Protons and Neutrons form a nucleus.

Electrons

Atomic Number and Mass Number Atomic Number Is the Number of Protons of the Nucleus

Mass Number Is the Number of Particles of the Nucleus mass number – atomic number = number of neutrons In this example, the neon atom has 10 neutrons. number of protons and neutrons (mass number)  20 number of protons (atomic number)  10 number of neutrons  10

Current Electron Theory Different Elements Can Have the Same Mass Number Isotopes of an Element Have the Same Atomic Number Bohr’s Model Confines Electrons to Energy Levels

Electrons Act Like Both Particles and Waves

Electrons and Light

Quantum Numbers The present-day model of the atom, in which electrons are located in orbitals, is also known as the quantum model. To define the region in which electrons can be found, scientists have assigned four quantum numbers to each electron.

Quantum Numbers The principal quantum number, symbolized by n, indicates the main energy level occupied by the electron. The main energy levels can be divided into sublevels. These sublevels are represented by the angular momentum quantum number, l. The magnetic quantum number, symbolized by m, is a subset of the l quantum number. The spin quantum number, symbolized by +1/2 or −1/2 (↑ or ↓), indicates the orientation of an electron’s magnetic field

Electron configuaration Pauli’s exclusion principle states that two particles of a certain class cannot be in the exact same energy state The Aufbau principle states that electrons fill orbitals that have the lowest energy first. Hunds’ rule states that orbitals of the same n and l quantum numbers are each occupied by one electron before any pairing occurs.