ATOMS AND MOLECULES-Notes

Maharishi Kanad, an ancient Indian sage and philosopher, is credited as the founder of the Vaisheshika school, which developed one of the earliest atomic theories in human history. He proposed that everything in the universe is composed of tiny, indivisible, and eternal particles called "parmanu" (atoms). Kanad believed these parmanu cannot be further subdivided, and through different combinations and arrangements, they give rise to all matter and its properties.

Key Concepts of Kanad’s Atomic Theory

• The smallest particle of matter—parmanu—is indivisible and eternal.
• Atoms combine in various ways to form molecules and create different kinds of substances.
• Atoms are invisible to the naked eye and retain their identity through all transformations.
• Change in substance occurs due to external factors such as heat, leading to combinations and reactions among atoms.
• His system outlines six fundamental categories (padarthas) to describe everything in the universe: substance (dravya), quality (guna), action (karma), universality (samanya), particularity (visesa), and inherence (samavaya).
• Kanad’s theory also considered atoms to have two possible states—motion and rest.

Around the same era, ancient Greek philosophers – Democritus and Leucippus suggested that if we go on dividing matter, a stage will come when particles obtained cannot be divided further. Democritus called these indivisible particles atoms (meaning indivisible).

Antoine Lavoisier, often called the "Father of Modern Chemistry," formulated the law of conservation of mass, which states that mass is neither created nor destroyed in a chemical reaction. His precise experiments with chemical reactions demonstrated that the total mass of reactants equals the total mass of products, establishing a key foundation for atomic theory: atoms are not created or destroyed during chemical changes.

Key Principles of Lavoisier’s Theory

• Law of Conservation of Mass:
  In any chemical reaction, the total mass of the substances remains constant.
• Definition of Element:
  Lavoisier defined an element as a basic substance that cannot be further broken down by chemical means.
• Refinement of Chemical Nomenclature:
  He introduced a rational system for naming compounds, much of which is still in use today.
• Rejection of Phlogiston Theory:
  Through experiments, he demonstrated that combustion and respiration require oxygen, disproving the previously popular phlogiston theory
• First Modern List of Elements:
  Lavoisier created one of the earliest periodic tables, listing the known elements and distinguishing them from compounds and mixtures.

• Law of Conservation of Mass
  This law states that mass is neither created nor destroyed during a chemical reaction; the total mass of reactants equals the total mass of products.
• Law of Definite Proportions (or Constant Proportions)
  This law asserts that a chemical compound always contains the same elements in the same proportion by mass, regardless of its source or method of preparation.

According to Dalton’s atomic theory, all matter, whether an element, a compound or a mixture is composed of small particles called atoms. The postulates of this theory may be stated as follows:

• All matter is made of very tiny particles called atoms, which participate in chemical reactions.
• Atoms are indivisible particles, which cannot be created or destroyed in a chemical reaction.
• Atoms of a given element are identical in mass and chemical properties.
• Atoms of different elements have different masses and chemical properties.
• Atoms combine in the ratio of small whole numbers to form compounds.
• The relative number and kinds of atoms are constant in a given compound.

An atom is the smallest unit of matter that can't be broken down chemically. It's made up of protons, neutrons, and electrons.

Structure

• The nucleus of an atom is made up of protons and neutrons.
• The electrons orbit the nucleus in a cloud-like shape.
• The nucleus is positively charged because protons have a positive charge and neutrons have no charge.
• The electrons are negatively charged, which binds them to the nucleus.

Properties

• All atoms of the same element are identical.
• Different elements have different types of atoms.
• Atoms can attach to other atoms to form chemical compounds like molecules or crystals.

Size

• Atoms are extremely small, about 100 picometers across.
• A human hair is about a million carbon atoms wide.
• Atoms are smaller than the shortest wavelength of visible light.

Symbols

• The modern day symbols of atoms of different elements are usually one- or two-letter abbreviations derived from their English or Latin names and are standardized by IUPAC (International Union of Pure and Applied Chemistry). These symbols provide a universal shorthand for representing elements in chemical equations and the periodic table.

Examples of Modern Atomic Symbols

IUPAC stands for the International Union of Pure and Applied Chemistry. It is the globally recognized authority responsible for standardizing chemical nomenclature, terminology, symbols, and atomic weights, including the naming of new elements on the periodic table.

Atomic mass is the total mass of an atom, which is made up of the mass of its protons, neutrons, and electrons. It is usually measured in unified atomic mass units (u). How is atomic mass calculated?

• The atomic mass of an atom is the sum of the masses of its protons and neutrons.
• The atomic mass of an element is the average mass of its naturally occurring isotopes.
• Carbon-12 isotope was chosen as the standard reference for measuring atomic masses. One atomic mass unit is a mass unit equal to exactly one-twelfth (1/12th) the mass of one atom of carbon-12.

Atomic masses of a few elements

A molecule is in general a group of two or more atoms that are chemically bonded together, that is, tightly held together by attractive forces. A molecule can be defined as the smallest particle of an element or a compound that is capable of an independent existence and shows all the properties of that substance. Atoms of the same element or of different elements can join together to form molecules.

