Ch 04 - STRUCTURE OF THE ATOM9th Science

The Pioneers of Subatomic Particles:

• E. Goldstein (1886): Discovered the presence of positively charged radiations in a gas discharge, which he named canal rays. This foundational work ultimately led to the discovery of the proton.
• J.J. Thomson (1897): Formally identified the very first sub-atomic particle, the electron (which he originally referred to as 'corpuscles').

Discovery & Properties:

• Discovered by James Chadwick in 1932.
• It is a sub-atomic particle with no electrical charge (it is strictly neutral).
• Its mass is nearly identical to that of a proton ($1 \text{ u}$).

Boundary Condition:

• Neutrons are present in the nucleus of all naturally occurring atoms in the universe except Hydrogen (specifically the Protium isotope, $^1\text{H}$, which has 1 proton and 0 neutrons).

Electron ($e^-$):

• Charge: $-1$ (Negative unit charge)
• Mass: Considered to be negligible ($~1/2000$ of a proton). It does not significantly contribute to the atom's total mass.

Proton ($p^+$):

• Charge: $+1$ (Positive unit charge)
• Mass: Taken as $1 \text{ u}$ (Unified atomic mass unit).
• Relative Comparison: A single proton is approximately 2000 times as massive as an electron.