Revision: Atoms and Molecules Science English Medium Class 9 CBSE

Metalloid: Metalloid is a chemical element that exhibits some properties of metals and some of non-metals. Metalloids are generally semi-conductors.

Ex.: Silicon. Arsenic, Antimony and Boron.

An atom is the smallest particle of a chemical element that retains its chemical properties.

Metal:  A chemical element that is an effective conductor of electricity and heat can be defined as a metal.

Ex.: Copper, Iron, Silver, etc.

Chemical bond— A chemical bond is the binding force between two or more atoms of a molecule.

Element: It is a substance that cannot be broken down into simpler substance by chemical means

Ex.: Oxygen, Hydrogen, Gold & Helium.

An atom is the smallest particle of an element which retains its chemical identity in all physical and chemical changes.

Radicals : A radical is an atom of an element or a group of atoms of different elements that behaves as a single unit with a positive or negative charge on it.

An Atom: Smallest particle of an element that can exist and have properties of an element.

Relative atomic mass— Relative atomic mass is the mass of an atom of an element as a multiple of the standard atomic mass unit.

The relative atomic mass of an element is the ratio between the average mass of its isotopes to 1/12th part of the mass of a carbon – 12 atoms. It is denoted as A_r.

Relative atomic mass = `" Average mass of the isotopes of the element"/(1"/"12^{"th"}" of the mass of one Carbon- 12 atom")`

Compound: A compound is a pure substance that is formed when the atoms of two or more elements combine chemically in definite proportions.

Ex: H20, NaCl.

Non-Metal: Non-metal is an element that doesn’t have the characteristics of metal including, (i.e.) ability to conduct heat or electricity luster or flexibility.

Ex. Carbon Iodine, Sulphur.

Mass number— Mass number is the sum of the number of protons and neutrons present in the nucleus of an atom. It is denoted by A.

An atom which becomes charged by losing or gaining electrons is called an ion.

Atom: An atom is the smallest indivisible unit of an
OR
Atom is the smallest unit of matter.

Molecule : Molecule is the smallest unit of a compound (or an element) which always has an independent existance.

Covalent bond— When atoms of different non-metals neither donate nor accept electrons and hence no ions are formed, such a bond is called covalent bond.

Compounds : A compound is a pure substance formed by the chemical combination of two or more elements in a fixed ratio by mass.

Alchemy was a form of chemistry studied in the middle age, which was concerned with trying to discover ways to change ordinary metals into gold.

Elements : An element is the basic form of matter that cannot be broken down into simpler substances by chemical reactions.

The mass of a single atom of an element is called the atomic mass.

Relative atomic mass is defined as the ratio of the average atomic mass to the unified atomic mass unit.

Relative atomic mass (A_r) = `"Average mass of the atom"/"Unified atomic mass"`

The relative atomic mass or atomic weight of an element is the number of times one atom of the element is heavier than A times of the mass of an atom of carbon-12.

Relative Atomic Mass = \[=\frac{\text{Mass of 1 atom of the element}}{\frac{1}{12}th\text{ the mass of one C-12 atom}}\]

Atomic number— Atomic number refers to the number of protons present in an atom. It is denoted by Z.

Atomic number : Number of protons present in the nucleus of an atom.

Mass number : Numbr of protons and neutrons present in the nucleus of an atom.

Atomic weight : It is the ratio that tells how many times an atom of an element is heavier than atom of Hydrogen

The weighted average of the masses of all its isotopes in a sample of that element is called the average molecular mass.

The sum of the atomic masses of all atoms in a molecule is called the molecular mass.

The quantity of the element which weighs equal to its gram atomic mass is called one gram atom of that element.

Mole is the amount of a substance containing elementary particles like atoms, molecules or ions in 12 g of carbon - 12.

