Atomic Models and Structure: From Dalton to Quantum Mechanics
Plum Pudding Model
J.J. Thomson’s identification of negatively charged subatomic particles (electrons) through his study of cathode rays led him to propose a model of the atom. This model, known as the plum pudding model, described electrons as negatively charged “plums” embedded in a “pudding” of positive matter.
Rutherford Model
Based on experiments bombarding thin metal films with alpha particles, Rutherford established the nuclear atomic model. This model describes the atom as consisting of two parts: the nucleus and the electron cloud. The nucleus is a small, dense, central region containing all the positive charge and virtually all the atom’s mass. The deflection of alpha particles in the gold foil experiment is attributed to the nucleus’ positive charge. The electron cloud is the vast space surrounding the nucleus, where electrons with negligible mass and negative charge reside. Like a miniature solar system, electrons orbit the nucleus, held in place by the electrostatic attraction between opposite charges.
Bohr Model
In 1913, Bohr explained the atomic spectrum of hydrogen. Building on Max Planck’s quantum theory, Bohr proposed that atoms can only exist at specific energy levels. Electrons occupy specific orbits with fixed radii. These orbits are stationary; electrons do not emit energy while in them. The electron’s kinetic energy perfectly balances the electrostatic attraction between the nucleus and the electron. Electrons can only possess energy values corresponding to these orbits. Transitions between energy levels correspond to the emission or absorption of electromagnetic energy (photons of light). However, the Bohr model could not explain the spectra of more complex atoms. The idea of electrons orbiting in fixed paths was eventually replaced by quantum mechanics, which Bohr helped develop.
Dalton’s Atomic Theory
Around 1808, Dalton defined atoms as the fundamental units of elements, reviving ancient Greek atomistic ideas. His theory, published in 1808 and 1810, can be summarized as follows:
- Matter consists of indivisible particles called atoms.
- Atoms of the same element are identical in all properties, including weight.
- Different elements are made of different atoms.
- Chemical compounds are formed by combining two or more atoms of different elements in simple numerical ratios.
- Atoms are indivisible and retain their identity during chemical reactions.
- In chemical reactions, atoms combine in simple numerical ratios.
- Atoms separate and combine in chemical reactions. No atoms are created or destroyed, and no atom of one element becomes an atom of another element.
While Dalton’s theory had limitations, it significantly advanced our understanding of matter’s structure. Acceptance of Dalton’s model was not immediate, with many scientists initially resisting the idea of atoms.
Dalton used symbols to represent atoms and molecules. However, he did not propose a structure for atoms. Other laws consistent with Dalton’s theory include:
- Law of Conservation of Mass: Matter is neither created nor destroyed, only transformed.
- Law of Definite Proportions: A pure compound always contains the same elements combined in the same proportions by mass.
- Law of Multiple Proportions: When two elements (A and B) form more than one compound, the masses of A that combine with a fixed mass of B are in ratios of small whole numbers.
Valences of Elements
Alkali Metals
Li (Lithium): 1, Na (Sodium): 1, K (Potassium): 1, Rb (Rubidium): 1, Cs (Cesium): 1, Fr (Francium): 1
Alkaline Earth Metals
Be (Beryllium): 2, Mg (Magnesium): 2, Ca (Calcium): 2, Sr (Strontium): 2, Ba (Barium): 2, Ra (Radium): 2
Transition Metals
Ti (Titanium): 3, 4; Cr (Chromium): 2, 3, 6; Mn (Manganese): 2, 3, 4, 6, 7; Fe (Iron): 2, 3; Co (Cobalt): 2, 3; Ni (Nickel): 2, 3; Cu (Copper): 1, 2; Zn (Zinc): 2; Pd (Palladium): 2, 4; Ag (Silver): 1; Cd (Cadmium): 2; Pt (Platinum): 2, 4; Au (Gold): 1, 3; Hg (Mercury): 1, 2
Actinide Metals
U (Uranium): 3, 4, 5, 6; Pu (Plutonium): 3, 4, 5, 6
Boron Family
B (Boron): 1, 3; Al (Aluminum): 3; Ga (Gallium): 3; In (Indium): 3; Tl (Thallium): 3
Carbon Family
C (Carbon): -4, 2, 4; Si (Silicon): 4; Ge (Germanium): 4; Sn (Tin): 2, 4; Pb (Lead): 2, 4
Nitrogen Family
N (Nitrogen): -3, 3, 5; P (Phosphorus): -3, 3, 5; As (Arsenic): -3, 3, 5; Sb (Antimony): -3, 3, 5; Bi (Bismuth): 3, 5
Oxygen Family
O (Oxygen): -2; S (Sulfur): -2, 2, 4, 6; Te (Tellurium): -2, 2, 4, 6; Po (Polonium): -2, 2, 4, 6
Halogen Family
F (Fluorine): -1; Cl (Chlorine): -1, 1, 3, 5, 7; Br (Bromine): -1, 1, 3, 5, 7; I (Iodine): -1, 1, 3, 5, 7
Lewis Structure of Order
