Question

Write the Lewis symbols that represent the ions in each ionic compound. $$ { a. NaF }\quad \text { b. } \mathrm{CaO} \quad \text { c. SrBr }_{2} \quad \text { d. } \mathrm{K}_{2} \mathrm{O} $$

Short answer

a. Na+ is 'Na' with a plus sign; F- is 'F' surrounded by eight dots with a minus sign. b. Ca2+ is 'Ca' with a 2+ sign; O2- is 'O' surrounded by eight dots with a 2- sign. c. Sr2+ is 'Sr' with a 2+ sign; Br- is 'Br' surrounded by eight dots with a minus sign. d. Each K+ is 'K' with a plus sign; O2- is 'O' surrounded by eight dots with a 2- sign.

Step-by-step solution

Identify the Charge on Each Ion

Determine the charge of the ions based on their position in the periodic table. Group 1 elements (e.g., Na, K) typically form +1 ions, group 2 elements (e.g., Ca, Sr) form +2 ions, halogens (e.g., F, Br) form -1 ions, and O, from group 16, forms a -2 ion.

Write Lewis Symbol for Sodium Ion (Na+)

For NaF: The sodium ion has lost one electron compared to its neutral state. Represent the Na+ ion as the symbol 'Na' with a plus sign to indicate the loss of one electron.

Write Lewis Symbol for Fluoride Ion (F-)

For NaF: The fluoride ion has gained one electron. Represent the F- ion as the symbol 'F' surrounded by eight dots representing its electron configuration and a minus sign indicating the extra electron.

Write Lewis Symbol for Calcium Ion (Ca2+)

For CaO: The calcium ion has lost two electrons. Represent the Ca2+ ion as the symbol 'Ca' with a 2+ sign to indicate the loss of two electrons.

Write Lewis Symbol for Oxide Ion (O2-)

For both CaO and K2O: The oxide ion has gained two electrons. Represent the O2- ion as the symbol 'O' surrounded by eight dots and a 2- sign.

Write Lewis Symbol for Strontium Ion (Sr2+)

For SrBr2: The strontium ion has lost two electrons. Represent the Sr2+ ion as the symbol 'Sr' with a 2+ sign.

Write Lewis Symbol for Bromide Ions (Br-)

For SrBr2: Each bromide ion has gained one electron. Represent each Br- ion as the symbol 'Br' surrounded by eight dots and a minus sign.

Write Lewis Symbols for Potassium Ions (K+)

For K2O: Each potassium ion has lost one electron. Represent each K+ ion as the symbol 'K' with a plus sign.

Key concepts

Ionic Compounds

Ionic compounds are substances formed by the chemical bonding of atoms through the transfer of electrons. They consist of positively charged ions called cations and negatively charged ions called anions. This electrical charge difference leads to a very strong electrostatic force that holds the ions together in a lattice structure, a repeating pattern that maximizes the ionic bond strengths and minimizes repulsion between ions.

For example, in the compound NaF (sodium fluoride), the sodium (Na) atom donates an electron to become a Na+ cation, while the fluorine (F) atom accepts an electron to become a F- anion. The resulting compound, NaF, is held together by ionic bonds between these ions. Ionic compounds have distinct properties: they typically have high melting and boiling points, are solid at room temperature, and can conduct electricity when dissolved in water or melted, as the ions are free to move and carry a current.

Understanding ionic compounds is fundamental to chemistry, as it helps explain how substances are held together and how they'll interact with other chemicals.

Electron Configuration

Electron configuration refers to the distribution of electrons in an atom or ion among the various orbitals around the nucleus. Electrons fill orbitals in a way that minimizes the energy of the atom or ion, which explains why certain elements tend to lose or gain electrons to achieve a more stable electron configuration.

Atoms aim to achieve the electron configuration of the nearest noble gas, which is a state with filled energy levels that grants them maximum stability. When sodium (Na) loses one electron, for instance, it achieves the electron configuration of neon (Ne), a noble gas. Similarly, when fluorine (F) gains an electron, it attains the configuration of neon. For ions, their Lewis symbols reflect these changes: a plus sign for cations indicates fewer electrons than the neutral atom, representing an electron lost, while a minus sign for anions denotes the gain of one electron.

Each electron in a Lewis symbol is represented by a dot surrounding the element's symbol, signaling the arrangement of its outermost electrons after it has formed an ion, known as its valence electrons. This configuration is important because it determines the chemical behavior and reactivity of the ion.

Periodic Table Charge Trends

The periodic table is not just a tool to list elements, but also a map for understanding the trends in properties of atoms, including their tendencies to form specific charges when becoming ions. These trends are predictable based on an element's position in the periodic table.

Elements in Group 1 (the first column) of the periodic table, such as sodium (Na) and potassium (K), typically lose one electron to form +1 ions. Moving to Group 2, like calcium (Ca) and strontium (Sr), atoms generally lose two electrons, resulting in a +2 charge. Conversely, nonmetals on the right-hand side of the periodic table, such as the halogens (fluorine and bromine), tend to gain one electron, becoming -1 ions, while oxygen, from Group 16, gains two electrons to form a 2- ion.

This pattern occurs because atoms 'want' to achieve a full outer shell of electrons, which often matches the electron configuration of the nearest noble gas. Recognizing these charge trends is crucial for predicting the formation of ions and understanding the composition of ionic compounds. When students grasp these trends, they not only learn to predict the charges of ions but also comprehend the underlying reasons for their formation.