Elements & Compounds

Matter

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Pure Substance

  • A substance that cannot be separated by any physical process i.e., element or molecule.

The Periodic Table

  • A chart that organizes all know chemical elements by atomic number, arranged by rows (periods) and columns (groups) to show periodic trends in their properties, with elements in the same group sharing similar characteristics.

    • Organization: Elements are ordered by atomic number (number of protons).

    • Periods (Rows): Horizontal rows where properties gradually change.

    • Groups (Columns): Vertical columns where elements have similar chemical behaviors (e.g., Group 1 alkali metals, Group 18 noble gases).

    • Element Data: Each element's square shows its symbol, atomic number, and average atomic mass. 

    • Classification: Divides elements into metals, nonmetals, and metalloids, revealing broader trends.

    • Periodic Law: The arrangement reflects the principle that element properties repeat periodically when ordered by atomic number.

Pure Substances

  • A substance that cannot be separated by any physical process i.e., element or molecule.

Element

  • A pure chemical substance that cannot be broken down into smaller parts or changed into another substance.

    • Elements are represented on the periodic table as one capitalized letter, or as one capitalized letter and Lower case letter.

Atom

  • Particles that combine to form matter, and are the fundamental and smallest unit of a chemical element that cannot be divided into smaller parts. (Fundamental building block of all matter).

    • Protons

    • Neutrons

    • Electrons

Molecules

  • When atoms connect with each other using bonds to hold them together, then molecules are formed.

    • We use the symbols for the atoms involved to show what atoms are bonded together and the numbers represent the numbers of atoms of each element that are in the molecule.

      •  Ex. Carbon with two oxygens attached to it is represented as CO2.  We don't need to say C1O2 because the 1 is understood.

  • Molecules can still be an element if all atoms are the same, such as O2 or S8.  Remember, an element is made up of only one kind of atom.

  • Molecules can only experience a physical change (cannot be separated into other materials by any physical process), i.e., water frozen into ice or water boiling into a gas.

Compounds

  • When molecules contain different types of atoms, like in CO2 or H2O, this is called a compound. 

  • Compounds behave very differently from the original elements from which they are formed.  

  • Compounds are pure substances. They are made up of only one kind of molecule.   

Mixtures

  • A mixture is made up of more than one kind of atom or molecule (pure substances), that can be separated by physical processes. Mixtures can be homogenous or heterogenous i.e., boil salt water and your left with salt.

    • Homogenous: All the substances are distributed evenly and every section look the same i.e., sugar and water.

    • Heterogenous: All the substance are not evenly distributed and every section looks differently i.e., oil and water.

  • A compound is not a mixture because it is made up of molecules that are all the same.  

  • Mixtures are new substances made of chemical bonds

Physical Change and Chemical Change

  • Physical Change: A change in properties such as shape, texture, or state.

  • Chemical Change: The formation of a new substance after atoms are rearranged in a chemical reaction.