Periodic tables

atomic number

the number of protons in the nucleus of an atom

atomic mass

The average mass of all the isotopes of an element

mass number

the sum of the number of neutrons and protons in an atomic nucleus

Order of elements on historic periodic table

Elements were ordered by atomic weight

Order of elements on modern periodic table

Elements are ordered by atomic number

Dmitri Mendeleev's ideas

Ordered elements by atomic weight BUT changed the order/left gaps to keep elements with similar properties in the same collumn

Advantages of Mendeleev's ideas

He was able to predict the properties of elements in the gaps.

Group

A vertical column of elements with the same number of outer shell electrons

Period

A horizontal row of elements with the same number of electron shells

Physical properties

observed or measured characteristics of a substance

Chemical properties

How a substance reacts with other substances.

Groups missing from Mendeleev's periodic table

Transition metal, Lanthanides, Actinoids