Elements Molecules and the Structure of the Atom

Definition of Elements: Elements are considered the fundamental building blocks of all complex substances in the universe.
Diversity and Discovery:
- There are currently $118$ known elements identified by scientists.
- Scientists are actively working to discover additional elements.
- Through various transformations and combinations of these limited basic elements, millions of distinct compounds have been created.
Examples of Elements: Common examples include gold, silver, carbon, oxygen, and helium.
Composition of Matter: All kinds of matter differ due to variations in the specific atoms and molecules that compose them.

Definition of a Molecule: A molecule is a group of two or more atoms held tightly together by chemical bonds or attractive forces.
Smallest Independent Unit: A molecule is the smallest particle of an element or a compound that can exist independently.
Representation of Properties: Molecules exhibit all the physical and chemical properties of the substance they constitute.
Molecules of Elements: These are composed of identical atoms.
- Monatomic Molecules: Some elements consist of molecules made of only one atom. Examples include Argon ( $Ar$ ) and Helium ( $He$ ).
- Diatomic and Polyatomic Molecules: Some elements consist of molecules made of two or more atoms of the same element. For instance, Oxygen ( $O_2$ ), Nitrogen ( $N_2$ ), and Hydrogen ( $H_2$ ) are made of identical atoms.
- Specific Case: A molecule of oxygen is composed of two oxygen atoms and is specifically called a diatomic molecule ( $O_2$ ).
Molecules of Compounds: These consist of two or more different elements chemically bonded together.
- Examples: Examples include Sodium Chloride ( $NaCl$ ), Ammonia ( $NH_3$ ), and Water ( $H_2O$ ).

Fundamental Role: Atoms, along with molecules, are the fundamental building blocks of all matter.
Definition of an Atom: The smallest particle of an element that participates in a chemical reaction without being divided.
Identity and Purity:
- All atoms within a specific element are identical to one another.
- Conversely, atoms of different elements differ from each other. For example, the iron atoms in a nail are identical to each other but are distinctly different from the atoms of copper.
- An element is defined as a pure substance because it contains only one kind of atom.

Critical Exploration Questions:
1. What is the smallest particle of an element?
2. Does an atom consist of smaller particles?
3. Do the atoms of all elements take part in chemical reactions?
4. What makes the atom of one element different from another?

Physical Shape and Size: According to scientific models, an atom is a tiny sphere with a diameter of approximately $10^{-10}\,m$ .
Subatomic Particles: Discovery in the 19th century revealed that atoms are not indivisible but contain smaller subatomic, elementary, or fundamental particles. The three main particles are:
1. Electrons
2. Protons
3. Neutrons
The Nucleus: Protons and inhibited neutrons are located at the center of the atom, forming the nucleus.
Charge: The nucleus is positively charged due to the presence of protons.

Plasma: A state of matter that becomes highly ionized.
Bose-Einstein Condensate (BEC): A state of matter achieved when matter is cooled to approximately $-273\,^{\circ}C$ (which corresponds to $0\,K$ ).