Atoms, Molecules, and Laws of Chemical Combinations

Posted on Sep 7, 2024 in Chemistry

Atoms and Molecules

Laws of Chemical Combinations

Before Dalton’s concept, the atom was a mere philosophical idea. Dalton explained the atom based on the Laws of Chemical Combinations. There are three laws of chemical combination:

  1. Law of Conservation of Mass
  2. Law of Constant Proportions
  3. Law of Multiple Proportions

Law of Conservation of Mass

Antoine L. Lavoisier, a French scientist, established the Law of Conservation of Mass. This law states: “Mass can neither be created nor destroyed in a chemical reaction”. According to this law, the mass of an isolated system will remain constant over time. This means when mass is enclosed in a system and none is allowed in or out, its quantity will never change. That is, mass will be conserved, and hence this is called the Law of Conservation of Mass. This means the total mass of products is always equal to the total mass of reactants. As there is no loss of mass of substances, i.e., mass is conserved, that’s why Lavoisier called this the Law of Conservation of Mass.

Reactants and Products

In a chemical reaction, the substances that combine or react are known as reactants, and the new substance/substances formed are called product or products. A chemical reaction can be represented in general as follows:

Reactant + Reactant → Product

Example:

Calcium oxide (56 g) + Water (18 g) → Calcium hydroxide (74 g)

Law of Constant Proportions

The Law of Constant Proportion states that a chemical compound always contains exactly the same proportion of elements by mass. This law is also known as the Law of Definite Proportions. Joseph Louis Proust gave this law; hence, this law is also known as Proust’s Law.

Explanation of the Law

Compounds are formed by the combination of two or more elements. In a compound, the ratio of the atoms or elements by mass remains always the same irrespective of the source of the compound. This means a certain compound is always formed by the combination of atoms in the same ratio by mass. If the ratio of the mass of constituent atoms is altered, a new compound is formed.

Examples:

Water is formed by the combination of hydrogen and oxygen. The ratio of masses of hydrogen and oxygen is always 1:8 in water, irrespective of the source of water. Whether you collect the water from a well, river, pond, or anywhere, the ratio of their constituent atoms by mass will always be the same.

Dalton’s Atomic Theory

John Dalton, a British chemist and scientist, gave the Atomic Theory in 1808. This theory is popularly known as Dalton’s Atomic Theory in honor of John Dalton. He gave the theory based on the Laws of Chemical Combination and explained them properly.

Main Postulates of Dalton’s Atomic Theory

  1. Elements are made of extremely small particles called atoms.
  2. Atoms of a given element are identical in size, mass, and other properties.
  3. Atoms of different elements differ in size, mass, and other properties.
  4. Atoms cannot be subdivided, created, or destroyed.
  5. Atoms of different elements combine in simple whole-number ratios to form chemical compounds.
  6. In chemical reactions, atoms are combined, separated, or rearranged.

Atoms

Based on Dalton’s Atomic Theory, “The smallest particles of matter are called Atoms”.

Characteristics of Atoms
  1. An atom is the smallest particle of matter.
  2. All elements are made of tiny particles called atoms.
  3. Atoms are very small in size and cannot be seen with naked eyes.
  4. Atoms do not exist in a free state in nature but take part in chemical reactions.
  5. The properties of matter depend upon the characteristics of atoms.
  6. Atoms are the building blocks of an element, similar to bricks that combine to make a building.
  7. The size of an atom is indicated by its radius.
  8. In ancient times, atoms were considered indivisible.

Symbols of Atoms of Elements

For this marvelous work, Berzelius, together with John Dalton, Antoine Lavoisier, and Robert Boyle, is considered the Father of Modern Chemistry.

Name and Symbol of Some Elements

Several elements are named after the place where they were discovered. For example, ‘Copper’ was discovered in Cyprus. Thus, it got its name similar to Cyprus.

Some elements are named after their color. For example, ‘Gold’ means yellow.

Atomic Mass

The mass of an atom is called atomic mass. Since atoms are very small, the actual mass of an atom is also very small. For example, the actual mass of one atom of hydrogen is equal to 1.673 x 10-24g. This is equal to 0.000000000000000000000001673 grams. Dealing with such small numbers is very difficult. Thus, for convenience, relative atomic mass is used.

Carbon-12 is considered the unit to calculate atomic mass. Carbon-12 is an isotope of carbon. The relative mass of all atoms is found with respect to C-12.

One atomic mass unit = 1/12 of the mass of one atom of C-12. This means the atomic mass unit = 1/12th of carbon-12.

Thus, atomic mass is the relative atomic mass of an atom with respect to 1/12th of the mass of a carbon-12 atom. ‘amu’ is the abbreviation of Atomic Mass Unit, but now it is denoted just by ‘u’.

