Trends in the Periodic Table
So far in this web site, we have discussed or mentioned several quantitative measurements of some properties of the elements, including atomic radius, electronegativity, electron affinity, and ionization energy. When one looks at a Periodic Table with these values overlaid, certain trends become apparent. It is important to be familiar with these trends, because being able to estimate a value when one is not readily available is a useful skill.
The first trend in the Table that we'll discuss is atomic radius, which dealt with in the "States of Matter" chapter regarding crystal structures and density. As an atom's atomic number increases, more protons are added to the nucleus, while more electrons are stacked in the shells and subshells around the atom. These extra electrons naturally take up space, which implies that the atomic radius of atoms will increase as one moves down the Table. Going across the Table, electrons are being added to existing subshells (s, p, and sometimes d and f), which takes up little extra space, but the increasing positive charge of the nucleus exerts a greater inward pull on the electrons. Hence, atomic radius decreases as one moves from left to right along a given period. Note that the atomic radius first shrinks quickly as one goes right, but slows greatly (for example, the atomic radius of potassium is 227 picometers, while that of gallium two elements to the right is 135, and krypton, at the far end of the period, measures only 112 picometers). To summarize, atomic radius increases as one goes down and left along the Periodic Table, and decreases as one goes up and right.
Electronegativity is measured using a negative scale, which means that as the number gets smaller (more negative), the specified atom's desire for electrons increases. Along a period, electronegativity becomes more negative, culminating with the halogens and trailing off to zero at the noble gases. Because the positive charge of the nucleus is spread over a much larger space in bigger atoms, electronegativity increases (becomes less negative, signifying less attraction) moving down the Table.
Electron affinity is closely tied to electronegativity, but its measurements are not as uniform. Its units are also calibrated in a negative scale, and the energy released as an atom gains an electron increases to the top and right of the Periodic Table (excepting the noble gases). Conversely, atoms at the left and bottom of the Table release little energy when receiving electrons.
Since ionization energy is the amount of energy required to remove an electron, and electron affinity is the amount of energy released when an electron is gained, it should not be surprising that these two values have similar trends. Atoms become increasingly harder to ionize as one moves upward and to the right, whereas the leftmost and downward atoms (the alkali metals and alkaline earth metals, especially) are relatively easy to strip of an electron.
Remembering these trends shouldn't be too difficult if you keep some key facts in mind: the alkali metals (especially those near the bottom) are large and like to be ionized, whereas the halogens (especially near the top) are small and like to gain electrons. These trends are gradual, so any atom near the center (both horizontally and vertically) is likely to have "average" values for any trend-dependent property.
These trends also allow you to predict the properties of ions, to some extent. Negative ions have gained electrons, so they will be significantly larger than the normal atom, mostly because the positive nucleus is now overpowered by the negative electrons, weakening its hold on then, and because electron-electron repulsion has increased. Negative ions, due to their extra electrons, have much larger (less negative) electronegativities and lower ionization energies. Positive ions are much smaller than their original atoms, for the same reasons described above, but with opposite effects. Positive ions also harder to ionize and often have smaller (more negative) electronegativities than their "parent" atoms.