describe the relationship between an element's placement in the periodic table and the number of electrons in its outermost energy level
The name for the electrons on the outer-most energy level is referred to as valence electrons. When talking about the Group A (Groups 1,2, and 13-18) elements, they increase from left-to-right across the periodic table. These generally don't apply to Group B (groups 3-12), or transition metals. Also, it is key to note Helium, which is on the far right in Group 8A (18),has only 2 valence electrons.
The relationship between an element's placement in the periodic table and the number of electrons in its outermost energy level can be explained by the concept of the periodic law. The periodic table is organized based on the increasing atomic number of elements, which represents the number of protons in an atom's nucleus.
The outermost energy level of an atom is called the valence shell or valence energy level. The number of electrons in the valence shell determines an element's chemical properties and how it interacts with other elements to form compounds.
In general, elements in the same column, or group, of the periodic table have similar properties because they have the same number of electrons in their outermost energy level. For example, all elements in Group 1 (the alkali metals) have one valence electron, while elements in Group 17 (the halogens) have seven valence electrons.
Moving across a row, or period, of the periodic table, the number of valence electrons increases by one with each element. So, elements in the same period have the same number of energy levels but different numbers of valence electrons.
For instance, elements in the second period (row) of the periodic table have two energy levels, and their outermost energy level can hold a maximum of eight electrons. The element with the lowest atomic number in the second period, lithium (Li) has three valence electrons, while the element at the end of the period, neon (Ne), has eight valence electrons.
Therefore, an element's placement in the periodic table directly correlates with the number of electrons in its outermost energy level, and this arrangement allows us to predict an element's chemical behavior.