Electrons are arranged around the nucleus of the atom in orbitals or electron shells. Each orbital can hold a maximum of two electrons.
If you think of an inverted pyramid, the first floor has one room fit for double occupancy, the second floor has four rooms, the third floor has nine rooms, the fourth floor has sixteen rooms etc. Each room has double occupancy capabilities. Remember also, that the orbitals closest to the nucleus have the least energy.
The table below shows an overview of the energy levels, orbitals and number of electrons: Energy LevelOrbitalsNumber of Electrons 112 248 3918 41632
With this information, the total number of electrons can be calculated. As an example, sodium has an atomic number of 11, therefore it has 11 protons in its nucleus and 11 electrons in total. Since the first electron shell can hold 2 electrons, and the second electron shell can hold 8 electrons, there is only 1 valence electron in the third electron shell
This method of finding the number of valence electrons in an atom works well except for the transition metals.
It is also important to realize that if there are not enough electrons to complete the third shell of 18, the shell will fill to 8, and the next electrons will go to the next electron shell.
Look at the following example of potassium. Electrons available in a potassium atom:
The valence or charge of an ion is determined by the number of electrons in its outer shell or orbital Atoms with fewer than half the number of electrons necessary to complete their outer ring tend to lend these electrons to other atoms which have spaces in their outer rings. They have positive charges
Atoms with outer rings more than half full of electrons tend to borrow electrons to fill the vacant spaces. They would acquire negative charges as a result Atoms with few electrons in the outer ring are metals; those with many (more than half) in the outer ring are non metals
Atoms having the outer rings approximately half full or exactly half full show either a negative or positive charge depending on the element with which they combine. These elements are neither typically metals or non metals
Atoms with only one or a few electrons in the outer ring are very active and readily combine with the atoms of other elements The atoms of elements with the outer shells completely full (8 electrons) are very inactive and only rarely combine with other atoms. These are known as “Noble Gases”
An atom with a positive charge is known as a cation At atom with a negative charge is known as an anion.