ATOM AND MOLECULES
1. Atoms
- Definition: An atom is the smallest unit of matter that retains the properties of an element. Atoms combine to form molecules.
- Structure:
- Nucleus: Located at the center of the atom, the nucleus contains positively charged protons and neutral neutrons.
- Electrons: Negatively charged particles that orbit the nucleus in different energy levels (also called electron shells or orbitals).
- Subatomic Particles:
- Protons: Positively charged, found in the nucleus.
- Neutrons: No charge, found in the nucleus.
- Electrons: Negatively charged, found in shells surrounding the nucleus.
- Atomic Number: The number of protons in an atom’s nucleus. It determines the element (e.g., Hydrogen has 1 proton, Oxygen has 8 protons).
- Mass Number: The total number of protons and neutrons in an atom’s nucleus.
- Isotopes: Atoms of the same element that have the same number of protons but different numbers of neutrons. They have different mass numbers (e.g., Carbon-12 and Carbon-14).
2. Elements
- Definition: A pure substance made of only one type of atom. Examples include Hydrogen (H), Oxygen (O), Carbon (C), etc.
- Periodic Table: A table that organizes all known elements based on their atomic number, electron configuration, and recurring chemical properties.
3. Molecules
- Definition: A molecule is a group of two or more atoms bonded together. Molecules can be made up of atoms of the same element (e.g., O₂) or different elements (e.g., H₂O).
- Types of Molecules:
- Diatomic Molecules: Composed of two atoms, which may be the same (e.g., O₂, N₂) or different (e.g., CO).
- Polyatomic Molecules: Composed of three or more atoms (e.g., H₂O, CO₂, C₆H₁₂O₆).
- Molecular Formula: Represents the number and types of atoms in a molecule. For example, H₂O indicates that water consists of 2 hydrogen atoms and 1 oxygen atom.
4. Chemical Bonds
- Definition: The forces that hold atoms together in molecules. There are three main types of chemical bonds:
- Covalent Bonds: Formed when atoms share electrons. It occurs between nonmetals (e.g., H₂O).
- Ionic Bonds: Formed when electrons are transferred from one atom to another, creating charged ions. This bond occurs between metals and nonmetals (e.g., NaCl).
- Metallic Bonds: Formed by the attraction between metal ions and free electrons in metals (e.g., in copper, iron).
- Polar and Nonpolar Covalent Bonds:
- Polar Covalent Bond: When electrons are shared unevenly between atoms, resulting in a molecule with slight positive and negative ends (e.g., water, H₂O).
- Nonpolar Covalent Bond: When electrons are shared equally between atoms, resulting in a balanced charge distribution (e.g., oxygen, O₂).
5. Chemical Reactions
- Definition: A process where atoms and molecules interact and rearrange to form new substances. Chemical reactions involve the breaking of old bonds and the formation of new ones.
- Reactants and Products:
- Reactants: The starting substances in a chemical reaction.
- Products: The substances formed after the reaction.
6. States of Matter
- Solid: Atoms or molecules are tightly packed and only vibrate in place.
- Liquid: Atoms or molecules are close together but can move around.
- Gas: Atoms or molecules are spread out and move freely.
7. Atomic Theory
- John Dalton (1803): Proposed that atoms are indivisible and that each element is made of atoms that are identical in mass and properties.
- J.J. Thomson (1897): Discovered the electron and proposed the “plum pudding” model of the atom.
- Ernest Rutherford (1911): Discovered the atomic nucleus through his gold foil experiment, leading to the “nuclear” model.
- Niels Bohr (1913): Proposed that electrons orbit the nucleus in specific energy levels.
- Modern Atomic Theory: Describes atoms with a central nucleus made up of protons and neutrons, with electrons in probabilistic orbitals (quantum mechanical model).
- Chemical Symbols: Each element is represented by one or two letters (e.g., H for hydrogen, O for oxygen).
- Chemical Formulas: Express the composition of a substance in terms of the number of atoms of each element. For example:
- H₂O for water (2 hydrogen atoms and 1 oxygen atom).
- NaCl for sodium chloride (1 sodium atom and 1 chlorine atom).
8. Chemical Symbols and Formulas
9. Molecular vs. Empirical Formula
- Molecular Formula: The actual number of atoms of each element in a molecule (e.g., C₆H₁₂O₆ for glucose).
- Empirical Formula: The simplest whole-number ratio of atoms in a compound (e.g., CH₂O for glucose).
- source : https://chatgpt.com/
10. Mole Concept
- Mole: A unit used to measure the amount of substance. One mole of any substance contains approximately 6.022 × 10²³ entities (Avogadro’s number).
- Molar Mass: The mass of one mole of a substance, measured in grams per mole (g/mol).