The student is expected to use the Periodic Table to identify and explain periodic trends, including atomic and ionic radii, electronegativity, and ionization energy.
Periodic trends are patterns that occur across a row (from left to right) or down a column of the Periodic Table and can be used to predict certain properties of elements in their atomic or ionic form.
The atomic radii of atoms will decrease when moving from left to right on the Periodic Table due to the increasing number of positive protons within the nuclei pulling on the valence electrons. The atomic radii of atoms will increase moving down a Group in the Periodic Table due to additional electron shells between the nucleus and repulsion among electrons.
Ionic radii can be compared based on their numbers of protons and electrons. A greater number of protons will cause an ion to become smaller due to attractive forces between the nucleus and the valence electrons, while a greater number of electrons will cause an ion to become larger due to electron repulsion. Moving down a Group, atoms will have an increase in ionic radius for both positive and negative ions due to the addition of an electron shell.
The electronegativity of an element reflects its ability to attract electrons to itself in a chemical bond. Electronegativity increases moving from left to right across a Period and moving up a Group, making fluorine the most electronegative element.
Ionization energy is the amount of energy required to remove an electron from a neutral atom. Ionization energy generally increases as you move from left to right across a Period and as you move up a Group. The ionization energy of elements within the same chemical family decreases with increasing atomic number.
Predict Properties with Trends
Chemical elements in their atomic or ionic form have periodic trends that are characteristics and patterns that occur across a row (period) or down a column (group) of the Periodic Table. These trends can be used to predict certain properties of elements in their atomic or ionic form. These characteristic properties of elemental families can be attributed to the valence electrons in the outermost electron shell of each element. Having the same number of valence electrons means having similar chemical properties, as the atoms of each element gain or lose electrons to achieve a stable octet of electrons. The exceptions are hydrogen and helium, which will achieve a duet. All periodic trends can be explained by three simple concepts: attraction of the nucleus for the valence electrons (opposite charges attract), the repulsion of the electrons from each other (like charges repel), and the concept of energy levels within the atom (valence electrons become further from the nucleus as energy levels are added).
Atomic Radii
The radii of atoms follow a very specific trend.
For instance, the atomic radii of atoms will decrease when moving from left to right on the Periodic Table. This change is due to the increasing number of positive protons within the nuclei pulling on the valence electrons. Also, in the horizontal direction, the atomic radii of atoms will increase moving down a group in the Periodic Table due to additional electron shells. The increase in electrons means more repulsion of electrons from each other. As an example, the alkali metals from lithium to cesium are in the same group, and hence the corresponding radii of the atoms increase as one moves down the group.
A shielding effect occurs when the inner electrons shield the outer valence electrons from the positive pull of the nucleus.
Ionic Radii
Cations: Ionic radii follow another predictable trend. The ionic radii of ions are based on the numbers of protons and electrons found within that ion. A neutral atom will have the same number of protons and electrons. If electrons are removed from a neutral atom, there will be a greater number of protons than electrons. The resulting ion will therefore have a positive charge, known as a cation. This will cause the ion to become smaller due to attractive forces between the nucleus and the valence electrons.
Anions: If electrons are added to a neutral atom, there will be a greater number of electrons than protons. Therefore, the resulting ion will have a negative charge, known as an anion. This result will cause the ion to become larger due to electron repulsion.
In review, the size of cations will decrease from left to right across a period, as will the size of anions. Ionic radii will increase moving down a group. In other words, the ionic radius of a cation is smaller than its corresponding atomic radius. The ionic radius of an anion is larger than its corresponding atomic radius.
Electronegativity
The electronegativity of an element reflects its ability to attract electrons to itself in a chemical bond. Electronegativity increases when moving from left to right across a period and moving up a group. In following this trend, this makes fluorine the most electronegative element. The noble gases are not included in the electronegativity values that are often given because the noble gases are considered to have very low chemical reactivity and therefore do not form many compounds.
Ionization Energy
Another important periodic trend is ionization energy, which is the amount of energy required to remove an electron from a neutral atom. The ionization energy of elements within the same chemical family decreases with increasing atomic number. Ionization energy generally increases as one moves from left to right across a period and as one moves up a group. Having a good understanding of the periodic trends can help one understand the properties of the chemical elements.