Electron arrangement of Atoms

• Electrons are arranged or configured in a specific way that follows a pattern.

• The total number of electrons must equal the atomic number of the element.

• We can use the period (row) of the Periodic Table that an element is in to determine the number of electron shells or energy levels. Period 1 elements have 1 shell or energy level of electrons. Period 2 elements have 2 shells or energy levels of electrons. Period 3 elements have 3 shells or energy levels of electrons. Period 4 elements have 4 shells or energy levels of electrons.

• We can use the Group number that an element is in to determine the number of outer shell or valence electrons. Group 1 elements have 1 valence electron, Group 2 elements have 2 valence electrons, Group 13 elements have 3 valence electrons, Group 14 elements have 4 valence electrons, Group 15 elements have 5 valence electrons, Group 16 have 6 valence electrons, Group 17 elements have 7 valence electrons, and Group 18 have a full shell or energy level of electrons (usually 8, but helium, He has 2 electrons in its valence shell).

• Shells or energy levels are filled from the inner shell first. This inner shell can hold a maximum or 2 electrons.

• Once the inner shell or energy level is filled then the first shell or energy level is filled next. It can hold a maximum of 8 electrons.

• The 3rd & 4th shells or energy levels are filled in turn. The 3rd shell or energy level is full when it has 8 electrons.

• When we write the electron configuration or arrangement there is a convention to follow.

Examples of electron configurations or atoms

Li (2, 1) this shows that a lithium atom has 2shells with 2 electrons in the inner shell and 1 valance electron.

Na (2, 8, 1) this shows that a sodium atom has 3 shells with 2 electrons in the inner shell, 8 electrons in the second shell/energy level and 1 valance electron.

Cl (2, 8, 7) this shows that a chlorine atom has 3 shells with 2 electrons in the inner shell, 8 electrons in the second shell/energy level and 7 valance electrons.

Ca (2, 8, 8, 2) this shows that a calcium atom has 4 shells with 2 electrons in the inner shell, 8 electrons in the second and third shells/energy levels and 2 valance electrons.

The representations below show the electron configuration (arrangement) of the first 20 elements.

Video - Electron Configuration DIagrams