9TH GRADE · ATOMIC STRUCTURE
Atom, nucleus, spectra and radioactivity
Atomic models, nucleons, isotopes, radiation and particle registration.
The entire theory of the lesson is collected here; you can read it without an account. In interactive mode, the same theory appears as cards, followed by questions. with checking the answer and task, and progress is saved.
Basic Concepts
Composition of the atomic nucleus
The nucleus is made up of protons and neutrons, collectively called nucleons, held together by nuclear forces.
The number of protons determines the element, the sum of protons and neutrons determines the mass number, and different isotopes differ in neutrons.
Atomic models and spectra
The nucleus-electron model explains particle scattering and discrete line emission and absorption spectra.
An atom emits or absorbs a photon when transitioning between allowed energy states.
Radioactivity and registration
Unstable nuclei spontaneously transform, emitting alpha, beta particles or gamma quanta.
The camera, counter and track detector record the ionization of the substance, and the penetrating power of the radiation varies.
Physical quantities
Charge number
The number of protons in the nucleus that defines a chemical element.
Unit: dimensionless quantity
Mass number
The total number of protons and neutrons in the nucleus.
Unit: dimensionless quantity
Half-life
The time during which, on average, half of the initial unstable nuclei decay.
Unit: second ·
Number of neutrons
The number of neutrons in the nucleus of the selected isotope.
Unit: dimensionless quantity
Number of undecayed nuclei
The number of radioactive nuclei that have not yet experienced decay.
Unit: dimensionless quantity
Lesson formulas
Number of neutrons
The number of neutrons is equal to the mass number minus the number of protons.
Law of Radioactive Decay
After each half-life, the number of undecayed nuclei is halved.
Related formulas
The same formulas in the reference, with symbols and units.