Evidence for Periodic Trends

The periodic properties are based on the repeating valence electron count of the atoms. Elements in the same group have the same number of valence electrons. For example, both \(\require{mhchem}\ce{Li}\) (electron configuration \(1s^2 2s^1\)) and \(\require{mhchem}\ce{Na}\) (electron configuration \(1s^2 2s^2 2p^6 3s^1\)) have \(1\) valence electron. This yields regular repeating properties in the periodic table.

Atomic size

Across a period, atomic size decreases. This is because the number of protons increases, causing an increase in net nuclear charge. The effect is that the electrons are pulled in more tightly.  Down a group, atomic size increases. Adding electrons into additional outer shells increases the size of the atom.

First ionization energy

The first ionization energy describes the process of losing an electron from a neutral atom (for example, \(\require{mhchem}\ce{Na(g) -> Na+(g) + e-}\)). The trend opposes atomic size, since it requires more energy to remove an electron that is closer to the nucleus.


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, as atoms that are more difficult to ionize prefer to pull electrons toward themselves in bonds.

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.

Which atom is most likely to form an anion (negatively charged ion)?

A. \(\require{mhchem}\ce{Na}\)

B. \(\require{mhchem}\ce{Al}\)

C. \(\require{mhchem}\ce{S}\)



C. \(\require{mhchem}\ce{S}\)

\(\require{mhchem}\ce{S}\) is a nonmetal with electron configuration \(1s^2 2s^2 2p^6 3s^2 3p^4\). It will gain two electrons to become the anion \(\require{mhchem}\ce{S^{2-}}\) (full subshell). This is because it has a relatively high ionization energy and is less likely to become a cation (positively charged ion). The other atoms have lower ionization energies and are more likely to lose electrons to become cations.

Example 2.

Which is most likely to form a cation with a \(2+\) charge?

A. \(\require{mhchem}\ce{K}\)

B. \(\require{mhchem}\ce{Sr}\)

C. \(\require{mhchem}\ce{Se}\)



B. \(\require{mhchem}\ce{Sr}\)

\(\require{mhchem}\ce{Sr}\) has two valence electrons (\(1s^2 2s^2 2p^6 3s^2 3p^6 4s^2 3d^{10} 4p^6 5s^2\)) and is more likely to lose both valence electrons due to the lower energy required. Removing a second electron from potassium requires removal of a core electron. Selenium is a nonmetal with a higher ionization energy, meaning that removing electrons requires input of more energy. 

Example 3.

Which is expected to have the largest radius: Cl or Cl-?

A. \(\require{mhchem}\ce{Cl-}\)

B. \(\require{mhchem}\ce{Cl}\)

C. \(\require{mhchem}\ce{Cl+}\)



A. \(\require{mhchem}\ce{Cl-}\)

\(\require{mhchem}\ce{Cl-}\) will have the largest radius. Anions are larger than their parent ions due to the addition of one or more electrons. The electrons decrease the net nuclear charge each electron experiences. This increases the size of the anion relative to the size of the neutral atom. Cations are smaller than their parent atoms due to a greater net nuclear charge that pulls each electron in more closely.