6.1 Organizing the Elements

• Chemists used the properties of elements to sort them into groups.
• Mendeleev arranged the elements in his periodic table in order of increasing atomic mass.
• In the modern periodic table, elements are arranged in order of increasing atomic number. The elements within a group in the table have similar properties.
• Three classes of elements are metals, nonmetals, and metalloids.

6.2 Classifying the Elements

• The periodic table displays the symbols and names of elements, along with information on the structure of their atoms.
• Elements can be sorted into noble gases, representative elements, transition metals, or inner transition metals based on their electron configurations.
• The periodic table can be divided into s, p, d, and f blocks that correspond to the highest occupied sublevels in atoms of elements.

6.3 Periodic Trends

• In general, atomic size increases from top to bottom within a group and decreases from left to right across a period.
• Positive and negative ions form when electrons are transferred between atoms.
• First ionization energy tends to decrease from top to bottom within a group and increase from left to right across a period.
• Cations are always smaller than the atoms from which they form. Anions are always larger than the atoms from which they form.
• In general, electronegativity values decrease from top to bottom within a group. For representative elements, the values tend to increase from left to right across a period.
• Trends in atomic size, ionization energy, ionic size, and electronegativity can be explained by variations in atomic structure. The increase in nuclear charge within groups and across periods explains many trends. Within groups an increase in shielding has a significant effect.