Periodic Trends

Properties of the elements exhibit trends in the periodic table. These properties include: atomic radius (or size), ionization energy, electron affinity, electronegativity, metallic and nonmetallic property.

ATOMIC SIZE

Atomic size refers to how big or how small an atom is. The size of an atom is determined by the distance of the valence electron (the outermost electron) from the nucleus of the atom. The shorter this distance is, the smaller is the atomic size. This distance is further dependent on other factors such as

a. the number of energy levels in an atom (the more is the number of energy levels, the bigger is the atom);

b. shielding effect (refers to the number of inner electrons that "shields" the attraction between the nucleus and the valence electrons-- the higher the shielding effect, the less is the attraction, therefore the bigger will be the size of the atom);

c. effective nuclear charge (the number of protons found inside the nucleus of the atom). The higher is the nuclear charge, the greater is the ability of the nucleus to pull electrons towards itself thus, decreasing the size of the atom.

The following "periodic table" shows a graphical representation of the sizes of the atoms down a group and across a period.

The absence of representations for those of the metals indicates that there is difficulty in establishing a clear trend for atomic size of these transition elements. Atomic size of the noble gases, Group 8A, are estimates from those of nearby elements.

IONIZATION ENERGY

When an atom loses an electron, it becomes a positive ion (cation). The removal of an electron from an atom requires an absorption of ionization energy. Ionization energy is the energy required to "knock-off" the most loosely held electron (the valence electron) from an atom.

The more tightly held is the electron to the nucleus (atomic size is small), the higher is the energy required to remove it. Thus a smaller atom will require higher ionization energy than a bigger one.

The following graph shows the ionization energies of the first 20 elements.

A more comprehensive survey of the ionization energies of the main group elements is shown below.

ELECTRON AFFINITY

Electron affinity is the energy released when an electron is added to a neutral atom to form a negative ion. If the electron affinity is low, the electron is weakly bound; if the electron affinity is high, the electron is strongly bonded. Generally, electron affinity increases from left to right across the period because of increase in nuclear charge and decrease in atomic size. This causes the incoming electron to experience a greater pull of the nucleus thus giving a higher electron affinity.

Electron affinity decreases down a group because of increasing number of energy levels (and atomic size). This causes the incoming electron not to experience much attraction to the nucleus thus giving a lower electron affinity.

The electron affinities of completely filled atoms (Group 8A) is almost zero. An atom does not accept an electron in its outermost energy level if it already has a stable configuration.

ELECTRONEGATIVITY

When two elements are joined in a chemical bond, the element that attracts the shared electrons more strongly is more electronegative (usually nonmetallic elements). Elements with low electronegativities (the metallic elements) are said to be electropositive. It is important to note that electronegativities are properties of atoms that are chemically bound to each other; there is no way of measuring the electronegativity of an isolated atom.

Electronegativity values increases across a period in the periodic table. This is because elements are becoming more nonmetallic (atomic size is decreasing) from left to right. Electronegativity decreases down a group because of increasing atomic size. Elements found below are more metallic thus they are more electropositive than those found above.

METALLIC and NONMETALLIC PROPERTIES

Metals are very good electron donors. In terms of atomic size, the bigger an atom is, the more metallic it is. Thus, across a period in the periodic table metallic property decreases. Down a group, where atomic size increases, metallic property also increases.

Nonmetals, on the other hand, are very good electron acceptor. The smaller is the size of the atom the easier for it to attract (or receive) an electron from an external source. Therefore across a period in the periodic table, nonmetallic property increases. Down a group, nonmetallic property decreases.