Elements and Compounds

• Matter exists in different forms, which can be broadly classified into elements, compounds, and mixtures.

Elements

• Elements are the simplest form of matter and cannot be broken down into simpler substances by chemical means.
• They are composed of only one type of atom.
• Each element is defined by its unique atomic number, which is the number of protons in the nucleus of its atoms.
• Examples of elements include hydrogen (H), oxygen (O), iron (Fe), and gold (Au).
• Elements are organized in the periodic table based on their atomic number and chemical properties.
• The periodic table groups elements with similar properties into vertical columns called groups or families.
• Horizontal rows in the periodic table are called periods.
• Elements can exist as single atoms or as molecules.
• Molecules of elements consist of two or more atoms of the same element bonded together, such as diatomic oxygen (O2) or triatomic ozone (O3).

Compounds

• Compounds are substances formed when two or more different elements are chemically bonded together in a fixed ratio.
• The properties of a compound are different from those of its constituent elements.
• Compounds can only be separated into their constituent elements by chemical reactions.
• Examples of compounds include water (H2O), sodium chloride (NaCl), and carbon dioxide (CO2).
• The chemical formula of a compound represents the types and numbers of atoms present in one molecule or formula unit of the compound.
• The smallest unit of a compound is a molecule (for molecular compounds) or a formula unit (for ionic compounds).
• Compounds are held together by chemical bonds, which are the attractive forces between atoms or ions.
• Ionic bonds involve the transfer of electrons between atoms, resulting in the formation of ions (positively charged cations and negatively charged anions).
• Covalent bonds involve the sharing of electrons between atoms.
• Compounds have a fixed composition by mass, which means the ratio of the masses of the elements in a compound is always the same.
• This is known as the law of definite proportions.

Mixtures

• Mixtures are combinations of two or more substances that are physically combined but not chemically bonded.
• The components of a mixture retain their individual properties.
• Mixtures can be separated by physical means, such as filtration, evaporation, distillation, or magnetism.
• There are two main types of mixtures: homogeneous and heterogeneous.
• Homogeneous mixtures have a uniform composition throughout.
• The components are evenly distributed and not easily visible.
• Examples of homogeneous mixtures include saltwater, air, and sugar dissolved in water.
• Homogeneous mixtures are also called solutions, where one substance (the solute) is dissolved in another (the solvent).
• Heterogeneous mixtures have a non-uniform composition.
• The components are not evenly distributed and are easily visible.
• Examples of heterogeneous mixtures include sand and water, oil and water, and a salad.
• The composition of a mixture can vary, meaning the amounts of the substances in a mixture can change.
• Unlike compounds, mixtures do not have a fixed ratio of components.
• Suspensions are heterogeneous mixtures in which solid particles are dispersed in a liquid but will settle out over time.
• Colloids are heterogeneous mixtures in which particles are dispersed throughout a substance but do not settle out due to their small size.
• The Tyndall effect is the scattering of light by the particles in a colloid or suspension.
• This effect is not observed in solutions because the particles are too small to scatter light.