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Structure of Matter

Grade 8 Science Worksheets

Elements, Molecules, and Compounds

Elements are the most basic chemical substances. They cannot be broken down further by chemical means. An element is a pure substance consisting of atoms which have the same number of protons or atomic number. The Periodic Table arranges all known elements in the order of their Atomic Numbers.

Atoms of the same element may combine to form Molecules. For example, two oxygen atoms

combine to form an oxygen molecule. Atoms of two different elements may also combine to

form a molecule. For example, two atoms of hydrogen combine with one atom of oxygen to

form a molecule named water. A molecule is the smallest particle that retains all the properties of the substance from which it is obtained. Further division of the molecule will yield only atoms of individual elements.

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When atoms of different elements combine in a fixed ratio, the result is nameda Compound. Water is thus a compound – it always has two atoms of hydrogen and one atom of oxygenin each molecule, no more no less. Each compound has its own unique properties, which are different from the properties of the individual elements from which it is made. Compounds may be solids, liquids, or gases.

Compounds hold their atoms together in many ways, such as – 

  1. Covalent compounds like water and methane are held together by covalent bonds
  2. Ionic compounds like sodium chloride (table salt) and sodium fluoride (found in toothpaste) are held together by ionic bonds
  3. Metallic compounds like gold and aluminum are held together by metallic bonds


Compounds have an identifiable structure based on how their atoms are arranged. For example, carbon forms a range of Organic Compounds with hydrogen, such as Methane, Ethane, Propane, Butane, to name a few, each with its unique structure of atom arrangement. The diagrams show carbon atoms in green and hydrogen in grey.

All other compounds that lack Carbon-Hydrogen bonds are generally called Inorganic Compounds.

Water molecule with an oxygen atom (shown red) and two hydrogen atoms (shown blue)

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Ions and Ionization

An atom or a group of atoms (a molecule) with an electric charge is called an Ion. Every atom has negatively charged electrons spinning around a heavy nucleus with positively charged protons. If the number of electrons and protons are equal, the atom is neutral. The chemical process of removing electrons from atoms or molecules to produce Positive Ions is called Ionization. These removed electrons may then join other atoms or groups of atoms to make them Negative Ions. The amount of charge that an ion has is determined by the number of electrons gained or lost by the atom or molecule.

Table salt, formed from sodium and chlorine, is an ionic compound. Each sodium atom loses an electron to become a positively charged sodium ion and each chlorine atom gains an electron to become a negatively charged chloride ion. (A negatively charged chlorine atom is called a Chloride). Table salt contains an equal number of positively charged sodium ions and negatively charged chloride ions.

An ion with a positive charge, as sodium in the example above, is called a Cation while an ion with a negative charge, as chloride in the example above, is called an Anion.

Sodium Chloridehas a fixed crystalline structure consisting of sodium cations (shown in purple) and chloride anions (shown in green).

Elements in the periodic table are grouped such as to identify atoms which will form cations or anions –

  • Halogens such as Fluorine, Chlorine, and Iodine, always form anions
  • Alkali metals such as Lithium, Sodium, Potassium, always form cations
  • Alkali earth metals such as Barium, Calcium, Magnesium, always form cations
  • Noble gases are generally unreactive and tend not to form ions

Most other metals such as Iron, Nickel, Silver, form cations, while most non-metals, such as Oxygen, Sulfur, Carbon, form anions.

Check Point

  1. Atoms of the same elements combine to form ______ while atoms of different elements combine to form ______.
  2. Organic Compounds are generally made up of ______ and ______.
  3. An atom with an electric charge is called _______.
  4. ______ is a negatively charged ion while ______ is a positively charged ion.
  5. In ionization process most metals lose electrons to form ______ while most non-metals gain electrons to form ______.

Answer Key

  1. Molecules, Compounds
  2. Carbon, Hydrogen
  3. Ion
  4. Anion, Cation
  5. Cations, Anions

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