Module 3.03 Periodic Trends

Periods – rows of the periodic table To Review: Periods – rows of the periodic table Groups – columns of the period table

Periodic Table

Trends are useful in making predictions Periodic table is arranged based on the properties of the elements Reading across each period and down each group you will see repeated trends in properties

Effective Nuclear Charge Effective Nuclear Charge – the charge (from the nucleus) felt by the valence electrons after the number of shielding electrons are taken into account. Subtract the number of shielding (core) electrons from the total nuclear charge (number of protons) gives the effective nuclear charge.

Effective Nuclear Charge Example: Nitrogen 7 protons (atomic number of 7) -2 core electrons (1s2 electrons) -------- 5 is the effective nuclear charge Each of the valence electrons in nitrogen feels an effective nuclear charge of about +5.

Effective Nuclear Charge Across a Period: The effective nuclear charge felt by an atom’s valence electrons increases by one for each element from left to right in a period Down a Group: The effective nuclear charge felt by an atom’s valence electrons stays constant for each element going down a group

Atomic Radius Atomic Radius – half the distance between the centers of two atoms that are bonded together.

Atomic Radius Across a Period: There is a gradual decrease in atomic radii from left to right across a period. Down a Group: There is a general increase in atomic radii going down each group on the periodic table.

Atomic Radius

Ionization Energy Ionization energy – energy required to remove on electron from an element, resulting in a positive ion. Elements with a lower effective nuclear charge felt by their electrons will give up an electron easier than other elements.

Ionization Energy Across a Period: Ionization energy has a general increase for elements across a period from left to right. Down a Group: Ionization energy of elements decreases going down a group because the atomic radius of the atoms increases.

Ionic radius – ½ the diameter of an ion. Note: Metals naturally form cations (positive ions) by losing one or more electrons. Nonmetals naturally form anions (negative ions) by gaining one or more electrons.

Atomic Radius vs Ionic Radius

Ionic Radius Across a Period: Within each period, the metals at the left form cations and the nonmetals at the right form anions. There is a decrease in the ionic radii of the cations from left to right and a decrease in the ionic radii of the anions from left to right. Down a Group: Ionic radii increase down a group, following the same trend as atomic radii.

Electronegativity Electronegativity – measure of the attraction of an atom for the electrons in a chemical bond.

Electronegativity Across a Period: Electronegativity increases from left to right across a period because of the effective nuclear charge. Down a Group: Electronegativity decreases down a group as a result of a greater atomic radius.

Module 3.03 Periodic Trends