Question

Which of the following are considered compounds (as opposed to elements)? a. $\mathrm{He}$ b. ${\mathrm{F}}_2$ c. $\mathrm{HCl}$ d. ${\mathrm{S}}_8$

Short answer

Only $\mathrm{HCl}$ is a compound, as it consists of two different elements, hydrogen and chlorine, combined chemically. The others, $\mathrm{He}$, ${\mathrm{F}}_2$, and ${\mathrm{S}}_8$, are all elements, as they consist of only one type of atom.

Step-by-step solution

Understand Elements and Compounds

Elements are substances made of only one type of atom. Compounds are formed from two or more different elements that are chemically combined.

a. $\mathrm{He}$

Helium ($\mathrm{He}$) is an element because it consists of only one type of atom – helium.

b. ${\mathrm{F}}_2$

Fluorine (${\mathrm{F}}_2$) is an element. Although there are two atoms of fluorine bonded together, they are still the same type of atom, thus making it an element.

c. $\mathrm{HCl}$

Hydrochloric acid ($\mathrm{HCl}$) is a compound because it is formed from two different elements - hydrogen ($\mathrm{H}$) and chlorine ($\mathrm{Cl}$) - that have chemically combined.

d. ${\mathrm{S}}_8$

Sulfur (${\mathrm{S}}_8$) is an element. Similar to ${\mathrm{F}}_2$, there are multiple atoms of sulfur bonded together, but since they are all sulfur atoms, it is considered an element.

Conclusion

Out of the given options, only $\mathrm{HCl}$ is a compound, while the rest are elements.

Key concepts

Chemical Bonding

Chemical bonding is the process where atoms combine to form more complex structures, like compounds. There are different types of bonds that hold these atoms together: typically, covalent and ionic bonds dominate the scene.
In covalent bonding, atoms share electrons to achieve stability or a noble gas configuration. For example, in $\mathrm{HCl}(Hydrochloricacid)$, hydrogen and chlorine atoms share electrons. This sharing creates a covalent bond that holds them together.
Ionic bonding is slightly different. It occurs when one atom donates an electron to another, creating charged ions that attract each other. Although $\mathrm{HCl}$ is covalent, ionic bonds are common in many different compounds.

• Atoms join by sharing or transferring electrons.
• This process helps in achieving stability.
• Both covalent and ionic bonds are essential in forming compounds.

Understanding these bonds is crucial to know how compounds form and behave.

Atomic Structure

The atomic structure is fundamental to understanding chemistry. Each atom comprises three basic particles: protons, neutrons, and electrons. Protons and neutrons reside in the nucleus, the atom's core, while electrons orbit this nucleus.

• Protons are positively charged.
• Neutrons have no charge.
• Electrons are negatively charged and orbit around the nucleus.

Each element is defined by the number of protons in its atoms, known as the atomic number. For instance, hydrogen has one proton. Atoms of the same element will share this atomic number.
Elements combine to form compounds when atoms rearrange and bond in pursuit of achieving a more stable configuration. Often, they try to attain a complete outer electron shell, similar to noble gases, which is why understanding atomic structure is key in grasping how elements interact and form compounds like $\mathrm{HCl}$.

Chemical Compounds

Chemical compounds are formed when two or more different elements chemically bond together. This differentiates them from elements, which consist of only one type of atom. A compound always contains at least two different types of atoms.
For example, hydrochloric acid $\mathrm{HCl}$, mentioned in the original problem, is a compound because it consists of hydrogen and chlorine atoms bonded together. Unlike pure elements like helium $\mathrm{He}$ or sulphur ${\mathrm{S}}_8$, compounds have unique chemical properties distinct from their constituent elements.

• Compounds result from chemical reactions between different elements.
• They have distinct physical and chemical properties.
• Formulas like $\mathrm{HCl}$ express the ratio and types of atoms in the compound.

Understanding the concept of chemical compounds helps explain how substances form and function naturally or in industrial applications.