Atomic Theory and Chemical Bonds: A Comprehensive Study
Postulates of Dalton’s Atomic Theory
- The elements that constitute matter are composed of indivisible and indestructible particles called atoms.
- Atoms of a particular element are equal in mass and properties.
- Atoms of different elements differ in mass and properties.
- Compounds are formed by the union of atoms of the corresponding elements in constant and simple fractions. The set of atoms that constitute a compound gives this compound some characteristic properties, and the atomic mass of the combination in the composite is constant.
J.J. Thomson’s Atomic Model (1903)
Thomson proposed that the atom was a sphere with a positive charge and electrons embedded like raisins in a cake.
Rutherford’s Atomic Model
- The mass of atoms is practically concentrated in the nucleus. The positive load also lies in the nucleus.
- Most of the atom is a large empty space.
Atomic Spectra
Atomic spectra are like a fingerprint, characteristic of each atom; no two are alike.
- Continuous spectra: Overlapping radiation between the ends.
- Batch spectra: Some contain only certain radiation (caused by emission or absorption).
Bohr’s Atomic Model (Three Principles)
- The electron orbits the nucleus in well-defined circular orbits without releasing or absorbing energy. It does not vary in speed and does not rush about the nucleus. The atomic radius is constant.
- Only circular orbits with certain specific energy values are allowed for the electron. It is said that the energy values of electrons are quantized. The electron absorbs energy and moves to a more distant orbit around the nucleus, where more energy is wasted (excited state).
- The passage of the electron from one orbit to another causes the emission or absorption of energy in accordance with the energy difference between the two energy levels. For an electron to pass from a lower to a higher orbit, it needs to absorb energy; it receives and emits energy as electromagnetic radiation.
Heisenberg’s Uncertainty Principle
It is impossible to know simultaneously and precisely the amount of movement and the position of a particle.
An orbital is the area of space around the nucleus in which there is a high probability of finding the electron.
- Principal quantum number (n): Determines the size of the orbit.
- Secondary quantum number (l): Determines the shape of the orbit. It depends on the principal quantum number, ranging from 0 to n-1.
- Magnetic quantum number (m): Determines the orientation of the orbital in space.
- Spin quantum number (s): Is associated with the magnetic properties of the electron.
Chemical Bonds
Chemical bonds are the unions between the same or different atoms.
Ionic Bond
An ionic bond occurs when one ion loses an electron and another ion gains an electron, creating an attraction between opposite charges.
Properties of Ionic Compounds
- All are solid.
- They are water-soluble.
- Melting and boiling points are very high.
- They have high hardness.
- The bond is very fragile and breaks easily.
Covalent Bond
A covalent bond is the union of two atoms through electron sharing. In the covalent bond, electrons of each atom are shared. The global electronic cloud is common to the atoms forming the link, and this creates a strong bond. (Non-metals combine through covalent bonds.)
Properties of Molecular Substances
- They have low melting and boiling points.
- Molecular solids are soft and do not have much mechanical strength.
- Molecular solids are not good conductors.
- Substances with low molecular weight or nonpolar substances are practically insoluble in polar solvents like water.
Properties of Atomic Substances
- They have very high melting points.
- They are tough.
- They do not conduct heat or electric current.
- They are insoluble.
Metallic Bond
A metallic bond is the union between atoms of metallic elements that share valence shell electrons. Metallic crystalline networks are very compact arrangements in which each positive ion of metal is attached to the rest by moving electrons.