The atomic mass of a hydrogen atom = 1u. This means one hydrogen atom is 1 time heavier than 1/12th of a carbon atom.

The atomic mass of oxygen is 16u. This means one atom of oxygen is 16 times heavier than 1/12th of a carbon atom.

Absolute Mass or Actual Atomic Mass

It is found that the actual atomic mass of a carbon-12 atom is equal to 1.9926×10-23g.

∴ 1u = 1.9926×10-23/12 g

⇒ 1 u = 1.6605×10-24g

Thus, by multiplying the relative atomic mass with 1.6605 × 10-24g, we can get the absolute or actual mass of an atom.

Existence of Atoms

Atoms of most elements exist in the form of molecules or ions since they are most reactive. For example, hydrogen, oxygen, chlorine, etc. However, atoms of some elements, which are non-reactive, exist in a free state in nature. For example, helium, neon, argon, etc.

Usually, atoms exist in the following two forms:

  • In the form of molecules
  • In the form of ions

Molecule

  1. A molecule is the smallest particle of a compound.
  2. Atoms exist in free states in the form of molecules.
  3. A molecule may be formed by the combination of two or more similar atoms of an element, such as an oxygen molecule formed by the combination of two oxygen atoms, or a molecule of hydrogen formed by the combination of two hydrogen atoms.
  4. Molecules may be formed by the combination of atoms of two or more different elements. For example, a molecule of water is formed by the combination of two atoms of hydrogen and one atom of oxygen. A molecule of Nitric oxide or nitrogen monoxide is formed by the combination of one nitrogen atom and one oxygen atom.
  5. A molecule takes part in a chemical reaction.
  6. Most atoms exist in the form of molecules. Molecules are formed by the combination of two or more elements. Example: Molecule of hydrogen (H2), Molecule of oxygen (O2), Molecule of nitrogen (N2), etc.

Molecules can be of two types:

  1. Molecules of elements
  2. Molecules of compounds

Molecules of Elements

When two or more atoms of the same element combine to form a molecule, these are called molecules of the element.

Example:

Hydrogen molecule (H2). A molecule of hydrogen is formed by the combination of two hydrogen atoms.

Molecules of some non-reactive elements are formed by a single atom. For example – helium, neon, argon, etc. molecules: Molecules of metals are formed as a big cluster of atoms. They are represented by their symbols simply. For example: Iron (Fe), Copper (Cu), Zinc (Zn), etc. These molecules are known as giant molecules.

Carbon is a non-metal, but it also exists as a giant molecule and is represented by its symbol ‘C’.

Atomicity

Monoatomic

When a molecule is formed by a single atom only, it is called a monoatomic molecule. Generally, noble gases form monoatomic molecules. For example: Helium (He), Neon (Ne), Argon (Ar), Kr (Krypton), Xenon (Xe), Radon (Rn).

Diatomic

When a molecule is formed by the combination of two atoms, it is called a diatomic molecule. For example: Hydrogen (H2), Oxygen (O2), Nitrogen (N2), Chlorine (Cl2), etc.

Triatomic

When a molecule is formed by the combination of three atoms, it is called a triatomic molecule. For example: a molecule of ozone (O3).

Tetra-atomic

When a molecule is formed by the combination of four atoms, it is called a tetra-atomic molecule. For example: Phosphorus molecule (P4).

Polyatomic

When a molecule is formed by the combination of more than two atoms, it is called a polyatomic molecule. For example: Sulphur molecule (S8).

Molecules of Compounds

When a molecule is formed by the combination of two or more atoms of different elements, it is called the molecule of a compound.

Example:

Molecule of water (H2O). A molecule of water is formed by the combination of two hydrogen and one oxygen atoms.

Ions

Atoms of several elements exist in the form of ions. Atoms or molecules with a negative or positive charge over them are called ions.

Example: Sodium ion (Na+), Potassium ion (K+), Chloride ion (Cl–), Fluoride ion (F–), etc.

Cations

Ions having a positive charge over them are called cations.

Example: Sodium ion (Na+), Potassium ion (K+), Hydrogen ion (H+), etc.

Anions

Ions having a negative charge over them are called anions.

Example: Chloride ion (Cl–), Fluoride ion (F–), Carbonate ion (CO3– –), etc.

Monoatomic Ions

Ions formed by one atom are called monoatomic ions.

Example: Sodium ion (Na+), Potassium ion (K+), Chloride ion (Cl–), Fluoride ion (F–), etc.

Polyatomic Ions

Ions formed by two or more atoms are called polyatomic ions.

Example: Hydroxide ion (OH–), Ammonium ion (NH4+), Carbonate ion (CO3– –), etc.