Elements, atoms, compounds

Atoms

• The basic building blocks of matter.

• Consist of protons, neutrons, and electrons.

  • Protons: Positively charged particles in the nucleus.

  • Neutrons: Neutral particles in the nucleus.

  • Electrons: Negatively charged particles orbiting the nucleus in shells or energy levels.

• Atoms are electrically neutral because they have an equal number of protons and electrons.

Elements

• A substance made of only one type of atom. All atoms of an element have the same number of protons (atomic number).

• Elements are represented by chemical symbols (e.g., H for hydrogen, O for oxygen, Na for sodium).

• Organized in the Periodic Table based on their atomic number and chemical properties.

  • Metals: Typically shiny, malleable, ductile, and good conductors of heat and electricity. Located on the left side of the Periodic Table.

  • Non-metals: Typically dull, brittle, and poor conductors of heat and electricity. Located on the right side of the Periodic Table.

  • Metalloids: Have properties of both metals and non-metals. Located along the staircase line on the Periodic Table.

• Key groups in the Periodic Table:

  • Group 1: Alkali Metals: Highly reactive metals.

  • Group 2: Alkaline Earth Metals: Reactive metals.

  • Group 7: Halogens: Highly reactive non-metals.

  • Group 0/8: Noble Gases: Inert (unreactive) gases due to having a full outer electron shell.

Compounds

• A substance made up of two or more different elements chemically combined in a fixed ratio.

• Compounds have different properties from their constituent elements.

• Chemical formulas represent compounds (e.g., H_2O for water, NaCl for sodium chloride).

Types of Compounds

1. Ionic Compounds

   • Formed by the transfer of electrons from a metal to a non-metal.

   • Consist of ions: charged particles (positive cations and negative anions).

   • Held together by strong electrostatic forces.

   • Typically have high melting and boiling points, and conduct electricity when molten or dissolved in water.

   • Example: Sodium Chloride (NaCl)

2. Covalent Compounds

   • Formed by the sharing of electrons between two non-metals.

   • Consist of molecules: groups of atoms held together by covalent bonds.

   • Typically have low melting and boiling points, and do not conduct electricity.

   • Example: Water (H_2O)

Chemical Formulas

• Show the number and type of atoms in a molecule or compound.

  • Example: H_2O indicates two hydrogen atoms and one oxygen atom.

• Empirical Formula: The simplest whole-number ratio of atoms in a compound.

• Molecular Formula: The actual number of atoms of each element in a molecule.

Chemical Equations

• Represent chemical reactions using chemical formulas.

• Must be balanced to satisfy the Law of Conservation of Mass (number of atoms of each element must be the same on both sides of the equation).

• Example: 2H2 + O2 \rightarrow 2H_2O

Key Concepts for IGCSE Chemistry

1. Atomic Structure

   • Nucleus contains protons and neutrons.

   • Electrons orbit the nucleus in specific energy levels or shells.

   • The number of electrons in the outermost shell determines the chemical properties of an element.

   • Electronic configuration: arrangement of electrons in energy levels (e.g., sodium: 2,8,1).

2. Ions

   • Atoms gain or lose electrons to form ions.

   • Metals typically lose electrons to form positive ions (cations).

   • Non-metals typically gain electrons to form negative ions (anions).

3. Isotopes

   • Atoms of the same element with the same number of protons but different numbers of neutrons.

   • Isotopes have the same chemical properties but different physical properties (e.g., mass).

   • Relative atomic mass (Ar): the weighted average mass of