce{Rb}, \;\ce{O}, \;\ce{Se}$$.

A. $$\require{mhchem}\ce{K}< \ce{Rb} <\ce{O} <\ce{Se}$$

B. $$\require{mhchem}\ce{O}< \ce{Se} <\ce{K} <\ce{Rb}$$

C. $$\require{mhchem}\ce{Rb}< \ce{K} <\ce{Se} <\ce{O}$$

Solution

B. $$\require{mhchem}\ce{O}< \ce{Se} <\ce{K} <\ce{Rb}$$

Based on their positions on the periodic table, oxygen has the smallest atomic size and rubidium has the largest atomic size.

Example 2.

Place the following atoms in order of decreasing first ionization energy: $$\require{mhchem}\ce{Na}, \;\ce{Li}, \;\ce{C}, \;\ce{N}$$.

A. $$\require{mhchem}\ce{Na}> \ce{Li} >\ce{C} >\ce{N}$$

B. $$\require{mhchem}\ce{N}> \ce{C} >\ce{Li} >\ce{Na}$$

C. $$\require{mhchem}\ce{Li}> \ce{C} >\ce{N} >\ce{Na}$$

Solution

B. $$\require{mhchem}\ce{N}> \ce{C} >\ce{Li} >\ce{Na}$$

Based on their positions on the periodic table, a nitrogen atom has the largest first ionization energy and sodium atom has the smallest first ionization energy.

Example 3.

Which  elements is most electronegative?: C, O, Si, S?

A. $$\require{mhchem}\ce{C}$$

B. $$\require{mhchem}\ce{O}$$

C. $$\require{mhchem}\ce{S}$$

Solution

B. $$\require{mhchem}\ce{O}$$

Oxygen is the element closest to the upper right hand corner of the periodic table; therefore, it is the most electronegative.