Section 5: Periodic Trends Chapter 9: Electrons in Atoms and the Periodic Table

Learning Goals Identify and understand periodic trends such as atomic size, ionization energy, and metallic character.

Chemical Properties The chemical properties of elements are largely determined by the number of valence electrons they contain. Their properties vary in a periodic fashion because the number of valence electrons is periodic.

Chemical Properties The noble gases are chemically stable because they have 8 valence electrons. Elements with electron configurations close to the noble gases are the most reactive because they can attain noble gas configurations by losing or gaining a small number of electrons.

Atomic Size As you move right across a period, atomic size decreases. The atomic size of an atom is determined by the distance between the outermost electrons and the nucleus. With each step across a period, the number of protons in the nucleus increases. This results in a greater pull on the electrons from the nucleus.

Atomic Size As you move down a column, atomic size increases. Since the size of an orbital increases with each energy level, the electrons that occupy the outermost orbitals are farther from the nucleus as you move down a column.

Ion Size When an atom loses an electron, it forms a cation. Cations are smaller than their respective atoms. When an electron is lost, the electron- electron repulsion is weaker and protons are better able to pull the electrons in.

Ion Size When an atom gains an electron, it forms an anion. Anions are much larger than their respective atoms. When an electron is gained, the electron-electron repulsion is stronger and pushes the electrons further apart.

Ionization Energy Ionization energy is the amount of energy needed to remove an electron from an atom. Ionization energy increases as you move to the right across a period and decreases as you move down a column in the periodic table.

Metallic Character Metals tend to lose electrons in their chemical reactions. As you move across a period, electrons are less likely to be lost in chemical reactions. Metallic character decreases as you move right, and increases as you move down a column.

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

Electronegativity Electronegativity tends to increase as you move right across a period and decreases as you move down a column.