Studied by 1 personatomic 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
Note 
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