1. Subatomic Particles
An atom contains three fundamental particles that are important in introductory chemistry: protons, neutrons, and electrons.
Proton
Positively charged particle found inside the nucleus.
Neutron
Electrically neutral particle found inside the nucleus.
Electron
Negatively charged particle found in regions around the nucleus.
2. Atomic Number and Mass Number
The identity of an element is determined by its number of protons. This number is called the atomic number.
Z = number of protons
The mass number represents the total number of protons and neutrons in the nucleus.
A = Z + N
Here, A is the mass number, Z is the atomic number, and N is the number of neutrons.
3. Development of Atomic Models
Our understanding of the atom developed through several models. Each model improved upon limitations of earlier ideas.
Matter was described as being composed of indivisible atoms.
The discovery of the electron showed that atoms contained smaller charged particles.
Scattering experiments provided evidence for a small, positively charged nucleus.
Electrons were described as occupying specific energy levels.
Modern chemistry describes electrons using probability distributions called orbitals rather than fixed circular paths.
4. Isotopes
Isotopes are atoms of the same element that contain the same number of protons but different numbers of neutrons.
Example: Carbon
Carbon-12 and Carbon-14 both contain six protons.
Carbon-12 has six neutrons, while Carbon-14 has eight.
They therefore have the same atomic number but different mass numbers.
5. Electronic Configuration
Electrons occupy regions around the nucleus according to their energy. These regions are organised into shells and subshells.
The four commonly introduced subshell types are s, p, d, and f.
The s subshell can contain 2 electrons, p can contain 6, d can contain 10, and f can contain 14.
s² p⁶ d¹⁰ f¹⁴
6. Quantum Numbers
Quantum numbers describe the state of an electron in an atom. Four quantum numbers are used to describe an electron.
Indicates the main energy level or shell.
Describes the subshell and orbital shape.
Describes the orientation of an orbital.
Describes the spin state of an electron.
7. Rules of Electronic Arrangement
Three important principles help determine how electrons are arranged in atoms.
Electrons generally occupy lower-energy orbitals before higher-energy orbitals.
No two electrons in an atom can have the same set of four quantum numbers.
Electrons occupy degenerate orbitals singly before pairing, when possible.
Key Ideas
It equals the number of protons in the nucleus.
They belong to the same element but have different mass numbers.
Particularly important are the electrons in the outermost occupied energy levels.