Atomicity is the number of atoms in a molecule of an element. For example, an oxygen molecule has two oxygen atoms, so its atomicity is 2.

Types of atomicity

• Monoatomic :
  A molecule made up of one atom. For example, neon \(\mathrm{(Ne)}\), helium \(\mathrm{(He)}\), and argon \(\mathrm{(Ar)}\) are monatomic.
• Diatomic :
  A molecule made up of two atoms. For example, hydrogen \(\mathrm{(H_2)}\), nitrogen \(\mathrm{(N_2)}\), oxygen \(\mathrm{(O_2)}\), fluorine \(\mathrm{(F_2)}\), and chlorine \(\mathrm{(Cl_2)}\) are diatomic.
• Triatomic :
  A molecule made up of three atoms. For example, ozone \(\mathrm{(O_3)}\) is triatomic.
• Polyatomic :
  A molecule made up of more than three atoms. For example, phosphorus \(\mathrm{(P_4)}\) and sulfur \(\mathrm{(S_8)}\) are polyatomic.

Examples of atomicity

An ion is an atom or a molecule that carries a net electrical charge due to the loss or gain of electrons. This charge imbalance occurs when the number of electrons is not equal to the number of protons present in the species.

Types of Ions

• Cation: A positively charged ion formed when an atom or molecule loses one or more electrons.
• Anion: A negatively charged ion formed when an atom or molecule gains one or more electrons.

Ions can also be grouped as:

• Monatomic ions:
  consisting of a single atom (e.g., \(\mathrm{Na^⁺}\), \(\mathrm{Cl^⁻}\)).
• Polyatomic ions:
  consisting of two or more atoms bonded together with a net charge (e.g., \(\mathrm{(SO_4)^2⁻, (NH_4)^⁺}\)).

Valency is the combining capacity of an atom or element. It's also known as the valence number.

How is valency determined?

• Valency is determined by the number of electrons in an atom's outermost shell.
• If the number of valence electrons is less than or equal to 4, then the valency is equal to the number of valence electrons.
• If the number of valence electrons is greater than 4, then the valency is equal to 8 minus the number of valence electrons.

Rules for Writing Chemical Formula

• The valencies or charges on the ion must balance.
• When a compound consists of a metal and a non-metal, the name or symbol of the metal is written first. For example: calcium oxide (CaO), sodium chloride (NaCl), iron sulphide (FeS), copper oxide (CuO), etc., where oxygen, chlorine, sulphur are non-metals and are written on the right, whereas calcium, sodium, iron and copper are metals, and are written on the left.
• in compounds formed with polyatomic ions, the number of ions present in the compound is indicated by enclosing the formula of ion in a bracket and writing the number of ions outside the bracket. For example, \(\mathrm{Mg(OH)_2}\). In case the number of polyatomic ion is one, the bracket is not required. For example, NaOH.

The simplest compounds, which are made up of two different elements are called binary compounds.

While writing the chemical formulae for compounds, we write the constituent elements and their valencies as shown below. Then we must crossover the valencies of the combining atoms.

Examples

Formula of hydrogen chloride

Formula of hydrogen sulphide

Formula for aluminium oxide

Formula of ammonium sulphate

The molecular mass of a substance is the sum of the atomic masses of all the atoms in a molecule of the substance. It is therefore the relative mass of a molecule expressed in atomic mass units (u).

Example

Example 3.1 (a) Calculate the relative molecular mass of water \(\mathrm{(H_2O)}\).
(b) Calculate the molecular mass of \(\mathrm{HNO_3}\).

Solution:
(a) Atomic Mass of Hydrogen = 1u
Atomic Mass of Oxygen = 16u
Molecular Mass = Mass of Hydrogen + Mass of Oxygen
Molecular Mass
= 1 x 2 + 16
= 18u

(b) Atomic Mass of Hydrogen = 1u
Atomic Mass of Nitrogen = 14u
Atomic Mass of Oxygen = 16u
Molecular Mass = Mass of Hydrogen + Mass of Oxygen + Mass of Nitrogen
Molecular Mass
= 1 x 3 + 16 + 14
= 63u

The formula unit mass of a substance is a sum of the atomic masses of all atoms in a formula unit of a compound. Formula unit mass is calculated in the same manner as we calculate the molecular mass. The only difference is that we use the word formula unit for those substances whose constituent particles are ions.

• During a chemical reaction, the sum of the masses of the reactants and products remains unchanged. This is known as the Law of Conservation of Mass.
• In a pure chemical compound, elements are always present in a definite proportion by mass. This is known as the Law of Definite Proportions.
• An atom is the smallest particle of the element that cannot usually exist independently and retain all its chemical properties.
• A molecule is the smallest particle of an element or a compound capable of independent existence under ordinary conditions. It shows all the properties of the substance.
• A chemical formula of a compound shows its constituent elements and the number of atoms of each combining element.
• Clusters of atoms that act as an ion are called polyatomic ions. They carry a fixed charge on them.
• The chemical formula of a molecular compound is determined by the valency of each element.
• In ionic compounds, the charge on each ion is used to determine the chemical formula of the compound.