Atom   |   Isotope   |   Xray   |   Accelerator   |   Half-Life   |   Geiger Counter   |   Fusion & Fission   |   Power

It's a Question of Physics: What is an atom?


(click for larger image)

The Rutherford Atom for Carbon
Ernest Rutherford's original atomic model is now understood to be inaccurate, but it retains its meaning as an icon today. The nucleus consisting of protons and neutrons, here shown in red, is surrounded by orbiting electrons, shown in blue.


(click for larger image)

The Schrodinger Atom for Carbon
Erwin Schrodinger's model is a more correct representation. It includes electron clouds.
An atom is the smallest particle into which an element can be divided without losing its chemical identity. Atoms consist of a heavy central nucleus surrounded by a cloud of negatively charged particles called electrons. The nucleus contains positive particles (protons) and electrically neutral particles (neutrons). The number of protons is called the atomic number. This number uniquely identifies each chemical element. In turn, protons and neutrons are composed of quarks.

An element is a chemical substance that is made up of single kind of atom. Iron, carbon, and hydrogen are all elements. A molecule is formed when two or more atoms of any kind of element are joined together chemically. If a molecule contains two or more different elements, it is known as a compound. A water molecule is a compound of the elements hydrogen and oxygen.

If an atom or molecule becomes electrically charged by gaining or losing one or more electrons, it becomes an ion. If the atom gains electrons, it has a negative charge. If it loses electrons, it has a positive charge.

(click for larger image)

Bohr Model of Calcium
Bohr originally developed his atomic model to demonstrate the way in which electrons of hydrogen atoms changed orbits. Physicists today know that this version is not completely accurate, but it is still used because it offers a clear explanation of atomic structure at an introductory level.


(click for larger image)

Bohr Model of a Calcium Ion
The calcium molecule above has gained two negatively charged electrons to become a calcium ion.
« Back to Questions