Periodic Trends

Trends in Atomic Size

• Atomic Size (Atomic Radius): The atomic size of an atom, also called the atomic radius, refers to the distance between an atom's nucleus and its valence electrons. Remember, the closer an electron is to the nucleus, the lower its energy and the more tightly it is held.

Trends in Atomic Size

Group Trends Moving Down a Group • The atomic radius increases moving down

a group. Once again protons are added moving down a group, but so are new energy shells of electrons. The new energy shells provide shielding, allowing the valence electrons to experience only a minimal amount of the protons' positive charge.

Trends in Atomic Size

Moving Across a Period • Moving from left to right across a period, the

atomic radius decreases. The nucleus of the atom gains protons moving from left to right, increasing the positive charge of the nucleus and increasing the attractive force of the nucleus upon the electrons. True, electrons are also added as the elements move from left to right across a period, but these electrons reside in the same energy shell and do not offer increased shielding.

Trends in Atomic Size

Trends in Atomic SizeIonic Radius• A cation is positively charged, meaning

that it is an atom that has lost an electron or electrons. The positive charge of the nucleus is thus distributed over a smaller number of electrons and electron-electron repulsion is decreased, meaning that the electrons are held more tightly and the atomic radius is smaller than in the normal neutral atom.

Trends in Atomic Size

Ionic Radius• Anions, conversely, are negatively

charged ions: atoms that have gained electrons. In anions, electron-electron repulsion increases and the positive charge of the nucleus is distributed over a large number of electrons. Anions have a greater atomic radius than the neutral atom from which they derive.

Ionization Energy and Electron Affinity

• The process of gaining or losing an electron requires energy. There are two common ways to measure this energy change: ionization energy and electron affinity.

Ionization Energy

• The ionization energy is the energy it takes to fully remove an electron from the atom.

• When several electrons are removed from an atom, the energy that it takes to remove the first electron is called the first ionization energy, the energy it takes to remove the second electron is the second ionization energy, and so on.

Ionization Energy• In general, the second ionization energy is greater

than first ionization energy. This is because the first electron removed feels the effect of shielding by the second electron and is therefore less strongly attracted to the nucleus.

• If a particular ionization energy follows a previous electron loss that emptied a subshell, the next ionization energy will take a rather large leap, rather than follow its normal gently increasing trend.

• This fact helps to show that just as electrons are more stable when they have a full valence shell, they are also relatively more stable when they at least have a full subshell.

Ionization Energy• First ionization energy• Na --> Na+ + e-• The equation for the second ionization energy is: • Na+ --> Na2+ + e-

Ionization Energy Across a Period

• Ionization energy predictably increases moving across the periodic table from left to right. Just as in the case of atomic size, moving from left to right, the number of protons increases. The electrons also increase in number, but without adding new shells or shielding.

• From left to right, the electrons therefore become more tightly held meaning it takes more energy to pry them loose.

Ionization Energy Across a Period• This fact gives a physical basis to the octet rule,

which states that elements with few valence electrons (those on the left of the periodic table) readily give those electrons up in order to attain a full octet within their inner shells, while those with many valence electrons tend to gain electrons.

• The electrons on the left tend to lose electrons since their ionization energy is so low (it takes such little energy to remove an electron) while those on the right tend to gain electrons since their nucleus has a powerful positive force and their ionization energy is high.

Ionization Energy Down a Group

• Ionization energy decreases moving down a group for the same reason atomic size increases: electrons add new shells creating extra shielding that supersedes the addition of protons. The atomic radius increases, as does the energy of the valence electrons. This means it takes less energy to remove an electron, which is what ionization energy measures.

Ionization Energy

Electron Affinity

• An atom's electron affinity is the energy change in an atom when that atom gains an electron.

Electron Affinities Across a Period • Electron affinities becoming increasingly negative

from left to right. Just as in ionization energy, this trend conforms to and helps explain the octet rule.

• The octet rule states that atoms with close to full valence shells will tend to gain electrons. Such atoms are located on the right of the periodic table and have very negative electron affinities, meaning they give off a great deal of energy upon gaining an electron and become more stable.

• Noble gases have full valence shells, are very stable, and do not want to add more electrons: noble gas electron affinities are positive.

• Similarly, atoms with full subshells also have more positive electron affinities (are less attractive of electrons) than the elements around them.

Electron Affinities Down a Group • Electron affinities change little moving down a

group, though they do generally become slightly more positive (less attractive toward electrons).

Electronegativity

• Electronegativity refers to the ability of an atom to attract the electrons of another atom to it when those two atoms are associated through a bond.

• Electronegativity is based on an atom's ionization energy and electron affinity. For that reason, electronegativity follows similar trends as its two constituents measure.

Electronegativity

• Electronegativity generally increases moving across a period and decreases moving down a group.

• Flourine (F), in group VII and period 2, is the most powerfully electronegative of the elements.

• Electronegativity plays a very large role in the processes of Chemical Bonding.

Periodic Trends