Question

Write formulas for the following anions: (a) fluoride (h) oxide (b) acetate (i) dichromate (c) iodide (j) hydrogen carbonate (d) carbonate (k) phosphate (e) sulfide (1) sulfate (f) nitrate (m) nitride (g) phosphide (n) chloride

Short answer

F-, O^2-, CH3COO- or C2H3O2-, Cr2O7^2-, I-, HCO3-, CO3^2-, PO4^3-, S^2-, SO4^2-, NO3-, N^3-, P^3-, Cl-

Step-by-step solution

- Writing the formula for fluoride

The fluoride anion is derived from fluorine. It gains one electron to achieve a full octet, resulting in the ion having a -1 charge. The formula is written as F-.

- Writing the formula for oxide

The oxide anion comes from oxygen. Oxygen gains two electrons to fill its outer shell, creating an ion with a -2 charge. The formula is expressed as O^2-.

- Writing the formula for acetate

Acetate is a polyatomic anion with the formula CH3COO- or C2H3O2-, where the total charge is -1.

- Writing the formula for dichromate

Dichromate is a polyatomic ion with the chemical formula Cr2O7^2-, where the total charge is -2.

- Writing the formula for iodide

The iodide ion comes from iodine, which gains an electron to achieve a full outer shell. The ion has a -1 charge and the formula is written as I-.

- Writing the formula for hydrogen carbonate

Hydrogen carbonate, also known as bicarbonate, is a polyatomic ion with the formula HCO3-, bearing a -1 charge.

- Writing the formula for carbonate

Carbonate is a polyatomic ion with a formula of CO3^2-, where the overall charge is -2.

- Writing the formula for phosphate

Phosphate is a polyatomic ion with the chemical formula PO4^3-, where the total charge is -3.

- Writing the formula for sulfide

The sulfide anion originates from sulfur. It receives two electrons to fill its valence shell, resulting in an ion with a -2 charge. The formula is S^2-.

- Writing the formula for sulfate

Sulfate is a polyatomic anion with the formula SO4^2-, where the total charge is -2.

- Writing the formula for nitrate

Nitrate is a polyatomic ion with the formula NO3-, carrying a -1 charge.

- Writing the formula for nitride

The nitride ion stems from nitrogen, which acquires three electrons to complete its valence shell. This generates an ion with a -3 charge. Its formula is N^3-.

- Writing the formula for phosphide

The phosphide ion is Phosphorus based, and accepts three electrons to achieve a complete outer shell, with a -3 charge. The formula is P^3-.

- Writing the formula for chloride

The chloride ion is derived from chlorine, gaining one electron to complete its valence shell, creating an anion with a -1 charge. The formula is Cl-.

Key concepts

Anions

Anions are atoms or groups of atoms that have gained one or more extra electrons, giving them a net negative charge. This happens because anions are typically formed when elements with high electron affinities, or a strong desire to gain electrons, come in contact with a source of electrons, like metal atoms during the formation of an ionic bond. In the example of the fluoride anion (F-), the fluorine atom gains an electron to achieve a stable electronic configuration. Other examples from the exercise include oxide (O^2-), iodide (I-), sulfide (S^2-), and chloride (Cl-), all of which have gained electrons corresponding to their needs to fill their valence shell, which is the outermost electron shell.

Understanding anions is fundamental in the study of chemistry, as they are key components of various compounds and play essential roles in chemical reactions, including the formation of salts when they combine with cations, which are positively charged ions.

Polyatomic Ions

Polyatomic ions consist of two or more atoms covalently bonded together, that function as a single ion with a positive or negative charge. Unlike monatomic anions that are formed from a single atom, polyatomic ions contain multiple elements. An illustrative example provided in the exercise is the acetate ion (CH3COO- or C2H3O2-), a common polyatomic anion consisting of carbon, hydrogen, and oxygen atoms. Other examples are the dichromate (Cr2O7^2-), hydrogen carbonate (HCO3-), carbonate (CO3^2-), phosphate (PO4^3-), and sulfate (SO4^2-) ions.

These complex ions play crucial roles in various chemical processes, including buffering solutions to maintain stable pH levels, forming precipitates, and acting as electrolytes in batteries. It is essential for students to get comfortable with recognizing and writing the chemical formulas of polyatomic ions, as they often appear in chemical equations and laboratory experiments.

Valence Electrons

Valence electrons are the electrons in the outermost shell of an atom that are available for bonding with other atoms. These electrons play a pivotal role in determining an element’s chemical properties and reactivity. In simple terms, the number of valence electrons corresponds to an element’s group number on the periodic table for groups 1 to 18. For instance, halogens like fluorine and chlorine have seven valence electrons and typically gain one more electron to form anions with a -1 charge, such as fluoride (F-) and chloride (Cl-).

Having a firm grasp on the concept of valence electrons is crucial for students as it helps to predict how atoms will interact to form chemical bonds, whether they will share electrons to form covalent bonds, or transfer electrons to form ionic bonds. It is also essential for understanding the octet rule, which is a guiding principle in chemistry that highlights the stability of atoms when they have eight valence electrons.

Ionic Charge

The ionic charge of an atom or molecule is an important concept that describes the electric charge an ion possesses due to the loss or gain of electrons. Atoms become cations (positive charge) by losing electrons and anions (negative charge) by gaining electrons. The charge on an ion influences how it will interact with other ions and molecules in a compound. For example, the ionic charge can affect the ratios in which ions combine to form neutral compounds. Sodium (Na), which typically loses one electron to become Na+, will combine with chloride (Cl-), which gains one electron, in a 1:1 ratio to form sodium chloride (NaCl).

Understanding ionic charge is important for students when they learn to balance chemical equations and predict the formulas of ionic compounds. Correct knowledge of an ion's charge ensures that students can correctly write formulas for compounds such as sulfide (S^2-) or phosphate (PO4^3-), which were examples provided in the exercise.