Atomic Model Evolution and Chemical Bonding

Atomic Model Evolution

Democritus's Atomic Theory

Democritus proposed that matter could not be divided into smaller pieces forever. He believed that there were tiny, eternal, invisible, and indivisible particles called atoms (meaning indivisible in Greek) that constituted all matter.

Dalton's Atomic Theory (1808)

• Matter is made of indivisible and indestructible particles called atoms.
• All atoms of an element are identical.
• Atoms of different elements combine in fixed proportions to form compounds.

Thomson's Atomic Model (1897)

J.J. Thomson experimentally verified the existence of the electron. He discovered that the electron's mass was much lower than that of atoms, suggesting that atoms possessed a negative charge and were divisible. His model described the atom as a sphere of positive charge with embedded electrons.

Rutherford's Atomic Model (1911)

Ernest Rutherford found that some alpha particles fired at thin gold sheets were deflected at angles greater than 90 degrees. This led him to propose a model where most of the atom's mass and positive charge resided in a very small central region called the nucleus, with electrons orbiting around it. The nucleus contains protons and neutrons, while electrons are located in the outer shells. Electrons have a mass approximately 1000 times smaller than protons and neutrons.

Electron Distribution

Electrons are arranged in energy layers around the atomic nucleus. Each layer can hold a maximum number of electrons, with the outermost layer (valence shell) typically holding up to 8 electrons. Atoms with few valence electrons tend to lose them, forming positive ions (cations), while atoms with a number of valence electrons close to 8 tend to gain electrons, forming negative ions (anions).

Molecules, Elements, and Compounds

An element is a pure substance formed by identical atoms. A compound is a pure substance formed by different atoms of chemical elements combined in simple, fixed numerical ratios. Compounds can be separated into their constituent elements.

Chemical Bonding

Ionic Bond

This bond occurs between atoms of two elements with very different electronegativity, typically a metal and a non-metal. Electrons are transferred from the metal to the non-metal.

Covalent Bond

Atoms of two elements share electrons to achieve the electron configuration of a noble gas.

Metallic Bond

Atoms of metals release their valence electrons, forming a structure where each node of the network is occupied by a metal cation. The electrons form a cloud and have greater freedom of movement.