Modern Periodic Table 5.2
The Periodic Law In the modern periodic table, elements are arranged by increasing atomic number (number of protons) Properties of elements repeat in a predictable way when atomic numbers are used to arrange elements into groups. This is called periodic law.
PARTS OF THE PERIODIC TABLE Period – horizontal row Group – vertical columns; they are numbered 1 – 18 Family – specific name describing one or more groups; ex: Column 18 is the noble gases Elements in the same group have the same number of valence electrons (electrons in their outer shell)
Atomic Mass Atomic mass depends on the distribution of an element’s isotopes in nature and the masses of those isotopes. The atomic mass in grams is extremely small, so scientists developed a unit based on the carbon atom Atomic mass unit (amu) – equal to 1/12 the mass of a carbon-12 atom
CLASSES OF ELEMENTS Metals – left and center part of periodic table Conduct heat and electricity, have luster, malleable, ductile All are solid at room temperature (except mercury)
CLASSES OF ELEMENTS Transition elements – groups 3 – 12 They are metals Unpredictable reactivity and properties
CLASSES OF ELEMENTS Lanthanides – atomic number 57 – 70 a. Rare earth elements – less than 0.01% of the elements found on Earth
CLASSES OF ELEMENTS Actinides – Atomic number 89 – 102 All radioactive Only 90, 91, and 92 occur naturally
CLASSES OF ELEMENTS Nonmetals – right side of the periodic table Very abundant in nature Poor conductors of electricity and heat, brittle, many are gases
CLASSES OF ELEMENTS Metalloids – some properties of metals and some properties of nonmetals They are found along the border between metals and nonmetals They are semiconductors
Exception to periodic law Hydrogen is placed in group 1, even though it is a gas. Hydrogen’s location is related to its valence electrons, not its physical properties Elements in a group have similar properties because they have the same number of valence electrons Valence electrons determine how reactive an atom is.