"The relative atomic mass or atomic weight of an element is the number of times one atom of the element is heavier than `1/12` times of the mass of an atom of carbon - 12".
Relative atomic mass = Mass of 1 atom of the element `1/12` of the mass of one C¹² atom.

A mole is the amount of pure substance containing the same number of chemical units as there are atoms in exactly 12 grams of carbon -12.

Avogadro's number is defined as the number of atoms present in 12 g (gram atomic mass) of C-12 isotope, i.e., 6·022 x 1023 atoms.

OR

Avogadro's number is the number of elementary units, i.e., atoms, ions or molecules present in one mole of a substance. It is denoted by N_A.

One mole of any gaseous molecules occupies 22.4 dm³ (litre) or 22400 cm³ (ml) at standard temperature and pressure (STP). This volume is known as the molar volume.

Avogadro’s number is defined as the number of atoms present in 12g of ₆C¹² isotope i.e. 6.023 × 10²³ atoms.

The relative molecular mass of a compound is the number that represents how many times one molecule of the substance is heavier than `1/12` of the mass of an atom of carbon ₆C¹².

The average atomic mass accounts for the different isotopes of an element and their natural abundances.

\[M_{\mathrm{avg.}}=\frac{M_{1}\times r_{1}+M_{2}\times r_{2}+M_{3}\times r_{3}}{r_{1}+r_{2}+r_{3}}\]

where M₁, M₂, M₃ are atomic masses of isotopes and r₁, r₂, r₃ are their relative abundances.

Five fundamental laws govern how elements and compounds combine chemically:

Law 1 — Law of Conservation of Mass (Antoine Lavoisier)

Mass is neither created nor destroyed during any chemical reaction. The total mass of reactants always equals the total mass of products.

Law 2 — Law of Definite Proportion (Joseph Proust)

A specific chemical compound always contains its elements combined in a fixed ratio by weight, regardless of where the compound comes from or how it was made.

Exception: This law does not hold for compounds made from different isotopes of an element.

Law 3 — Law of Multiple Proportion (John Dalton)

When two elements combine to form more than one compound, the different masses of one element that combine with a fixed mass of the other are always in a simple whole-number ratio. Example: CO and CO₂.

Law 4 — Gay Lussac's Law of Gaseous Volumes

When gases react or are produced in a chemical reaction, their volumes bear a simple whole-number ratio to each other — provided temperature and pressure remain the same.

Law 5 — Avogadro's Law

At the same temperature and pressure, equal volumes of all gases contain the same number of molecules, regardless of the type of gas.

"A given compound always contains exactly the same proportion of elements by mass, regardless of its source."

• Proposed by Joseph Proust in 1797.
• e.g. Pure water always has H : O mass ratio = 1 : 8, regardless of its source.
• e.g. Cupric carbonate (CuCO₃) found naturally or prepared synthetically always has the same percentage of Cu, C, and O.

Carbonic acid

According to the given information, water has been taken from different sources and still decomposes into the same amount of oxygen and hydrogen; therefore, this is an example of the law of constant proportions.

• The word atom comes from the Greek word atomos meaning uncuttable or indivisible.
• Generally, the size of an atom is about 10⁻¹⁰ m (this distance is the average distance between the nucleus and the outermost shell carrying electrons).
• Atomic number (Z) = Number of protons = Number of electrons (in neutral atom).
• Atomic mass = Number of neutrons + Number of protons.
• Atomic mass = Equivalent mass × Valency = 6.4 × Specific heat (cal).

Dalton's atomic theory laid the foundation of modern chemistry with four core postulates:

1. All matter is made up of extremely small particles called atoms.
2. Atoms of the same element are identical to each other in mass and properties; atoms of different elements differ.
3. Atoms can neither be created nor destroyed — they are indestructible.
4. Atoms combine in fixed, simple whole-number ratios to form compound atoms (molecules).

Note: Modern discoveries have refined some postulates (e.g., isotopes show atoms of the same element can differ in mass), but the core framework remains foundational.