Laws of Chemical Combination

LAW OF CONSERVATION OF MASS

Law of conservation of mass states that mass can neither be created nor destroyed in a chemical reaction.

LAW OF CONSTANT PROPORTIONS

This law was stated as “In a chemical substance the elements are always present in definite proportions by mass”.

• Many compounds were made up of two or more elements, and no matter where they originated from or who made it, they all had the same elements in the same proportions.

• Eg In a compound such as water, the ratio of the mass of hydrogen to the mass of oxygen is always 1:8, whatever the source of water. Thus, if 9 g of water is decomposed, 1 g of hydrogen and 8 g of oxygen are always obtained.

Dalton’s atomic theory

All matter, whether an element, a compound or a mixture is composed of small particles called atoms.

POSTULATES OF DALTONS ATOMIC THEORY

• Atoms, the incredibly small building blocks of all matter, take part in chemical reactions.

• In a chemical reaction, atoms cannot be formed or destroyed since they are indivisible particles.

• The mass and chemical characteristics of each atom of a given element are the same.

• Distinct elements' atoms have different masses and chemical characteristics.

• Compounds are created when atoms join in a ratio of small whole numbers.

• In a particular compound, the relative number and types of atoms remain constant.

ATOMS

• An atom is a particle of matter that uniquely defines a chemical element. An atom consists of a central nucleus that is surrounded by one or more negatively charged electrons.

• Atomic radius is measured in nanometres.

Some elements' symbols are created by combining the name's first letter with a letter that follows it. Examples include (i)  zinc, Zn, (ii) chlorine, Cl, etc.Other symbols were derived from Latin, German, or Greek names of elements.For example, the symbol of iron is Fe from itsLatin name ferrum, sodium is Na from natrium

ATOMIC MASS

• The atomic mass was the most astonishing idea Dalton's atomic theory provided.

• He claimed that each element has a distinctive atomic mass. The theory's ability to properly describe the law of constant proportions led researchers to determine the atomic mass of an atom.

• Since determining the mass of an individual atom was a relatively difficult task, relative atomic masses were determined using the laws of chemical combinations and the compounds formed.

• The atomic mass of an element is the total sum of protons and neutrons present in the atom

• One atomic mass unit is one-twelfth (1/12th) the mass of one carbon-12 atom.

• The average mass of an atom, as measured against 1/12th the mass of one carbon-12 atom, is referred to as the relative atomic mass of that element.

HOW DO ATOMS EXIST?

The majority of elements' atoms cannot exist alone. Atoms create ions and molecules. The matter that we can see, feel, or touch is created when these molecules or ions come together in vast quantities.

MOLECULE

The smallest component of an element or compound that is capable of independent existence and exhibits all the characteristics of that substance is known as a molecule.

• 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 created by joining together atoms of the same element or of different elements.

MOLECULES OF ELEMENTS

• An element's molecules are made up of the same kind of atoms.

• Many elements have only one atom per molecule, including helium (He), argon (Ar), and others.

• But with the majority of nonmetals, this is not the case. For instance, a molecule of oxygen is known as a diatomic molecule, O2, since it only contains two oxygen atoms. Ozone, or O3, is created when 3 oxygen atoms combine into a molecule rather than the typical 2.

• The atomicity of a molecule refers to how many atoms make up that molecule.

• Metals and some other elements, such as carbon, do not have a simple structure but consist of a very large and indefinite number of atoms bonded together.

MOLECULES OF COMPOUNDS

Compound molecules are created when specific proportions of atoms from several elements combine.

Examples

COMPOUND CONSTITUENT ELEMENTS RATIO

• Carbon dioxide (CO2 ) Carbon, Oxygen 3:8

• Ammonia (NH3 ) Nitrogen, Hydrogen 14:3

• Water (H2O) Hydrogen, Oxygen 1:8

IONS

Charged species exist in combinations of metals and nonmetals. Ions are the names for the charged species. Ions can be made up of a single charged atom or a collection of atoms with a net charge.

• An ion can be negatively or positively charged.

• A negatively charged ion is called an ‘anion

• positively charged ion is called a ‘cation’.

• Consider sodium chloride (NaCl). Positively charged sodium ions (Na+) and negatively charged chloride ions (Cl-) make up its constituent particles. A polyatomic ion is an association of charged atoms.

Chemical Formulae

• The chemical formula of a compound is a symbolic representation of its composition

• The combining power (or capacity) of an element is known as its valency

• Valency can be used to find out how the atoms of an element will combine with the atom(s) of another element to form a chemical compound.

RULES FOR WRITING A CHEMICAL FORMULA

• The ion's valencies, or charges, must balance.

• When a compound contains a metal and a non-metal, the name or symbol of the metal comes first.For instance,copper oxide (CuO), ferrous sulphide (FeS), etc. The non-metals like oxygen, and sulphur are written on the right,and the metals like copper and iron are on the left.

• The number of ions present in a compound made up of polyatomic ions is expressed by enclosing the formula of the ion in brackets and writing the number of ions outside the brackets. For instance, Mg(OH)2( and it does not need a bracket if there is only one polyatomic ion.For instance, NaOH.)

Molecular Mass

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

• Consequently, it is a molecule's relative mass represented in atomic mass units (u).

Formula unit mass

• 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. For example, sodium chloride as discussed above, has a formula unit NaCl. Its formula unit mass can be calculated as– 1 × 23 + 1 × 35.5 = 58.5 u

Mole concept

• Mole=given mass of compound /sum of mass numbers of the elements present in a compound

• 1mole=6.023x10^23 entities(it is 6.023x10^23 atoms if an element is given and 6.023x10^23 molecules if a compound is given in question)

• volume occupied by 1mole of gas at standard temperature and pressure(stp)=22.4L

note-volume of 1mole of gas is 22.4L only when it is given in the question that the gas is at standard temperature and pressure(stp)