Caption: The periodic table of the (chemical) elements or the atoms.

Features:

1. An element is a species of atom. Thus, in a sense, atom is just the microscopic perspective on element. So element and atom can be used as synonyms in many contexts.

2. The atoms in the periodic table are arranged in a way that reflects their chemical properties and, more fundamentally, their electronic configuration or in more general perspective their electronic structure.

   Down the columns, the atoms tend to have similar chemical properties. Along the rows (AKA the periods), the chemical properties tend to vary in a systematic way. The whole set of trends in the periodic table are called the periodic trends (see Wikipedia: Periodic trends).

3. The chemistry of atoms (taking quantum mechanics, nuclear physics, and electron behavior as givens) is determined by the number of protons in the atomic nucleus. This number is called the atomic number (symbolized by Z).

   In other words, the atomic number determines the electronic configuration and chemical behavior. Examples: 1 proton gives chemical element hydrogen, 2 protons gives chemical element helium, etc.

4. Atoms were NOT well understood or accepted when the periodic table developed in the 19th century (Wikipedia: Periodic table: History). In particular, number of protons (i.e., atomic number) was NOT understood and the early versions of the periodic table had the elements ordered by increasing atomic mass which is almost the order by atomic number (see Wikipedia: Periodic table: Mendeleev's table).

5. The arrangement in the periodic table has only a limited correlation with the nuclear properties of the atomic nuclei. Of course, there is the major correlation that the atomic number is that the number of protons.

6. Atomic mass is essentially proportional to the combined number of protons and neutrons. There are smaller mass effects due to the additional mass of the electrons and mass reduction due to the binding energy of all the particles.

7. Atoms with the same number protons, but different numbers of neutrons are isotopes of each other.

   The isotopes of an atom are nearly identical chemically, but can have very different nuclear properties.

   Many isotopes are unstable to radioactive decay. The half-life of a radioactive isotope is a kind of mean life before radioactive decay of an atomic nucleus.

8. Lead (Pb) (atomic number 82) is the highest atomic-number element with absolutely stable isotopes: Pb-204,Pb-207,Pb-208 and Pb-206 (see Wikipedia: Lead; List of elements by stability of isotopes).

   But many of the higher atomic-number elements have isotopes with half-lives of billions of years, and so are exist on Earth in nearly constant abundances over human history,

   Uranium (U) (atomic number 92) has the highest atomic number among primordial elements (i.e., elements that have existed since the formation of Solar System: Solar System age = 4.5682 Gyr by a standard definition). Its longest lived isotope is U-238 with half-life 4.46 Gyr.

9. Currently, Oganesson (Og, element 118) has the highest atomic number, but like all the high atomic-number elements, it doesn't occur in observable nature and is only seen in minute quantities after creation in particle accelerators. Such elements typically only survive for a fraction of a second before they undergo radioactive decay.