Image description

The image shows the structure and the component parts of an atom, respectively, the central nucleus formed by protons and neutrons, around which the electrons gravitate in orbits.

The nucleus or the center positively charged by the protons, is rendered by a thickened central circle.

In the middle of the circle are round particles of protons, rendered by small circles, cut in the center by a plus, and neutrons, which have no charge, highlighted by small blank circles.

Around the nucleus are illustrated invisible orbits or paths or closed trajectories, to which are attached particles that rotate around the nucleus, marked by thinner circular lines.

On these orbits are electrons with a negative electric charge, rendered by small circles, embossed.

General information

An atom is the smallest constituent unit of ordinary matter that constitutes a chemical element.

Every solid, liquid, gas, and plasma is composed of neutral or ionized atoms.

Atoms are extremely small, typically around 100 picometers across (tenth billionth of a meter).

Atoms do not have well-defined limits and there are different ways to define the size, which each give different values, but close in value.

Every atom is composed of a nucleus and one or more electrons bound to the nucleus.

The nucleus is made of one or more protons and usually a similar number of neutrons. Protons and neutrons are called nucleons. More than 99.94% of an atom’s mass is in the nucleus. The protons have a positive electric charge, the electrons have a negative electric charge, and the neutrons have no electric charge. If the number of protons and electrons are equal, then the atom is electrically neutral. If an atom has more or fewer electrons than protons, then it has an overall negative or positive charge, respectively, and is called an ion. 

The electrons of an atom are attracted to the protons in an atomic nucleus by this electromagnetic force. The protons and neutrons in the nucleus are attracted to each other by another force, called the nuclear force, which is usually stronger than the repelling electromagnetic force that acts between positively charged protons.

The number of protons in the nucleus defines to which chemical element the atom belongs. For example, all copper atoms contain 29 protons. The number of neutrons defines the isotope of the element.

The number of electrons influences the magnetic properties of an atom. Atoms can attach to one or more other atoms through chemical bonds to form chemical compounds, such as molecules.Atoms can attach to one or more other atoms by chemical bonds to form chemical compounds such as molecules. The ability of atoms to associate and dissociate is responsible for most of the physical changes observed in nature, and the discipline that studies that changes is Chemistry.

The Bohr model of the atom, with an electron making instantaneous ,,quantum leaps” from one orbit to another. This model is obsolete.

In 1913, the physicist Niels Bohr proposed a model in which the electrons of an atom were assumed to orbit the nucleus but could only do so in a finite set of orbits, and could jump between these orbits only in discrete changes of energy corresponding to absorption or radiation of a photon.

This quantization was used to explain why the electrons’ orbits are stable  and why elements absorb and emit electromagnetic radiation in discrete spectra.

Later in the same year, Henry Moseley provided additional experimental evidence in favor of Niels Bohr’s theory. These results refined Ernest Rutherford’s and the model of Antonius van den Broek’s model, which proposed that the atom contains in its nucleus a number of positive nuclear charges that is equal to its (atomic) number in the periodic table. Until these experiments, the atomic number was not known to be a physical and experimental quantity. That it is equal to the atomic nuclear charge remains the accepted atomic model today.


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