Chemistry Notes on Atomic Structure
Structure of the Atom
An atom has a nucleus at its center, significantly smaller than the atom itself.
The nucleus contains most of the atom's mass and carries a positive electrical charge.
It is surrounded by negatively charged particles called electrons.
The atomic size is determined by the outer electrons.
Dalton's Atomic Theory
Based on known laws, Dalton presented several hypotheses:
An element is composed of many minute particles called atoms.
Atoms are indivisible and cannot be divided into smaller particles by any means.
Atoms cannot be created or destroyed.
The smallest indivisible particle that can take part in a chemical reaction is called an atom.
The term atom is derived from the Greek word 'a-tomio', which means indivisible.
Modern Atomic Theory
Atoms are not indivisible; they are composed of subatomic particles such as electrons, protons, and neutrons, which can also be divided.
Isotopes and Isobars
Atoms of the same element can have different masses; these are referred to as isotopes (e.g., hydrogen has three isotopes: protium, deuterium, tritium with atomic weights of 1, 2, and 3).
Isobars are atoms of different elements having the same atomic weight but different atomic numbers.
Fundamental Particles of an Atom
Electrons:
Discovered by J.J. Thomson in 1897.
Extremely small particle with a negative charge of -1.6 imes 10^{-19} coulombs.
Weight is rac{1}{1837} of hydrogen's weight, approximately 9.1 imes 10^{-31} ext{ kg}.
Protons:
Discovered by Rutherford in 1919.
Positive particle with a charge of +1.6 imes 10^{-19} coulombs.
Weight is nearly equal to hydrogen's atomic weight, represented by 'p' or 'H'.
Neutrons:
Discovered by Chadwick.
Electrically neutral particle with no charge.
Weight is approximately 1.6750 imes 10^{-27} ext{ kg}, about equal to hydrogen's atomic weight, represented by 'n'.
Rutherford's Nuclear Model
Conducted experiments in 1911 bombarding gold foil with alpha particles:
Majority of particles passed through the foil, indicating that most of the atom is empty space.
Neils Bohr's Atomic Model
Electrons revolve only in certain fixed orbits around the nucleus.
These orbits have a fixed radius and do not emit energy while in these stable orbits (energy levels).
Filling of Electrons in Orbitals
Pauli Exclusion Principle:
A maximum of 2 electrons can occupy an orbital, but their spins must be opposite.
Aufbau Principle:
Electrons fill the lowest energy levels first.
Order of filling: 1s < 2s < 2p < 3s < 3p < 4s < 3d < 4p < 5s < 4d < 5p < 6s < 4f < 5d < 6p < 7s.
Quantum Numbers
Used to define the position of electrons around the nucleus:
Principal Quantum Number: Indicates position and energy; can be any whole number except zero.
Azimuthal Quantum Number: Determines the shape of sub-orbitals, values range from 0 to (n-1); represents s, p, d, f.
Magnetic Quantum Number: Depends on the azimuthal number; can have values between -l and +l.
Spin Quantum Number: Indicates the direction of electron's spin; can be either +1/2 (up-spin) or -1/2 (down-spin).
Atomic Number and Mass Number
Atomic Number (Z): Discovered by Moseley in 1913; equal to the number of protons and, for neutral atoms, equal to the number of electrons.
Z = ext{number of protons} = ext{number of electrons (for neutral atom)}.
Mass Number (A): Equal to the sum of protons and neutrons in the nucleus.
A = ext{number of protons} + ext{number of neutrons}.
Characteristics of atomic representation:
Atomic number = number of protons,
Mass number = atomic number + neutrons.
Elements represented with their atomic number and mass number.
Key Points on Atom Representation
An atom is often symbolically represented with its atomic and mass number.