Bohr Rutherford Diagrams
Electrons are arranged in shells or orbitals around the nucleus. The placement of the
electrons is determined as follows:
The first (inner shell) can hold up to 2 electrons
The second shell can hold up to 8 electrons
The third shell can hold up to 18 electrons * However, when it contains 8 electrons, the
electrons then fill into the 4th shell first
The fourth shell can hold up to 32 electrons
Bohr model
Modified on the Rutherford model, electrons travel in fixed pathways (energy levels)
around the nucleus and contain a certain number of electrons in each energy level
The nucleus is represented by a circle with a P or N, the electrons are outside in energy
levels
The pattern of electrons in the energy levels is 2,8,8,2 for the first twenty elements
Li
2
1e-
Rb
2
8
18
8
Br
2
8
18
7e-
1e-
Before placing electrons in the next shell, the previous shell must first be full. Atoms are
stable when their shells are full, this is called stable octet. The electrons in the last shell
are called valence electrons. Valence electrons are the electrons that are involved in
bonding.
Atoms will gain or loss electrons to make their outer shell full, the atom becomes
charged and is called an ion. An atom that loses an electron becomes positively charged
(cation), if an atom gains an electron is becomes negatively charged (anion).