2.3 Why elements react to form compounds

• Molecules:

  • A group of two or more atoms held together by chemical bonds
  • Examples include oxygen, nitrogen, chlorine, and carbon dioxide
  • Can contain multiple different elements
• Compounds:

  • Substances containing two or more different elements
  • Atoms are held together by chemical bonds
  • Elements in compounds are always found in the same proportions
  • Formulas for compounds use chemical symbols and numbers for each atom
  • Examples include water (H2O), carbon dioxide (CO2), and sulfuric acid (H2SO4)
• Water:

  • Compound containing hydrogen and oxygen
  • Formula: H2O (with the 2 as a subscript)
  • Always has two hydrogen atoms and one oxygen atom
• Carbon dioxide:

  • Compound containing oxygen and carbon
  • Formula: CO2
  • Has two oxygen atoms and one carbon atom
• Sulfuric acid:

  • Formula: H2SO4
  • Contains two hydrogen atoms, one sulfur atom, and four oxygen atoms
• Mixtures:

  • Consist of two or more substances not chemically combined
  • Can be separated using physical methods like filtration, crystallization, or distillation
• A compound is made up of two or more different types of atom joined together by strong bonds
• The chemical formula shows the number of atoms of each element present in one molecule or ion.
• Compounds have fixed ratios of atoms, which are shown in their chemical formulae
• To separate mixtures, we can use physical processes such as filtering, evaporation, and distillation.
• An ion is a charged particle, which can be a single atom like Na+ or a group of atoms like the hydroxide ion OH-
• Ions are formed when atoms gain or lose electrons to have a full outer shell and become more stable
• The group number in the periodic table indicates how many electrons the atoms of that group have in the outermost shell
• Atoms need to lose or gain electrons to achieve a full outer shell, but this process requires energy
• Atoms in group one (alkaline metals) easily lose one electron to become one plus ions
• Atoms in group two can easily lose two electrons to become two plus ions
• Atoms in group six gain two electrons to become two minus ions
• Atoms in group seven gain one electron to become one minus ions
• Elements in groups three, four, and five would have to lose or gain three or four electrons, which requires a lot of energy, so they don't often form ions
• Chemical equations can be written to show the gain or loss of electrons, with electrons placed on the right for positive ions and on the left for negative ions
• Atoms like magnesium in group two form a two plus ion by losing two electrons
• Atoms like oxygen in group six form a two minus ion by gaining two electrons
• In a compound, the atoms are held together by very strong forces called covalent bonds.
• When a substance undergoes a change as a result of a chemical reaction, it has been transformed into another substance with different properties
• Metals tend to have more protons than non-metals.
• Non-metals tend to have fewer protons than metals.
• Chemical bonds are forces that hold atoms together in molecules or compounds, contributing to the stability of the structure.
• A covalent bond forms when atoms share electrons, creating a stable arrangement and resulting in a molecule.
• Dot and cross diagrams illustrate the arrangement of electrons in atoms and the sharing or transfer of electrons in chemical bonding.
• An ion is an atom or molecule that has gained or lost electrons, resulting in a net electrical charge.
• The highest energy level refers to the outermost electron shell of an atom, which determines its reactivity and ability to form bonds.
• Labeling in chemistry involves identifying and assigning names or symbols to different components or parts of a chemical structure or reaction.
• Ionic bonds occur between ions of opposite charges, resulting in the formation of ionic compounds, such as salts.
• Ionic compounds are chemical compounds composed of ions held together by ionic bon
• A molecule consists of two or more atoms held together by covalent bonds, forming a distinct chemical entity.
• A molecule consists of two or more atoms held together by covalent bonds, forming a distinct chemical entity.
• State what a dot and cross diagram is.

  Showcase of the sharing or transfer of electrons in chemical bonding.
• Atoms have electrons arranged in different electron shells or energy levels around the nucleus, with the outermost shell containing electrons with the highest energy level.
• The number of electrons in each shell varies: the first shell can hold up to two electrons, the second shell up to eight electrons, and the third shell also up to eight electrons.
• The atomic number of an atom, which equals the number of protons in the nucleus, determines the number of electrons in the atom.