Atomic number is the number of protons in the nucleus
The origins on the periodic table begin with the work of John Newlands and Dimitri Mendeleev.
The atomic number of the elements provides information on the number of protons in the nucleus.
Atomic mass refers to the mass of both the protons and neutrons and is always the larger of the two represented in the periodic table.
Isotopes are elements with different numbers of neutrons in the nucleus but the same number of protons.
Gaps in the periodic table were first used by Dimtri Mendeleev when he could not find an element to fit the patterns of the groups and periods he was using. Mendeleev knew there would be undiscovered elements so used these gaps to prompt other scientists to start looking for them.
Groups in the periodic table are the columns where elements with similar properties are found grouped together.
The lanthanides are a series of chemicals in the expanded periodic table that have an expanded number of orbitals beyond that of the transition metals. The lanthanides are mostly soft metals with their hardness increasing across the period. Whilst they are metals and can conduct electricity the lanthanides have a relatively high resistivity.