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The Lewis symbol is a simplistic representation of an element and its valence electrons - those electrons which take part in chemical bonding. For a given element, the Lewis symbol uses dots placed around the chemical symbol to symbolize the number of valence electrons. The chemical behavior of an element is largely determined by its number of valence electrons, which is why the Lewis symbol is a useful tool in predicting how an element might interact with others during a chemical reaction.
To draw a Lewis symbol:

• Identify its valence electrons based on its group number in the periodic table.
• Represent each of these electrons with a dot placed around the element's chemical symbol.
• Spread the dots evenly around the four sides of the symbol just like arranging dots on dice.

For example, arsenic (As) has five valence electrons. Its Lewis symbol is represented by five dots placed around the 'As' symbol. Similarly, selenium (Se) has six valence electrons, so six dots represent its Lewis symbol.

Valence electrons are the electrons that reside in the outermost shell of an atom. They play a crucial role in bonding because they can be lost, gained, or shared with other atoms to achieve a complete outer shell, known as achieving a noble gas configuration.
Valence electrons are critical in determining:

• How atoms bond with each other.
• The chemical properties of elements.
• Most notably, the formation of ions.

For example, arsenic, with five valence electrons, wants to share or gain three more electrons to fulfill its outer shell, whereas selenium, with six valence electrons, wants to gain or share two electrons.
Understanding the number of valence electrons helps to predict the common charges that elements form as ions during chemical reactions. This forms the basis of many chemical bonding theories.

The As鲁鈦� ion is formed when a neutral arsenic atom loses three of its valence electrons. In its neutral state, arsenic has five valence electrons, so losing three leaves it with only two remaining. This loss of electrons results in a positive charge of +3.
The electron configuration changes because the ion loses electrons primarily from the highest energy level:

• Atoms will lose electrons starting with their outermost electrons, typically from the highest occupied subshell, to form cations.
• For As, electrons are removed from the 4p subshell, reducing electron configuration stability.

Therefore, the electron configuration for As鲁鈦� is 1s虏 2s虏 2p鈦� 3s虏 3p鈦� 4s虏 3d鹿鈦�, representing all shells filled to just below the 4p level.

The Se虏鈦� ion occurs when a selenium atom gains two additional electrons to complete its outer shell, forming a full valence shell similar to that of a noble gas. Selenium normally has six valence electrons. By acquiring two more, Se虏鈦� reaches a more stable electron configuration.
With the gain of these electrons:

• The electron configuration becomes 1s虏 2s虏 2p鈦� 3s虏 3p鈦� 4s虏 3d鹿鈦� 4p鈦�.
• This configuration fills the 4p subshell, exhibiting a highly stable arrangement corresponding to krypton's configuration.

The Se虏鈦� ion is notable for achieving complete stability with a full octet through this additional electronic influence, which is critical in forming stable ionic compounds.