Atoms, Elements, Compounds, and Molecules

Structure of an Atom 🏗️

Definition: An atom is the smallest particle of an element that can participate in a chemical reaction. Atoms are electrically neutral due to an equal number of protons and electrons. ⚖️

Fundamental Particles of an Atom

• Electron: Negatively charged with a charge of -1; found in shells around the nucleus, with a mass of approximately 0 atomic mass units (a.m.u). ⚡
• Proton: Positively charged with a charge of +1; located in the nucleus, with a mass of 1 a.m.u. 🔴
• Neutron: Neutrally charged with no charge; located in the nucleus, with a mass of 1 a.m.u. ⚪

Example Calculation

An atom of sodium has 11 protons and 12 neutrons. Calculate its mass number. 🧮

Solution:
A = Z + N = 11 + 12 = 23

Electron shells can accommodate electrons according to the formula 2n², where n is the shell number.

A nuclide is represented with the atomic number and mass number in the format:

Nuclide Notation: A = mass number, X = element symbol, Z = atomic number.

Isotopes

Isotopes are atoms of the same element with the same number of protons but different numbers of neutrons (and mass numbers). ⚖️

Examples of Isotopes

• Carbon:
  Carbon-12: p = 6, n = 6
  Carbon-13: p = 6, n = 7
  Carbon-14: p = 6, n = 8
• Hydrogen:
  Hydrogen-1 (H): p = 1, n = 0
  Deuterium (D): p = 1, n = 1
  Tritium (T): p = 1, n = 2

Applications of Isotopes

Isotopes are used in various applications such as:

• Radioisotopes: Used in cancer treatment, e.g., Cobalt-60. 💉
• Carbon-14: Used in radiocarbon dating to determine the age of fossils. ⏳
• Uranium-235: Used in nuclear energy and weapon manufacturing. ⚛️

Definition: The relative atomic mass of an element is the weighted average mass of the isotopes of that element, relative to 1/12th the mass of a carbon-12 atom.

Example Calculation

Problem: Chlorine has two isotopes, Cl-35 (75% abundance) and Cl-37 (25% abundance). Calculate the relative atomic mass of chlorine. 🧮

Solution:

• Cl-35: 35 × 0.75 = 26.25
• Cl-37: 37 × 0.25 = 9.25
• Total = 26.25 + 9.25 = 35.5

Definition of Bonding

Bonding: Chemical bonding is the process of atoms joining together through forces of attraction to form compounds. Only outermost shell electrons participate in bonding. ⚡

Atoms bond to achieve stability, usually by obtaining a full outer electron shell similar to noble gases. 🌌

Types of Chemical Bonds

• Ionic Bonding: Involves the transfer of electrons from a metal to a non-metal. ⚡
• Covalent Bonding: Involves the sharing of electrons between non-metal atoms. 🤝
• Metallic Bonding: Occurs between metal atoms, with a "sea of electrons" that allows metals to conduct electricity. ⚡

Noble gases have complete outermost electron shells, making them stable. Other atoms form bonds to reach this stable configuration, following the octet rule (eight electrons in the outer shell). 🔢

Examples of Noble Gases

Alternative Term: Electrovalent Bonding

Ionic bonding involves the transfer of electrons, typically from a metal to a non-metal, resulting in the formation of positively and negatively charged ions. ⚖️

Formation of an Ionic Bond

Example: Formation of Sodium Chloride (NaCl) 🧂

• Sodium Atom (Na): Loses one electron to become Na⁺.
• Chlorine Atom (Cl): Gains one electron to become Cl^-.
• The oppositely charged ions attract each other to form NaCl. 🧂

Characteristics of Ionic Compounds

• Composed of positive and negative ions. ⚖️
• High melting and boiling points. 🔥
• Soluble in water but insoluble in organic solvents. 💧
• Conductive in aqueous or molten form, due to free-moving ions. ⚡

Alternative Term: Molecular Bonding

Covalent bonding involves the sharing of electrons between non-metal atoms, allowing each atom to complete its outer shell and achieve stability. 🌌

Examples of Covalent Compounds

• Hydrogen (H₂): Two hydrogen atoms share one pair of electrons. 🤝
• Water (H₂O): Each hydrogen atom shares one electron with oxygen, forming a molecule. 💧
• Carbon Dioxide (CO₂): Carbon shares two pairs of electrons with each oxygen atom. 🌍

Characteristics of Covalent Compounds

• Low melting and boiling points due to weak intermolecular forces. ❄️
• Generally insoluble in water, but soluble in organic solvents. 💧
• Non-conductive in both solid and solution forms. 🚫

