Patterns in the Periodic Table

When one observes the periodic table, one can automatically see that there are specific patterns in which the elements have been placed. All of the elements on the left-hand side and in the middle of the periodic table (excluding hydrogen) are metallic elements, or metals.

The majority of the elements found within the periodic table are metals. Metals have numerous common characteristics such as luster and high electrical and heat conductivity. All metals, with the exception of mercury (Hg), are solids at room temperature.
The metallic elements are separated from the nonmetallic elements by the staircase-like line that starts from boron (B) and runs down to astatine (At) (click the link below to see the periodic table).

Even though hydrogen is found on the left side of the periodic table, it is a nonmetal. Nonmetals have varying physical properties at room temperature. Some of the nonmetals at room temperature are gaseous, some are liquids, and some are solids.

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There are some elements which exhibit both of the characteristics of metals and nonmetals. These elements, such as antimony (Sb), are found along the line that separates metals from nonmetals. These elements are referred to as metalloids.

Another group of nonmetals are found in group 8A of the periodic table. These elements are referred to as the inert or noble gases. As their name suggests, they are all gases at room temperature. They are all monoatomic, meaning, they are made up of single atoms rather than molecules.

The inert gases consist of helium (He), neon (Ne), argon (Ar), krypton (Kr), xenon (Xe), and radon (Rn). The high radioactivity of radon has inhibited the study of its chemistry. The reason why these elements are inert is to due to the fact that their outer electron shells are completely filled with electrons. As a result, they cannot accept more electrons and are unreactive.

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Frostburg University covers a number of general chemistry topics at General Chemistry Online!.

Chemical Reactivity in the Periodic Table

As we have already observed, the periodic properties of the elements can assist us in understanding the properties and characteristics of groups of the representative elements. We recognize the fact that elements within the same column of the periodic table have the same number of electrons in their valence orbitals (the outermost electron shell). The similarity of valence electron structure among elements leads to the similarities among elements in the same group.

There are differences among elements in the same group, however, which arise from the valence orbitals which are found in different shells.

Atoms are not hard spheres with well-structured electron shells, the way they are sometimes depicted in illustrations. That said, however, the current quantum mechanical theory of the atom proposes the notion of electron shells which are at certain distances from the nucleus. At these distances, there is a higher probability of finding electrons.

The relative size of an atom can be monitored by its atomic radius. The atomic radius is the measure of how large the atom's radius would be if it actually were a hard sphere. As we go down a column in the periodic table, the atomic radii increase. As we go from left to right in a row in the table, the atomic radii decrease.

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See a detailed discussion of trends in atomic radii at this Oklahoma State University Chemistry Department page.

The alkali metals (group 1A) are soft metals which have a low density ratio along with a low melting point. They also have the lowest ionization energy of the elements.

The ionization energy of an atom or an ion is the minimum amount of energy required to remove an electron from the lower energy state of the gaseous atom or ion. The alkali metals are very reactive towards nonmetals. During these reactions, they readily lose their outer shell electron to form 1+ ions.

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Ionization

The greater the ionization energy of an atom is, the more difficult it is to lose an electron. As we observe the elements listed on the periodic table, we can see that the ionization energies for an element increase in magnitude as successive electrons are removed. This trend comes about due to the fact that the positive nuclear charge that causes the attractive force remains unchanged, whereas the number of electrons, which provides the repulsive interactions, steadily decreases. This results in an increase of the effective nuclear charge experienced by the remaining electrons.

The alkaline earth metals (group 2A) are harder and denser than the alkali metals and have a higher melting point as well. Alkaline earth metals are extremely reactive toward nonmetals, although not as reactive as the alkali metals. The alkaline earth metals readily lose their outer shell electrons to form 2+ ions.

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Common trends

What common trends do we observe in ionization energy as we move from one element to another? In general, ionization energies decrease as we go down a column and increase as we go left to right in a row. The electron configurations help us understand these characteristics and deviations from them.

The electron affinity of an element is the energy change on that element as we add an electron to an atom. Adding an electron to an atom will form an anion, which is a negatively charged atom.

A negative electron affinity refers to the idea that the anion is stable; while a positive electron affinity means that the anion is not stable and will not form.

As we proceed from left to right in the periodic table toward the halogens, the electron affinities become more negative. Group 7A is known as the halogens. The halogens are nonmetals that exist as diatomic molecules (molecules made up of two atoms). The halogens have the most negative electron affinities of the elements since their chemistry is largely dominated by a tendency to form 1- ions, especially in reactions with metals. The electron affinities of the noble gases are all positive since the added electron would have to take up the space of a new, higher-energy subshell.

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How is chemical bonding determined on the periodic table?

Trends within the periodic table of elements can also be used to determine the number of electrons available for bonding. Atoms that have large electronegativity values, such as the halogens (i.e. chlorine and fluorine), and atoms such as oxygen attract electrons away from lower electron affinity atoms, such as the alkali metals.

The relationship of how evenly an electron pair is shared between atoms is determined through the difference in electronegativity between the two bonding atoms. Electrons will not be shared if there is a large difference in electronegativity between the atoms. This can lead to an ionic bond.

A small difference in electronegativity leads to a covalent bond with the electrons being more equally shared. Covalent bonds typically result between the reactivity of two nonmetal atoms. There is a direct relationship between electronegativity and ionization energy. Ionization energy refers to the energy required to remove an electron from an isolated gaseous atom, leaving a positively charged ion. As the electron affinity increases, so too does the ionization energy.

Click to see the Periodic Table.