2 - Atoms, elements and compounds

Elements, Compounds and Mixtures

Definitions

• Element:

  • A pure substance made up of atoms that cannot be broken down further by chemical means.

  • Defined by its atomic number, corresponding to the number of protons in its nucleus.

• Compound:

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

  • Exhibits unique properties that differ from the individual elements.

• Mixture:

  • A combination of two or more substances where each retains its individual properties.

  • Can be categorized as homogeneous (uniform composition) or heterogeneous (distinct, separate components).

Chemical Terms

• Ion:

  • A charged particle formed when an atom loses or gains one or more electrons.

  • Can be cations (positively charged) or anions (negatively charged).

• Molecule:

  • A group of atoms bonded together, representing the smallest unit of a chemical compound that retains its chemical properties.

  • Can consist of atoms of the same or different elements.

• Atom:

  • The smallest unit of an element that retains its chemical properties, made up of three subatomic particles:

    • Protons: Positively charged particles in the nucleus.

    • Neutrons: Neutral particles also located in the nucleus.

    • Electrons: Negatively charged particles orbiting the nucleus.

Atomic Structure

• Atoms contain a dense nucleus made up of protons and neutrons, surrounded by a cloud of electrons in quantized energy levels.

Atomic Composition

• Proton/Atomic Number:

  • The number of protons in an atom's nucleus, determining the element's identity and periodic table position.

• Mass/Nucleon Number:

  • The total count of protons and neutrons in the nucleus, which helps in estimating the atom's mass.

  • Mass number = protons + neutrons; protons and neutrons have much greater mass than electrons.

• Stable Atom:

  • In a neutral atom, the number of protons equals the number of electrons, resulting in electrical neutrality.

• Isotopes:

  • Variants of an element with the same number of protons but a different number of neutrons, affecting atomic mass but maintaining chemical behavior due to the same valence electrons.

• Relative Atomic Mass (Ar):

  • The weighted average mass of an element's isotopes relative to 1/12 of the carbon-12 atom.

• Relative Molecular Mass (Mr):

  • The sum of the relative atomic masses of all atoms in a molecule, indicating the molecule's mass relative to carbon-12.

Bonding Types

Definition of Bonding

• The process of forming a chemical bond between two or more atoms, molecules, or ions, crucial for determining substance properties.

Ionic Bonding

• Strong electrostatic attraction occurs between positively charged cations and negatively charged anions.

• Typically forms between metals (which lose electrons) and non-metals (which gain electrons).

• Results in the formation of an ionic lattice, characterized by a regular arrangement of ions.

Properties of Ionic Compounds

• High Melting and Boiling Points:

  • Strong electrostatic forces cause ionic compounds to be solid at room temperature.

• Electrical Conductivity:

  • Able to conduct electricity when dissolved in water (aqueous) or melted due to free-moving ions.

Covalent Bonding

• Definition:

  • Formed by the sharing of electron pairs between atoms, mainly seen in non-metals.

• Covalent Bonds:

  • Can be single (one pair of shared electrons) or double (two pairs of shared electrons).

Properties of Covalent Compounds

• Generally do not conduct electricity due to the lack of charged particles.

• Often exist as gases or liquids with lower melting and boiling points due to weak intermolecular forces.

Allotropes of Carbon

• Definition:

  • Different structural forms of the same element existing in the same physical state.

Examples:

• Diamond:

  • Each carbon atom is covalently bonded to four others in a three-dimensional lattice.

  • Results in extreme hardness and a very high melting point; does not conduct electricity.

• Graphite:

  • Each carbon atom is bonded to three others in layers with delocalized electrons.

  • Allows for electrical conductivity, with layers that can slide over each other, making it useful in lubricants and pencil leads.

Metallic Bonding

• Description:

  • Characterized by the attraction between positive ions in a giant metallic lattice and a sea of delocalized electrons.

• Characteristics:

  • Exhibits good electrical and thermal conductivity due to mobile electrons.

  • High melting and boiling points, malleable (shaped) and ductile (drawn into wires) due to its layered structure.

Forces within Molecules

• Intramolecular Forces:

  • Forces that hold the atoms together within a molecule, including covalent and ionic bonds.

• Intermolecular Forces:

  • Attractions between molecules that significantly impact physical properties like boiling and melting points, such as van der Waals forces, dipole-dipole interactions, and hydrogen bonding.