Definition: Metallic bonding is the force of attraction between positively charged metal ions and the sea of delocalized electrons surrounding them. 🌊

Characteristics of Metallic Bonding

• High melting and boiling points. 🔥
• Good electrical and thermal conductivity due to free-moving electrons. ⚡
• Malleable and ductile as atoms can slide over each other without breaking the metallic bond. 🔄

Formation Examples

Formation of Sodium Chloride (NaCl)

In the formation of sodium chloride, a sodium atom loses one electron to become a sodium ion (Na⁺), while a chlorine atom gains this electron to become a chloride ion (Cl^-). The Na⁺ and Cl^- ions attract each other to form the compound NaCl. 🧂

Formation of Magnesium Oxide (MgO)

A magnesium atom loses two electrons to form a Mg²⁺ ion, and an oxygen atom gains these two electrons to form an O^2- ion. The oppositely charged ions combine to form MgO. ⚖️

Definition: Valency is the number of electrons an atom loses, gains, or shares to attain a stable structure. ⚖️

Valencies of Some Common Elements

Steps for Writing Chemical Formulas

• Write the symbols of the elements or radicals. ✏️
• Determine the valency of each element or radical. 🔢
• Swap the valencies to write the formula. 🔄

Examples of Chemical Formulas

• Sodium Chloride: NaCl 🧂
• Potassium Carbonate: K₂CO₃ 🧪
• Ammonium Sulfate: (NH₄)₂SO₄ 🧪

Balancing a chemical equation involves making the number of atoms of each element equal on both sides of the equation. 🔄

Example Balancing Equations

• Combustion of Hydrogen: 2H₂ + O₂ → 2H₂O 🔥
• Formation of Magnesium Oxide: 2Mg + O₂ → 2MgO 🔥

Definition: Ionic equations show only the ions involved in a chemical reaction, omitting spectator ions. 🔄

Steps to Write Ionic Equations

• Construct a balanced chemical equation. ⚖️
• Split soluble ionic compounds into ions. 🔄
• Cancel out spectator ions that appear on both sides. 🚫

Example of an Ionic Equation

Reaction: AgNO₃ + NaCl → AgCl + NaNO₃

Ionic Equation: Ag⁺ + Cl^- → AgCl ⚡

A radical is a group of atoms that behaves as a single unit with a specific charge. Radicals often participate in reactions and can combine with other elements or compounds to form new substances. 🔄

Common Radicals and Their Valencies

Types of Chemical Equations

• Word Equations: Describe reactions in words (e.g., Magnesium + Oxygen → Magnesium Oxide). 📜
• Symbol Equations: Represent reactions using chemical symbols (e.g., Mg + O₂ → MgO). ⚗️

State Symbols in Equations

State symbols indicate the physical state of substances in a reaction:

• (s) - Solid ❄️
• (l) - Liquid 💧
• (g) - Gas 🌬️
• (aq) - Aqueous (dissolved in water) 💧

Writing Balanced Chemical Equations

Balancing a chemical equation means ensuring the same number of each type of atom on both sides of the reaction. This is achieved by adjusting coefficients without changing subscripts within formulas. ⚖️

Examples of Balanced Chemical Equations

• Mercury Oxide Decomposition: 2HgO(s) → 2Hg(l) + O₂(g) 🔥
• Combustion of Methane: CH₄ + 2O₂ → CO₂ + 2H₂O 🔥

Main Types of Reactions

• Synthesis Reaction: Two or more substances combine to form a single compound.
  Example: A + B → AB
• Decomposition Reaction: A single compound breaks down into two or more simpler substances.
  Example: AB → A + B
• Single Displacement Reaction: One element replaces another in a compound.
  Example: A + BC → AC + B
• Double Displacement Reaction: The ions of two compounds exchange places to form two new compounds.
  Example: AB + CD → AD + CB
• Combustion Reaction: A substance combines with oxygen, releasing energy as heat and light.
  Example: C_xH_y + O₂ → CO₂ + H₂O

Examples of Chemical Reactions

• Synthesis: 2H₂ + O₂ → 2H₂O 🔄
• Decomposition: 2HgO → 2Hg + O₂ 🔄
• Single Displacement: Zn + 2HCl → ZnCl₂ + H₂ 🔄
• Double Displacement: AgNO₃ + NaCl → AgCl + NaNO₃ 🔄
• Combustion: CH₄ + 2O₂ → CO₂ + 2H₂O 🔥