1. Modern Periodic Table (Section 5.2)

2. The PERIODIC TABLE arranges elements in columns, based on sets of properties that repeat from row to row
Elements are arranged by increasing atomic number

3. Periods A PERIOD is a row in the table of elements
The number of elements per period varies because the number of available orbitals increases from one energy level to the next

4. Groups A GROUP is a column on the periodic table
Properties repeat in a predictable way when atomic numbers are used to arrange elements into groups
Elements in groups have similar electron configurations and chemical properties
PERIODIC LAW is the pattern of repeating properties

5. Atomic Mass
Depends on the distribution of an element’s isotopes in nature and the masses of those isotopes
ATOMIC MASS UNIT (amu) is defined as 1/12 the mass of a Carbon-12 atom
Isotopes of chlorine
Chlorine-35 occurs 75.78% naturally
Chlorine-37 occurs only 24.22% naturally
Therefore the atomic mass of chlorine is is the WEIGHTED AVERAGE ( and not 35)
Weighted averages
Because one isotope occurs more than another you can’t just divide the sum of both masses by 2
For example chlorine-35 occurs more than chlorine-37 which has more mass so the atomic mass of chlorine is (a weighted average) instead of (what you would get if you took both masses and divided by 2)

6. Classes of Elements
SOLID, LIQUID, or GAS (based on their state at room temperature)
NATURALLY OCCURING vs. NOT NATURALLY OCCURING
METAL, NONMETAL, METALLOID

7. Metals Good conductors of electric current and heat
Solid at room temperature (except mercury)
Malleable (can be pounded into flat sheets) and ductile (drawn into thin wires)
Some are extremely reactive and some do not react easily

8. Transition Metals
Elements that form a bridge between the elements on the left and right sides of the table
Form compounds with distinctive colors

9. Nonmetals Poor conductors of heat and electric current
Low boiling points, many are gases at room temperature
All the gases in the periodic table are nonmetals
The ones that are solid at room temperature tend to be brittle
Some are extremely reactive, some hardly react at all, and some fall in between
Fluorine is the most reactive nonmetal

10. Metalloids Properties that fall between those of metals and nonmetals
Ability to conduct electricity varies with temperature
Silicon and germanium are good insulators at room temperature and good conductors at high temperatures

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