Stability and thermal dissociation
Every chemical substance possesses a certain degree of stability. When heated, organic molecules (such as potato carbohydrates) undergo thermal decomposition and char. However, at extreme temperatures — from 4,000 to 5,000 °C — even the most stable compounds are destroyed.
Thermal dissociation occurs: molecules break down into isolated atoms, forming a gaseous mixture. In nature, this state of matter is characteristic, for example, of the upper layers of the Sun. The process is completely reversible: upon cooling, the atoms recombine to form the original or new complex substances.
Planetary model and subatomic particles
Until the late 19th century, the atom was considered an indivisible solid sphere. In 1911, E. Rutherford conducted an experiment bombarding gold foil with $\alpha$-particles (having a charge of +2). It was found that almost all particles pass freely through the metal, and only a few deflect sharply.
This led to the creation of the planetary model of the atom:
- Most of the atom's volume is empty space.
- At the center lies an ultra-dense, positively charged nucleus.
- Electrons move in orbits around the nucleus.
| Particle | Symbol | Charge | Relative mass |
|---|---|---|---|
| Proton | $p^+$ | +1 | ~1 (1836 electron masses) |
| Neutron | $n^0$ | 0 | ~1 (1838 electron masses) |
| Electron | $e^-$ | –1 | 1 |
Nucleus and mass defect
The nucleus consists of nucleons: protons and neutrons. Despite the tremendous electrostatic repulsion between similarly charged protons, the nucleus does not disintegrate. Its stability is ensured by the strong nuclear force — a special attraction between particles at subminiature distances.
Precise measurements show that the mass of any atom is always slightly less than the sum of the masses of all its individual protons, neutrons, and electrons. This difference is called the mass defect.
Where does the mass go? According to Einstein's formula $E = m \cdot c^2$, this "excess" converts into the huge binding energy released during the formation of the nucleus. In chemical reactions, this mass defect is so negligible (fractions of a milligram per hundreds of thousands of tons of reactants) that the law of conservation of mass is considered absolutely applicable.
Quantum theory and spectra
Electrons in an atom do not behave merely like planets in orbits. They possess both corpuscular and wave properties (capable of diffraction, much like light).
An electron's energy can only take strictly defined (allowed) values. An electron changes its energy exclusively in discrete jumps by absorbing or emitting a photon — a quantum of electromagnetic radiation with energy $E = h\nu$. This is precisely why incandescent gases produce not a continuous spectrum, but a unique line spectrum (a set of dark or bright lines at specific wavelengths), which allows the chemical composition of a substance to be determined with precision.
Isotopes
The type of a chemical element is determined by the number of protons in its nucleus. However, the number of neutrons can vary. Atoms of the same element with different numbers of neutrons are isotopes. A prime example is hydrogen, which has three main isotopes:
- Protium (${}^1_1\text{H}$): 1 proton (no neutrons).
- Deuterium (${}^2_1\text{H}$): 1 proton, 1 neutron.
- Tritium (${}^3_1\text{H}$): 1 proton, 2 neutrons (radioactive).
Briefly: answers to topical questions
What quantum numbers describe the state of an electron in an atom?
The state of an electron in an atom is described by a set of quantum numbers, including the principal quantum number, azimuthal (angular momentum) quantum number, magnetic quantum number, and spin quantum number. The principal quantum number determines the electron's energy. The azimuthal quantum number determines the shape of the atomic orbital and takes integer values from 0 to n - 1. The magnetic quantum number characterizes the spatial orientation of the electron orbital and takes values from -l to +l. The spin quantum number characterizes the intrinsic spin of the electron around its axis and takes values of +1/2 and -1/2.
What are the shapes of s-, p-, and d-electron orbitals?
The azimuthal quantum number determines the shape of the atomic orbital. The value l = 0 corresponds to the s-sublevel, l = 1 to the p-sublevel, and l = 2 to the d-sublevel. The specific geometric shapes of s-, p-, and d-orbitals are not described in the cited sources. For s-, p-, and d-sublevels, respectively, 1, 3, and 5 spatial orientations of orbitals are possible.
What is the Pauli exclusion principle and Hund's rule when filling electron orbitals?
The Pauli exclusion principle defines the maximum number of electrons that can occupy a single orbital, stating that an orbital can hold no more than two electrons with opposite spins, forming an electron pair. Hund's rule states that the lowest-energy (most stable) state of an atom corresponds to the maximum number of electrons occupying degenerate (energy-equivalent) orbitals with parallel spin quantum numbers. This minimizes inter-electron repulsion and lowers the total energy of the system.
How does the quantum-mechanical model of the atom differ from Rutherford's planetary model?
Classical Rutherford's planetary model assumes electrons move along definite trajectories around the nucleus like planets, whereas the quantum-mechanical theory points to the quantum nature of processes and the wave properties of matter. According to quantum theory, the electron energy in an atom is not continuous, but takes only allowed values, changing exclusively in discrete jumps. This fundamental difference between electron motion and planetary motion is confirmed by the existence of line spectra and the wave nature of electrons.
What is the electron configuration of a carbon atom in its ground and excited states?
In its ground state, a carbon atom has two unpaired electrons. In the excited state, one electron transitions to a vacant orbital, resulting in four unpaired electrons. Therefore, carbon exhibits a valency of IV in compounds. The specific electron configuration of carbon in the form of 1s²2s²2p² and its excited state are not specified in the cited sources.
To find the number of neutrons (N) in a nucleus, use a simple rule: subtract the atomic number (Z, lower index) from the mass number (A, upper index). That is: N = A – Z.
Class and exam questions
Do atoms disappear during chemical reactions?
No. During chemical transformations, the atom is neither divided nor destroyed. Only the distribution of electrons between atoms changes, while the nucleus itself always remains unchanged.
Why are the numbers in the atomic mass table not integers?
In nature, elements exist as a mixture of various isotopes. The atomic mass in the periodic table is an averaged value calculated taking into account the mass fractions of the light and heavy isotopes of that element.
Are there nuclei where protons outnumber neutrons?
In the vast majority of cases, neutrons are equal to or more numerous than protons. The only exception among stable nuclides is the helium-3 nucleus (2 protons and 1 neutron).
What else to study on this topic
- History of atomism: from Democritus' hypotheses to J. Dalton's postulates
- Mathematical calculation of isotope mass fractions (using chlorine as an example)
- Operating principle of the mass spectrometer for separating particles by mass
- X-ray diffraction analysis of crystal lattices (Laue method)
Master the entire topic in the "Chemistry" course
This is a summary. The course covers everything tested in classes and exams.
- Complete study guide on "Atomic Structure" and 3,009 more pages of the subject
- Assignments and quizzes after every topic — just like in a colloquium
- AI tutor answers questions on the topic anytime
- 95 video lessons for the subject
Other topics in the section "Chapter 1. Substances. Properties and Structure"
Prepared by WhiteCoat Pro faculty in accordance with the medical school curriculum. For educational purposes only.