Physical Science:

Atomic theory is the foundation of modern chemistry and provides a basic understanding of the nature of matter. The theory states that all matter is made up of tiny particles called atoms, which are the smallest units of matter that retain the properties of an element. Atoms are indivisible: Atoms are the smallest units of matter that can exist and cannot be broken down into smaller units. Atoms are identical: Atoms of the same element are identical and have the same number of protons, electrons, and neutrons. Atoms have structure: Atoms have a small central nucleus, which contains positively charged protons and neutral neutrons, surrounded by negatively charged electrons in shells. Atoms are arranged in the periodic table: Atoms of different elements have different numbers of protons, electrons, and neutrons and are arranged in the periodic table according to their atomic number, which is equal to the number of protons in the nucleus. Atoms can bond to form molecules: Atoms can bond together to form molecules by sharing or exchanging electrons. The properties of molecules are determined by the type of atoms and the way they are bonded together. Atoms can be ionized: Atoms can gain or lose electrons to become positively or negatively charged ions. Ions play an important role in many chemical reactions.

Molecule: A molecule is the smallest unit of a substance that retains the chemical properties of that substance and can exist as a separate entity. It is composed of two or more atoms that are chemically bonded together. In a molecule, the atoms are held together by chemical bonds, which are formed by the sharing or exchange of electrons between the atoms. The type of bonds and the arrangement of atoms in a molecule determine the chemical and physical properties of the substance. Molecules can be formed from a single type of atom, such as a molecule of oxygen (\(O_2\)), or from different types of atoms, such as a molecule of water (\(H_2O\)). The number of atoms in a molecule and the type of atoms can also vary, giving rise to a vast array of different molecules, each with its own unique properties. Molecules are the building blocks of matter and play a crucial role in many physical and chemical processes, including chemical reactions, phase transitions, and the formation of materials. A deep understanding of the structure and properties of molecules is essential for many scientific and engineering disciplines, including chemistry, biology, materials science, and environmental science.

Difference between molecule and compound: A molecule is a group of two or more atoms that are chemically bonded together. A compound is a substance made up of two or more different elements that are chemically combined in a fixed ratio. The main difference between a molecule and a compound is that a molecule is a collection of atoms that are bonded together, whereas a compound is a substance that consists of molecules made up of different elements in a fixed ratio. For example, water (\(H_2O\)) is a compound made up of hydrogen and oxygen atoms in a fixed ratio of 2:1. The smallest unit of water that retains its chemical properties is a molecule of water, which is made up of two hydrogen atoms and one oxygen atom chemically bonded together. Another example is table salt (NaCl), which is a compound made up of sodium and chlorine atoms in a fixed ratio of 1:1. The smallest unit of salt that retains its chemical properties is a molecule of salt, which is made up of one sodium atom and one chlorine atom chemically bonded together.

Hydrogen gas is made out of \(H_2\) molecules. It is not a compound. Oxygen gas is made out of \(O_2\) molecules and is also not a compound. They are made out of two atoms of the same element. Similarly, Ozone gas (\(O_3\)), nitrogen gas (\(N_2\)), chlorine substance (\(Cl_2\)) are molecule. However, carbon dioxide gas (\(CO_2\)) is a compound and a molecule both, similarly, amonia (\(NH_3\)) is a compound and a molecule both. Because they are made out of two different elements. All compounds are molecules but all molecules are not compounds.

Some substances are elemental such as iron bar (Fe), liquid mercury (Hg), helium gas (He), gold jewlry (Au), etc., some are molecular such as hydrogen gas (\(H_2\)), oxygen gas (\(O_2\)), ozone gas (\(O_3\)), water (\(H_2O\)), methane (\(CH_4\)), carbon dioxide gas (\(CO_2\)), amonia (\(NH_3\)), diamond (\(C_4\)), glucose (\(C_6H_{12}O_6\)) etc. diamond is a pure form of carbon, but it is not an element. An element is a substance that cannot be broken down into simpler substances by chemical means, while a molecule is a group of two or more atoms that are chemically bonded together.