Structure atomique

Atoms consist of protons, neutrons, and electrons. Protons and neutrons cluster in the nucleus, each contributing one atomic mass unit to atomic mass. Protons carry a positive charge, neutrons are uncharged, and electrons are negatively charged particles that orbit the nucleus in a cloud, contributing negligible mass.

The number of protons in an atom equals the atomic number and uniquely identifies the element. All atoms of a given element contain the same number of protons, making this the defining characteristic that distinguishes one element from another. This fundamental property determines all of an element's chemical and physical properties.

A neutral atom contains an equal number of protons and electrons. Since protons carry positive charges and electrons carry negative charges, these opposite charges cancel each other out, resulting in no net electrical charge. This balance is essential for atomic stability and chemical behavior.

The nucleus is a dense central region containing protons and neutrons clustered together. Protons are positively charged, and neutrons are uncharged, giving the nucleus a net positive charge. This concentrated mass occupies only a tiny fraction of the atom's total volume despite containing most of its mass.

Electrons have negligible mass, so they contribute almost nothing to an atom's atomic mass. However, their negative charge is crucial for determining overall atomic charge. In a neutral atom, electrons balance the positive charge of protons, while in ions, unequal numbers of electrons and protons create a net charge.

The electron cloud is a region of mostly empty space surrounding the nucleus where negatively charged electrons are attracted and orbit. This cloud makes up the vast majority of an atom's volume despite containing very little mass. The electrons' positions within this cloud determine how atoms interact and form chemical bonds.

J.J. Thomson discovered electrons and proposed the plum pudding model. Ernest Rutherford's experiments revealed that most atomic mass concentrates in the nucleus, disproving Thomson's model. James Chadwick later discovered neutrons in 1932, completing our understanding of atoms' three fundamental subatomic particles and their arrangement.