The Inner Structure of Atoms: The NUCLEUS

At the center of every atom lies a core of positively charged protons and neutrons (no charge) called the NUCLEUS. The nucleus contains 99.99% of an atom's MASS. It is the number of positively charged PROTONS in the nucleus that indicate the identity of each type of atom. This "proton count" appears on every block of the Periodic Table (#1 - #118); it is referred to as the "Atomic Number". By example, a Hydrogen (#1) atom contains only one proton, Helium (#2) atoms have two protons, Lithium (#3) atoms have three protons, an Aluminum (#13) atom has 13 protons, and a Uranium (#92) atom contains 92 protons.

The larger number on each block of the Periodic Table is called the Atomic Weight (or Mass); it indicates the total number of PROTONS + NEUTRONS found in the nucleus.

The number of neutrons can vary slightly without affecting the identity of the atom. These variations in neutron counts within a specie of atom are called, "isotopes". The Atomic Weight listed is an average of naturally occurring isotopes, and therefore do not appear as integers. Instead you'll see atomic weight numbers like this: Carbon (#6): 12.011

Carbon has three isotopes: C-12, C-13, and C-14.

C-12 has 6 protons + 6 neutrons

C-13 has 6 protons + 7 neutrons

C-14 has 6 protons + 8 neutrons

Can you guess which is the most common naturally occurring isotope?

Scientists know with absolute certainty that they have discovered every type of element/atom in the Universe because there are no gaps in the proton counts - from one proton through 92 protons, all atoms/elements are accounted for.

Once scientists discovered the make up of the atomic nucleus, they were completely perplexed. How could protons and neutrons be held together? Why didn't the positively charged protons repel each other apart?

They concluded that there must be some internal force that was stronger than electromagnetic repulsion, and they named it the "Strong Nuclear Force". This is the same force unleashed in nuclear fission reactions that provide heat for nuclear power plants and the destructive force of an atomic bomb blast!