Atoms and the Periodic Table

Atomic Theory

The concept of the atom has evolved over centuries, beginning with philosophical ideas and culminating in scientific theories that form the foundation of modern chemistry.

• Democritus (5th century BCE): Proposed that matter consists of very small, indivisible particles called atoms.

• Dalton's Atomic Theory (early 19th century):

  • Elements are composed of extremely small particles called atoms.

  • All atoms of a given element are identical in mass and chemical properties.

  • Atoms of different elements have different properties (e.g., Cu atoms are different from Ag atoms).

  • Compounds are composed of atoms of more than one element, combined in simple whole-number ratios.

  • Chemical reactions involve the rearrangement of atoms; atoms are not created or destroyed (Law of Conservation of Mass).

Example: In sodium chloride (NaCl), the ratio of sodium to chlorine atoms is 1:1. In calcium chloride (CaCl2), the ratio is 1:2.

Chemical Reactions and Conservation of Mass

Chemical reactions involve the separation, combination, or rearrangement of atoms. The total mass remains constant during a chemical reaction, as stated by the Law of Conservation of Mass.

• Reactants combine to form products with different physical and chemical properties.

• Atoms are neither created nor destroyed in a chemical reaction.

Example: When phosphorus burns in air, it reacts with oxygen to form phosphorus oxide. The total mass before and after the reaction remains the same.

Structure of the Atom

Subatomic Particles

Atoms are composed of even smaller particles known as subatomic particles: electrons, protons, and neutrons.

• Electrons (e-): Negatively charged particles with a very small mass.

• Protons (p+): Positively charged particles found in the nucleus; mass is approximately 1,836 times that of an electron.

• Neutrons (n0): Electrically neutral particles found in the nucleus; mass is slightly greater than that of a proton.

Discovery of the Electron

• Cathode Ray Tube Experiments: When a high voltage is applied, an invisible ray (cathode ray) is emitted, which is deflected by electric and magnetic fields, indicating it consists of negatively charged particles (electrons).

• J.J. Thomson: Used the cathode ray tube to determine the charge-to-mass ratio of the electron:

• The unit of electric charge is the coulomb (C).

Additional info:

• The discovery of the electron led to the realization that atoms are not indivisible, but contain internal structure.