Basic Ideas About Atoms

Section 1: Fundamental Concepts of Atoms

Every element consists of tiny particles known as atoms. An atom is the smallest unit of an element that retains the chemical properties of that element.
Atoms are composed of subatomic particles: protons, neutrons, and electrons. Each of these particles has a different percentage of the total atomic mass and charge.
Protons are positively charged particles located in the atomic nucleus. The number of protons in an atom is referred to as the atomic number and it distinguishes one element from another.
Neutrons are neutral particles also located in the atomic nucleus alongside protons. They contribute to the mass of the atom but do not affect the atomic number.
Electrons are negatively charged particles which move in orbits around the nucleus. Their charge is equal but opposite to that of protons.
The relative charges are: electron = -1, proton = +1 and neutron = 0. The relative masses are: electron = ~0, proton = 1 and neutron = 1.

Section 2: Isotopes

Isotopes are variants of a particular chemical element which differ in neutron number, and consequently in nucleon number. They have the same atomic number but different mass numbers.
The relative atomic mass of an element is the weighted average mass of the isotopes of that element compared with 1/12 of the mass of an atom of carbon-12.

Section 3: Basic Unit Structures

A molecule is formed when two or more atoms are chemically bonded together. A molecule can either be an element or a compound.
An ion is an atom (or group of atoms) that has gained or lost electrons and therefore holds a net charge.

Section 4: Quantum Mechanics in Atomic Structure

Electrons occupy atomic orbitals, which are a region of space around the nucleus where there is a high probability of locating an electron.
An electron’s position in an atom is described by four quantum numbers: (i) the principal quantum number (n), which informs about the energy level; (ii) the azimuthal quantum number (l), which defines the shape of the orbital; (iii) the magnetic quantum number (ml), which specifies the orientation of the orbital; and (iv) the spin quantum number (ms), which indicates the spin direction of the electron.
The Pauli Exclusion Principle states that no two electrons in an atom can have the same set of four quantum numbers.