Calcium reacts with fluorine to form the compound \(\mathrm{CaF}_{2}\). In the reaction, each atom of one of the elements loses two electrons, and each atom of the other element gains one electron. (a) Atoms of which element gain electrons? (b) Atoms of which element lose electrons? (c) What is the charge on the atoms that gain one electron? (d) What is the charge on the atoms that lose two electrons? (e) Write the full atomic symbol, including charges, for each element. Give Ca 20 neutrons and F 10 neutrons.

The octet rule is foundational in understanding how atoms interact in chemical reactions. It states that atoms are generally most stable when they have eight electrons in their valence, or outermost, electron shell. This rule is based on the electron configuration of noble gases, which are inherently stable due to their filled valence shells.

Atoms like calcium (Ca), which have only two valence electrons, reach this stable arrangement by losing electrons. Conversely, atoms like fluorine (F), with seven valence electrons, tend to gain electrons to fill their shell. This exchange of electrons to satisfy the octet rule is a driving force behind the formation of chemical compounds, as seen in the reaction between calcium and fluorine to produce calcium fluoride (CaF₂).

Ion formation occurs when atoms gain or lose electrons, resulting in a charged particle known as an ion. This process is guided by an atom's desire to achieve a full octet and be energetically favored. When an atom loses electrons, it forms a positively charged ion or cation. For example, calcium loses two electrons to form a Ca²⁺ cation.

Alternatively, when an atom gains electrons, it becomes a negatively charged ion or anion, such as the fluorine atom gaining one electron to form an F^- anion. The ionic charges are crucial because they determine the electrostatic attraction between ions, leading to the formation of ionic compounds.

Chemical compounds are substances formed from two or more different elements that are chemically bonded together. In the case of calcium fluoride (CaF₂), calcium ions (Ca²⁺) and fluoride ions (F^-) combine in a 1:2 ratio to balance the total charge to zero, creating a stable compound. This ratio is determined by the charges of the ions, with the Ca²⁺ needing two F^- ions to achieve neutrality.

Compounds can have either ionic or covalent bonds. Ionic bonds occur between metals and nonmetals with significantly different electronegativities and involve the transfer of electrons. Covalent bonds, on the other hand, involve sharing of electrons and typically form between nonmetals.