History of the knowledge of the atom 3º ESO

Dalton´s atomic theory (1808) Matter is made of indivisible particles called atoms (we now know that atoms are not indivisible). Elements consist of the same kind of atoms, atoms that are exactly the same (we now know that there are atoms of the same element that are different, those are the isotopes). Atoms of different elements are different (they have different mass and properties). Chemical compounds are made up of molecules, in which atoms are combined according to a fix whole-number ratio. Atom is the Greek word for “indivisible”

It was soon discovered that atoms are not indivisible but are formed by smaller particles. The first particle that was discovered was the electron, a particle with a negative charge. As the atom was not indivisible new models of the structure of an atom were needed.

Thomson´s atomic model (1904) Atoms can be considered as uniform positively charged spheres in which electrons are embedded. The amount of positive and negative charge is always the same so that the atom is electrically balanced. Thomson´s model is also known as the plum pudding model.

Thomson´s atomic model (1904)

Rutherford´s experiment (1911) It consisted of shooting alpha particles (which are positively charged) at a thin gold foil. Some of the particles went through the gold foil without diverting, some of them were slightly diverted and some were strongly diverted. This experiment showed that the atom was not homogeneous.

Rutherford´s experiment (1911)

Rutherford´s atomic model (1911) Rutherford explained the results of his experiment proposing a new atom model: Atoms are divided into a tiny core, called nucleus, and an electron shell. Between the nucleus and the electron shell there is empty space. Most of the atom is empty space. The nucleus of an atom is positively charged and contains almost all the mass of the atom. We know now that it is composed of protons and neutrons. Electrons orbit around the nucleus in circular orbits. The nucleus holds the electrons by means of an electric force between positive and negative charges.

Rutherford´s atomic model (1911)

Rutherford´s atomic model (1911)

Rutherford´s experiment (1911) https://phet.colorado.edu/en/simulation/rutherford-scattering

Bohr´s atomic model (1913) Electrons orbit around the nucleus in different layers. In each layer there is a maximum number of electrons: In the first layer there are 2 electrons at most, 8 in the second layer, 18 in the third layer... In the layer n there are 2n2 electrons at most. The energy and the radius of each orbit is quantized, that means it has a specific value (it can´t be any number). Each layer has different energy. Inner layers have lower energy and outer layers have higher energy. An electron can absorb energy and go to a higher layer. Or it can go to a lower layer and release energy. As atoms absorb or release energy in the form of light, absorption and emission spectra of the elements are explained. The spectrum of each element is specific and unique for that element.

Bohr´s atomic model (1913)

Bohr´s atomic model (1913)

Particles of the atom In the nucleus: protons (discovered in 1919) and neutrons (discovered in 1932). In the shell, orbiting around the nucleus: electrons (discovered in 1897).

Current atomic model We don´t talk about orbits, we talk about orbitals: regions of space where the probability of finding an electron is very high. Each orbital is defined by a number (1, 2, 3, 4, 5...), which indicates the layer, and a letter (s, p, d or f), which indicates the shape of the orbital.

Current atomic model