Classroom Central

The Atom and Its Structure

What is an Atom?

An atom is the smallest particle of an element that can exist and still have the chemical properties of that element. It is the basic building block of all matter (solids, liquids, and gases).

Sub-atomic Particles

An atom is composed of three main sub-atomic particles:

Sub-atomic Particle	Location	Electric Charge	Relative Mass
Proton ( $p^{+}$ )	- In the Nucleus (center)	- Positive ( $+1$ )	- 1 (heavy)
Neutron ( $n^{0}$ )	- In the Nucleus (center)	- Neutral (no charge)	- 1 (heavy)
Electron ( $e^{-}$ )	- Orbiting the nucleus in shells	- Negative ( $-1$ )	- Very small (almost 0)

Simple Atomic Model

The atom has three main regions:

1. The Nucleus:
- This is the small, dense center of the atom.
- It contains the protons and neutrons (collectively called nucleons).
- It carries the overall positive charge and contains almost all of the atom's mass.
2. Electron Shells (Orbits):
- These are the regions or paths around the nucleus where the electrons revolve rapidly.
- Electrons are held in orbit by the attraction between their negative charge and the positive charge of the nucleus.

3. Electrical Neutrality: A normal, uncharged atom is electrically neutral because the number of protons (positive charges) is equal to the number of electrons (negative charges), causing the charges to cancel out.

Important Terms

*Term*	*Definition*
Element	A pure substance made up of only one type of atom (e.g., Oxygen, Iron).
Compound	A substance formed when atoms of two or more different elements are chemically combined in a fixed ratio (e.g., Water, $H_{2} O$ ).
Molecule	The smallest unit of an element or compound that can exist on its own and still keep the original properties (e.g., $O_{2}$ or $H_{2} O$ ).
Atomic Number	The number of protons in an atom's nucleus. It uniquely identifies an element.
Mass Number	The total number of protons and neutrons in an atom's nucleus.

History of Atomic Structure (Key Scientists)

*Scientist*	*Year (Approx.)*	*Key Contribution*
John Dalton	$1803$	Proposed the first Atomic Theory; atom was a solid, indivisible sphere.
J.J. Thomson	$1897$	Discovered the electron ( $e^{-}$ ); proposed the "Plum Pudding Model."
Ernest Rutherford	$1909-1911$	Discovered the nucleus and proton ( $p^{+}$ ) via the Gold Foil Experiment (Nuclear Model).
Niels Bohr	$1913$	Proposed that electrons orbit in fixed energy levels (shells) (Planetary Model).
James Chadwick	$1932$	Discovered the neutron ( $n^{0}$ ), completing the known particles of the nucleus.