an abbreviation that shows how many atoms are present in one molecule of a compound
Answers
→ Atoms of the same substance are identical in all aspects, i.e., they possess same size, shape, mass, chemical properties etc.
→ Atoms of different substances are different in all aspects, i.e., they possess different size, shape, mass etc.
→ Atom is the smallest particle that takes part in a chemical reaction.
→ Atoms of different elements combine with each other in a simple whole number ratio to form compound.
→ The relative number and kinds of atoms are constant in a given compound.
Try the following questions:
Answer:
Laws of Chemical Combination:
The process of combination of two or more elements to form new compounds is governed by certain laws called laws of chemical combination. These are:
1. Law of conservation of mass.
2. Law of constant proportions.
1. Law of conservation of mass (by Lavoisier in 1744):
This law states that mass can neither be created nor destroyed in a chemical reaction.
2. Law of constant proportions (by Proust in 1797):
This law states that in a chemical substance the elements are always present in definite proportions by mass.
For example, the ratio of hydrogen and oxygen in pure water is always 1: 8 by weight.
This law is also called law of definite proportions or law of constant proportions.
Dalton’s Atomic Theory
According to Dalton’s atomic theory, all matter, whether an element, a compound or a mixture is composed of small particles called atoms
Postulates of Dalton’s atomic theory:
→ Matter is made up of extremely small indivisible particles called atoms that can neither be created nor deQ. 3 g of carbon on burning in 8 g oxygen produces 11 g of carbon dioxide. What mass of carbon dioxide will be formed when 3 g of carbon is burnt in 50 g of oxygen?
2Mg + O2 → 2MgO
Atoms:
The smallest tiny particles of matter which can't be divided further is called atom, i.e., an atom is the smallest building block of matter.
For example: Sodium (Na), Hydrogen (H), Oxygen (O), etc.
Names of Atoms or Elements and Their Symbols:
→ IUPAC (International Union of Pure and Applied Chemistry) approves names of elements.
→ The abbreviation used for lengthy names of elements are termed as their symbols.
→ The symbol of an element is formed by writing only the first letter or first letter followed by the second or some other letter of English name or Latin name of the element.
→ While writing a symbol, the first letter is always capital and the second is always small.
→ Symbols used for some common elements are given below:
symbols of elements
Atomic Mass
Atomic mass of an element may be defined as the average relative mass of an atom of the element as compared with the mass of an atom of carbon (C-12 isotope) taken as 12 amu.
formula for atomic mass
Gram Atomic Mass: The atomic mass of an element expressed in grams is known as gram atomic mass.
Molecules:
A group of two or more than two atoms of the same or different elements that are chemically bonded together is called a molecule.
For example: Two atoms of hydrogen (H2) and one atom of oxygen (O2) react with each other and form one molecule of water.
Atomicity:
The number of atoms present in a molecule of an element or a compound is known as its atomicity.
For example, atomicity of oxygen (O2) is 2 while atomicity of ozone (O3) is 3.
Molecules of Elements:
The molecules of an element are constituted by the same type of atoms.
For example, a molecule of oxygen consists of two atoms of oxygen to form a diatomic molecule O2.
Molecules of Compounds:
Atoms of different elements join together in definite proportions to form molecules of compounds.
For example, a molecule of water consists of two atoms of hydrogen and one atom of oxygen to form a triatomic molecule H2O.
Ion:
It is an electrically charged atom or group of atoms. It is formed by the loss or gain of one or more electrons by an atom.
Ions are of two types:
(i) Cation: It is positively charged ion and is formed by the loss of one or more electrons from an atom
For example: sodium atom, loses one electron to form a sodium ion Na
Na – e− → Na+
(ii) Anion: It is a negatively charged ion and is formed by the gain of one or more electrons by an atom.
For example a chlorine atom gains one electron to form a chloride ion Cl−.
Cl + e− → Cl−
Valency:
It is defined by the combining power (or capacity) of an element.
Depending on their valency, elements can be classified as following:
(i) Monovalent cation: Having cationic valency of 1.
For example: Sodium ion (Na+ ). Potassium ion (K+ ), Hydrogen ion (H+ ).
Monovalent anion: Having anionic valency of -1.
For example: Chloride ion (Cl-), Bromide ion (Br-)
(ii) Divalent cation: Having cationic valency of 2.
For example: Magnesium ion (Mg2+), Ferrous ion (Fe2+).
Divalent anion: Having anionic valency of −2.
For example: Oxide ion (O2−), Sulphide ion (S2−).
(iii) Trivalent cations: Having cationic valency of 3.
For example: Aluminium ion (Al3+), Ferric ion (Fe3+).
Trivalent anion: Having anionic valency of -3.