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Atoms and Molecules / Counting Matter- Summary, Introduction

 Table of content

  • Summary
  • Introduction
  • Laws of Chemical Combination
  • FAQs

Laws of  Chemical Combination
(a) Law of Conservation of Mass – Mass can neither be created nor be destroyed in a chemical reaction.
(b) Law of Constant Proportions In a chemical substance, the elements are always present in definite proportions by mass.
♦ Dalton’s atomic theory – All matter is composed of small, indivisible particles called atoms.
♦ An atom is the smallest particle of the element that can 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.
♦ Atomic mass – The mass of an atom of a chemical element expressed in atomic mass units. It is approximately equivalent to the number of protons and neutrons in the atom (the mass number).
♦ Ion – Charged species are known as ions. A negatively charged ion is called an ‘anion’ and the positively charged ion is called a ‘cation’.
 A chemical formula of a compound shows its composition and the number of atoms of each  combining element.
♦ Valency – The combining power or capacity of an element.
♦ Rules for making a chemical formula – When a compound consists of a metal and a non-metal, the symbol of the metal is written first followed by that of the non-metal. Two metals do not join up to form compounds.
(i) Write the symbols.                                                                     K         O
(ii) Write the valency of each element.                                           1          2
(iii) Crossover the valency of the combining atoms.                    K2       O1

Formula of the compound is K2O.
In ionic compounds, the charge on each ion is used to determine the chemical formula of the
♦ Molecular mass – It is the sum of the atomic masses of all the atoms in a molecule of the substance.
 Formula unit mass – It is the sum of the atomic masses of all the atoms in a formula unit of a compound.  
Number of atoms in exactly 12g of carbon-12 = 6.022 x 10²³ (Avogadro number).
 1 mole (of anything) =  6.022 x 10²³ in number
♦ Molar mass = Mass of 1 mole of a substance


 All matter is composed of basic elements that cannot be broken down to substances with different chemical or physical properties.  
♦ Elements are substances consisting of one type of atom, for example Carbon atoms make up diamond, and also graphite. Pure (24K) gold is composed of only one type of atom, gold atoms.
 A compound is a substance that consists of two or more different elements chemically united in a definite ratio.
♦ Most of the Universe consists of matter and energy. Energy is the capacity to do work. Matter has mass and occupies space. The basic building blocks of the “normal” matter that we see in the Universe are atoms, and combinations of atoms are called molecules. That means atoms are the smallest particle into which an element can be divided.

 Constituents of Atoms     
Atoms are composed of three classes of constituents, as illustrated in the following table.

  Most of the mass of atoms resides in the neutrons and protons which occupy the dense central region called the nucleus.

♦ The number of protons (or the number of electrons) is called the atomic number for the atom. The total number of protons plus neutrons is called the atomic mass number for the atom.   
 Atoms are electrically neutral because the number of negatively-charged electrons is exactly equal to the number of positively-charged protons. The number of neutrons is approximately equal to the number of protons for stable light nuclei, and is about 1-2 times the number of protons for the heavier stable nuclei.
♦ Atoms having the same number of protons (and therefore the same number of electrons) but different numbers of neutrons are called isotopes of the element. Thus, the isotopes of an element have the same atomic number but differ in their atomic mass number.
♦ The properties of matter depend on the properties of atoms and molecules from which it is made.
By the end of the 18th century, scientists proved experimentally that matter exists in the atomic state and molecular state and tried to understand how and why elements combine and what happens when they combine. Antoine L. Lavoisier laid the foundation of chemical sciences by establishing important laws of chemical combination.

Laws of Chemical Combination

There are four laws of chemical combination. They are –
1. The law of conservation of mass
2. The law of constant (definite) proportions
3. The law of multiple proportions
4. The law of reciprocal proportions

Joseph Louis Proust (1754-1826) was popular for the law of definite proportion, otherwise called the Proust Law.

Frequently asked Questions –
1. What are the constituents of atom?
2. Who proposed the particulate nature of matter?
3. What is an atom?
4. What is a molecule?
5. How is an element different from a compound?

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Laws of  Chemical Combination – The law of conservation of mass