1. The Modern Periodic Table
Chapter 5
Section 2

2. The Periodic Law
In the modern periodic table, elements are arranged by increasing atomic number (number of protons).

3. Periods Each row of the table of elements is a period.
As the atomic number increases, the electrons increase as does the protons.
The first energy level has only one orbital. It will hold only 2 electrons. Row 1
The second energy level will hold 6 electrons. This is row 2.

4. Groups Each column on the periodic table is a group.
Properties of elements repeat in a predictable way when atomic numbers are used to arrange elements into groups.
Elements in a group have similar electron configurations.
An element’s electron configuration determines its chemical properties.
Periodic law is the pattern of the members of a group having similar chemical properties.

6. Atomic Mass
Atomic mass is a value that depends on the distribution of an element’s isotopes in nature and the masses of those isotopes.

7. Atomic Mass Units
Mass of an atom in grams is extremely small and not very useful because the samples of matter that scientist work with contain trillions of atoms.
Scientists assigned 12 atomic mass units to the carbon-12 atom, which has 6 protons and 6 neutrons.
An atomic mass unit (amu) is defined as one twelfth the mass of a carbon-12 atom.

8. Classes of Elements
Elements are classified as metals, nonmetals, and metalloids.
Metals are elements that are good conductors of electric current and heat.
Transition metals are elements that form a bridge between the elements on the left and right sides of the periodic table.
Nonmetals are elements that are poor conductors of heat and electric current.
Metalloids are elements with properties that fall between those of metals and nonmetals.

9. Variation Across a Period
Across a period from left to right, the elements become less metallic and more nonmetallic in their properties.