Atomic Theory: From Dalton to Bohr and Beyond
Dalton’s Atomic Theory
Dalton’s atomic theory states that:
- All matter is made up of atoms.
- Atoms are indivisible and indestructible.
- All atoms of a given element are identical in mass and properties.
- Compounds are formed by a combination of two or more different kinds of atoms.
- A chemical reaction is a rearrangement of atoms.
Atoms
Atoms are the smallest particles of everything. There are three basic parts of an atom: electrons, protons, and neutrons. The thing that makes each of those elements different is the number of electrons, protons, and neutrons. The protons and neutrons are always in the center of an atom. Scientists call the center of the atom the nucleus. The electrons are always found whizzing around the center in areas called orbitals. The electron always has a negative charge (-), and the proton always has a positive charge (+). If the charge of an entire atom is 0, that means there are equal numbers of negative and positive pieces, equal numbers of electrons and protons. The third particle is the neutron. It has a neutral charge, so a charge of zero.
Thomson Model
In the Thomson model, the atom is composed of electrons (which Thomson still called “corpuscles”) surrounded by a soup of positive charge to balance the electrons’ negative charges, like negatively charged “plums” surrounded by positively charged “pudding”.
Rutherford Model
Rutherford’s concentration of most of the atom’s mass into a very small core made a planetary model an even more likely metaphor than before, as such a core would contain most of the atom’s mass, in an analogous way to the Sun containing most of the solar system’s mass. Rutherford’s model was later corrected by Bohr.
The Bohr Atom Model
In the Bohr model, the neutrons and protons occupy a dense central region called the nucleus, and the electrons orbit the nucleus much like planets orbiting the Sun, although the orbits are not confined to a plane as is approximately true in the Solar System.
Elemental Notation
Every atom of a particular element has the same number of protons. However, a particular element may have several different types of atoms, with different numbers of neutrons. The atomic number represents the number of protons in the atom’s nucleus. The mass number represents the total number of protons and neutrons in an atom’s nucleus.
Isotopes
Isotopes are atoms with the same atomic number but different mass numbers.
Ions
Ions are atoms with either extra electrons or missing electrons.
Cation
A cation is an atom that loses electrons.
Anion
An anion is an atom that gains an electron.