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In the modern atomic model, an atom is a compact, positively charged nucleus orbited by a diffuse cloud of negatively charged electrons. The nucleus houses protons, which carry the positive charge, and neutrons, which are electrically neutral. The number of protons uniquely identifies each element and is called the atomic number. Unlike atomic mass or atomic weight, the atomic number is a fixed integer for a given element and does not vary with isotopic composition.
The atomic number equals the proton count in an element’s nucleus, determining its position on the periodic table. Atomic weight is an average of the masses of all naturally occurring isotopes and is typically larger than the atomic number.
The periodic table arranges elements in ascending order of atomic number. Scientists currently recognize 118 elements; the newest, oganesson (Og, atomic number 118), was added in 2015 and is produced only in laboratories. Hydrogen, with a single proton, occupies the first spot. Each element’s symbol on the table is accompanied by its atomic number, allowing quick identification of proton count without counting positions.
When you consult a modern periodic table, you’ll see a second numeric value next to the atomic number. This is the atomic weight, an average mass of all naturally occurring isotopes of that element expressed in atomic mass units (amu). It is distinct from the atomic mass of a single isotope, which sums only the masses of protons and neutrons in the nucleus. Electrons contribute negligibly to atomic mass.
Because isotopes of an element differ only in neutron count, each has a unique atomic mass. The atomic weight presented in the table is a weighted average of these masses, reflecting the natural isotopic distribution. Thus, while the atomic number is invariant, the atomic weight can vary with changes in isotopic composition.
