Discovering the electron
- When electricity flows in an aqueous solution of ionic compounds, the metal ions appears at the negative electrode (cathode). - The metal exists in the solution as positively charged particles known as ions. - A 1+ ion plus one unit of electricity gives one metal atom.
- Low pressures gases conduct electricity very well.
- The glass of containing vessel opposite the cathode (negatives electrode) glowed when the applied potential difference (voltage) was sufficiently high. - Solid objects cast shadows between the cathode and the glow was caused by rays coming from the cathode and called these cathode rays
[To be continued]
Atomic and mass number
Atomic number (Z)
- Throughout the period, each element differ each other by one atomic number, and this is the most important difference between atoms of different elements. - The number of protons in an atom determines the element to which the atom belongs. - The atomic number of an element shows:
* The number of protons in the nucleus of an atom of the element * The number of electrons in the neutral atom of that element * The position of the element in the Periodic Table
Mass number (A)
- For any atom, the mass number is the sum of the number of protons and the number of neutrons.
Particle name| Relative mass| Relative charge|
Electron| Negligible (about 1/2000th the mass of a proton)| -1| Proton| 1| +1|
Neutron| 1| 0|
- Same element could have different atomic masses.
- These atoms are isotopes, and this word means 'equal place'. - Occupying the same place in the Periodic Table and having the same atomic number. - Isotopes are atoms with same atomic number, but different mass numbers. - Atomic number (Z) = number of protons
- Mass number (A) = number of protons + number of neutrons
- E.g. U-235 and U-238
Counting atoms and molecules
- The relative atomic number, Ar, of...