Introduction to Atoms
Atoms are the fundamental building blocks of all matter. They are the smallest units of an
element that retain the chemical properties of that element. The concept of the atom dates back
to ancient philosophers but was scientifically formulated in the 19th century by John Dalton.
Dalton's Atomic Theory
Dalton proposed that:
Matter is made up of indivisible particles called atoms.
Atoms of a given element are identical in mass and properties.
Atoms of different elements differ in mass and properties.
Atoms combine in simple whole-number ratios to form compounds.
Atoms cannot be created or destroyed in chemical reactions.
Structure of the Atom
An atom consists of:
A central nucleus containing positively charged protons and neutral neutrons.
Negatively charged electrons orbiting the nucleus in defined energy levels or shells.
The nucleus contains most of the atom's mass.
Atomic Number and Mass Number
The atomic number (Z) is the number of protons in the nucleus and defines the element.
The mass number (A) is the total number of protons and neutrons.
Different atoms of the same element with varying neutrons are called isotopes.
Atomic Mass
Atomic mass is the weighted average mass of an element’s isotopes.
It accounts for the number of protons plus neutrons and their relative abundance.
Historical Atomic Models
Dalton’s Model: Atoms as solid spheres.
Thomson’s Plum Pudding Model: Electrons embedded in a positively charged sphere.
Rutherford’s Nuclear Model: Dense positive nucleus with electrons orbiting.
Bohr’s Model: Electrons orbit nucleus at fixed energy levels.
