Electron Configuration

& Periodic Properties By: Nicole and Savannah

Atomic Radii

A way to express at atom's radius is to measure the distance between the nuclei of two identical atoms that are chemically bonded together, then divide this distance by two.

atomic radius may be defined as one-half the distance between the nuclei of identical atoms that are bonded together.



Period Trends

The trend to smaller atoms across a period is caused by the increasing positive charge of the nucleus. A electrons add to s and p sublevels in the same main energy level, they are gradually pulled closer to the more highly charged nucleus. The increased pull decreases the atomic radii.


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Group Trends

As electrons occupy sublevels in successively higher main energy levels located farther from the nucleus, the sizes fo the atoms increase. In general, the atomic radii of the main-group elements increase down a group.


Ionization Energy

A + energy --> A+ + E-


A+ represents an ion of element. A with a single positive charge, referred to as a 1+ ion.

An ion is an atom or group of bonded atoms that has a positive or negative charge. Any process that results in the formation of an ion is referred to as ionization.

The energy required to remove one electron from a neutral atom of an element is the ionization energy, IE (or first ionization energy, IE1)

In general, first ionization energies increase across a period and decrease down a group.

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Period Trends

In general, ionization energies of the main-group elements increase across each period.


Group 1 elements have the lowest first ionization energies in their respective periods. Therefore, they lose electrons most easily. This increases their reactivity because the electrons are easier to loose. The low reactivity of the noble gases is partly based on this difficulty of electron removal.

Group Trends

Among the main-group elements, ionization energies generally decrease down the groups. Electrons removed from atoms of each succeeding element in a group are in higher energy levels, farther from the nucleus. Therefore, they are removed more easily.


Removing Electrons from Positive Ions

The energies for removal of additional electrons from an atom are referred to as the second ionization energy (IE2), third ionization energy (IE3), and so on.


Each successive electron removed from an ion feels an increasingly stronger effective nuclear charge (the nuclear charge minus the electron shielding).

Electron Affinity

Neutral atoms can also acquire electrons. The energy change that occurs when an electron is acquired by a neutral atom is called the atom's electron affinity.


A + e- ----> A- + energy

A + e- + energy -----> A-


Period Trends

In general, as electrons add to the same p sublevel of atoms with increasing nuclear charge, electron affinities become more negative across each period within the p block.


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Group Trends

As a general rule, electrons add with greater difficulty down a group. This pattern is a result of two competing factors. The first is a slight increase in effective nuclear charge down a group, which increases electron affinities. The second is an increase in atomic radius down a group, which decreases electron affinities. In general, the size effect predominates.


Adding Electrons to Negative Ions

Second electron affinities are all positive. Certain p-block nonmetals tend to form negative ions that have noble gas configurations.



Ionic Radii

A positive ion is known as a cation. The formation of a cation by the loss of one or more electrons always leads to a decrease in atomic radius because the removal of the highest-energy-level electrons results in a smaller electron cloud. Also, the remaining electrons are drawn closer to the nucleus by its unbalanced positive charge.


A negative ion is known as a anion. The formation of an anion by the addition of one or more electrons always leads to an increase in atomic radius. This is because the total positive charge of the nucleus remains unchanged when an electron is added to an atom or an ion. So the electrons are not drawn to the nucleus as strongly as they were before te addition of the extra electron. The electron cloud also spreads out because of greater repulsion between the increased number of electrons.

Period Trends

Cationic radii decrease across the period because the electron cloud shrinks due to the increasing nuclear charge acting on the electrons in the same main energy level. Anionic radii decrease across each period for the elements in Groups 15-18. The reasons for this trend are the same as the reasons that cationic radii decrease from left to right across the period.



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Group Trends

As they are in atoms, the outer electrons in both cations and anions are in higher energy levels as one reads down a group. Therefore, just as there is a gradual increase of atomic radii down a group, there is also a gradual increase of ionic radii.


Valence Electrons

Chemical compounds form because electrons are lost, gained, or shared between atoms.


The electrons available to be lost, gained, or shared in the formation of chemical compounds are referred to as valence electrons. Valence electrons are often located in incompletely filled main-energy levels.

Electronegativity

Electronegativity is a measure of the ability of the atom in a chemical compound to attract electrons from another atom in the compound.


Period Trends

Electronegativities tend to increase across each period, although there are exceptions. Electronegativities tend to either decrease down a group or remain about the same.


Periodic Properties of the d- and f-Block Elements

Recall that atoms of the d-block elements contain from zero to two electrons in the s orbital of their highest occupied energy level and one to ten electrons in the d sublevel of the next-lower energy level. Therefore, electrons in both the ns sublevel and the (n-1)d sublevel are available to interact with their surroundings. As a result, electrons in the incompletely filled d sublevels are responsible for many characteristic properties of the d-block elements.


Atomic Radii

The atomic radii of the d-block elements generally decrease across the periods. However, this decrease is less than that for the main-group elements because the electrons added to the n-1d sublevel shield the outer electrons from the nucleus.


Ionization Energy

Ionization energies of the d and f black elements generally increase across the periods. In contrast to the decrease down the main groups, however, the first ionization energies of the d block elements generally increase down each group. This is because the electrons available for ionization in the outer s sublevels are less shielded from the increasing nuclear charge by electrons in the incomplete n-1d sublevels.


Ion Formation and Ionic Radii

Among all atoms of the d and f block elements, electrons in the highest occupied sublevel are always removed first. For the d block elements, this means that although newly added electrons occupy the d sublevels, the first electrons to be removed are those in the outermost s sublevels.



Electronegativity

The d block elements all the electronegativities between 1.1 and 2.54. The f block elements all have similar electronegativites, which range from 1.1 to 1.5.