CHAPTER 9: ELECTRONS IN
ATOMS AND THE PERIODIC TABLE

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

• 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.

• 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.

• 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.

• 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.

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

• 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 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.

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

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