Question

Which of the following are elements, which are molecules but not compounds, which are compounds but not molecules, and which are both compounds and molecules? (a) \(\mathrm{SO}_{2},(\mathrm{~b}) \mathrm{S}_{8},(\mathrm{c}) \mathrm{Cs},(\mathrm{d}) \mathrm{N}_{2} \mathrm{O}_{5},(\mathrm{e}) \mathrm{O}\) (f) \(\mathrm{O}_{2},(\mathrm{~g}) \mathrm{O}_{3},\) (i) \(\mathrm{KBr}\) (h) \(\mathrm{CH}_{4}\) (j) \(\mathrm{S},(\mathrm{k}) \mathrm{P}_{4},(\mathrm{l}) \mathrm{LiF}\).

Short answer

Elements: Cs, O, S. Molecules but not compounds: S₈, O₂, O₃, P₄. Compounds and molecules: SO₂, N₂O₅, CH₄, KBr, LiF.

Step-by-step solution

Define Terms

Before solving the problem, understand the terms: - Elements: Pure chemical substances consisting of one type of atom, listed on the periodic table. - Molecules: Two or more atoms bonded together. - Compounds: A molecule that contains at least two different elements.

Identify Elements

Look for items in the list that are pure elements, meaning they consist of one type of atom. - (c) Cs: An element. - (e) O: An element. - (j) S: An element.

Identify Molecules but not Compounds

Find molecules that consist of more than one atom but are not composed of different elements. - (b) S₈: A molecule of sulfur atoms only. - (f) O₂: A molecule of oxygen atoms only. - (g) O₃: Ozone, a molecule of oxygen atoms only. - (k) P₄: A molecule of phosphorus atoms only.

Identify Compounds and Molecules

Identify items that are molecules composed of different elements. - (a) SO₂: Sulfur dioxide, consisting of sulfur and oxygen. - (d) N₂O₅: Dinitrogen pentoxide, consisting of nitrogen and oxygen. - (h) CH₄: Methane, consisting of carbon and hydrogen. - (i) KBr: Potassium bromide, consisting of potassium and bromine. - (l) LiF: Lithium fluoride, consisting of lithium and fluorine.

Classify Compounds but not Molecules

Since no compound can exist as a combination of different elements without being a molecule, this category remains empty in conventional chemistry.

Key concepts

Elements

Elements are the simplest chemical substances, each consisting of one type of atom. They are the fundamental building blocks of chemistry. You can find them on the periodic table, where each element is uniquely identified by its atomic number and symbol.
For example, in the exercise:

• Cs (Cesium) is an element because it consists only of cesium atoms.
• O (Oxygen) is an element when it refers to a single oxygen atom.
• S (Sulfur) is also an element because it consists only of sulfur atoms.

Understanding elements is crucial because they combine in numerous ways to form molecules and compounds, which make up all the matter around us.

Molecules

Molecules are chemical structures formed when two or more atoms bond together. They can consist of atoms from the same element or different elements. In simpler terms, a molecule is like a small unit of a substance without necessarily altering its chemical properties. Some molecules are not compounds, as they contain only one type of element.
Consider these examples:

• S₈: A molecule made entirely of sulfur atoms.
• O₂ and O₃ (Ozone): Both are molecules, consisting only of oxygen atoms.
• P₄: A molecule made purely of phosphorus atoms.

A molecule conveys the elemental makeup and can represent various states like liquids, gases, and solids. It's distinct from compounds when consisting only of identical elements.

Compounds

Compounds are a specific type of molecule that consists of atoms from at least two different elements that are chemically bonded. They exhibit distinct properties from the individual elements they are composed of, often resulting in new substances with unique characteristics.
In the mentioned exercise, these are some examples:

• SO₂: Made of sulfur and oxygen, forming sulfur dioxide.
• N₂O₅: A compound of nitrogen and oxygen atoms, forming dinitrogen pentoxide.
• CH₄: A compound consisting of carbon and hydrogen, known as methane.
• KBr and LiF: Composed of potassium and bromine, and lithium and fluorine, respectively.

Compounds are essential because they display diverse chemical behaviors, enabling the wide variety of chemical reactions and phenomena we observe in nature. They are always molecules, combining different elements together.