NEET Foundation Chemistry Notes For Chapter 3 Atoms And Molecules

NEET Foundation Chemistry Notes For Chapter 3 Atoms And Molecules

For ages, ancient Indian and Greek philosophers wondered about the unseen and unknown form of matter. Around 500 bc, India, the idea of divisibility of matter took shape.

Maharishi Kanad, An Indian philosopher, suggested that if we kept on diving matter (padarth), we will get smaller and smaller particles and ultimately, a time will come when the further division will not be possible and we will obtain the smallest particle of the matter.

He called these particles as parmanu. Pakudha Katyayama, another Indian philosopher, postulated that these exists in the combined form which is the reason for the varied forms of matter.

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All matters are made up of atoms. Atoms do not exist independently. Atoms are further divided into ions and molecules. Molecules and ions collectively in large number forms matter.

NEET Foundation Chemistry Notes For Chapter 3 Atoms And Molecules Laws of Chemical Combination

A molecule is a group of atoms that are chemically bonded together and held by intermolecular force. The tiniest particle of an element which exists independently by inheriting all the characteristics of that substance is called a molecule.

Matter transforms from one state to the other under certain circumstances. This phenomenon takes place due to combination of two types of matter. This process of combination of different elements to form compounds takes place under certain rule.

Chemistry is the study of the transformation of matter from one form to the other. These transformations often occur as a result of the combination of two different types of matter. The combination of different elements to form compounds is governed by certain basic rules. These rules are referred to as laws of chemical combination.

The rules are known as laws of chemical combination. The rules that aid the chemical combinations of elements are discussed below.

Law of Conservation of Mass

Law of conservation of mass was given by Antoine Lavoisier and verified by Landolt. According to this law, during a chemical reaction matter is neither created nor destroyed, it may change from one form to other. After a chemical reaction the total mass of materials is same as the total mass before reaction.

Greek Philosopher Democritus (460–370 bc) and Leucippus also worked on the idea of division of matter. According to them sub-division of matter is limited. Democritus named the individual particles as ’atoms’ meaning indivisible.

Law of Constant or Definite Proportion

Law of definite proportion was given by Joseph Proust who was a French chemist. According to the law, in a given compound the proportion of elements by weight will always remain same irrespective of the methods of preparation.

Law of Multiple Proportions

Law of multiple proportions was given by Dalton in 1803 and verified by Berzelius. According to this law, when more than one compound is formed by combining two elements the masses of these elements in the reaction are in the ratio of small whole numbers.

Law of Equivalent Proportion or Law of Reciprocal Proportion

Law of equivalent proportion was given by Ritcher. According to this law, the weights of two or more elements which separately react with same weight of a third element are also the weights of these elements which react with each other or in simple multiple of them.

Gay-Lussac’s Law of Gaseous Volumes

Gay-Lussac’s law of Gaseous volumes was given by Gay–Lussac in 1808 and is applicable only for gases. This law was based on experimental observation done by Gay-Lussac. According to the law, the production of gases takes place in a simple ratio by volume, temperature and pressure being constant.

It can be treated as another form of law of definite proportions. The only difference between these two is that Gay Lussac’s Law is stated with respect to volume while law of definite proportions is stated with respect to mass.

Avogadro’s Law

Avogadro in 1811 gave Avogadro’s law which states that equal volume of gas contains equal number of molecules, temperature and pressure being constant.

Atoms And Molecules Notes

Example: 2 litres of hydrogen contains same number of molecules as 2 litres of oxygen, temperature and pressure being constant of both gases.

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NEET Foundation Chemistry Notes For Modern Periodic Table

In recent times, IUPAC (International Union of Pure and Applied Chemistry) approves names of elements. Many of the symbols are the first one or two letters of the element’s name in English.

The first letterof a symbol is always written as a capital letter (uppercase) and the second letter as a small letter (lowercase).

NEET Foundation Chemistry Notes For Chapter 3 Atoms And Molecules About Molecule

Molecules of elements: A molecule is made up of atoms. Molecules of elements are made up of same types of atoms. Some molecules may be made up of a single atom.

Example: Helium, But in most of the cases the molecules are made up of two or more atoms. The number of atoms needed to make a molecule is called as atomicity of that element.

Molecules of compounds: A compound is a molecule made up of 2 or more different types of atoms.

Example: 1 molecule of water contains 2 molecules of hydrogen and 1 molecule of oxygen. Similarly, 1 molecule hydrochloric acid contains 1 molecule of hydrogen and 1 molecule of chlorine.

Molecules of some compounds

Atomicity

It refers to the number of atoms present in one molecule of an element or substance.

For example one molecule of HCl has one atom of hydrogen and one atom of chlorine. So, the atomicity of HCl is 2. The molecules are called Monatomic, Diatomic or triatomic depending on the number of atoms present in it.

Atomicity of some elements

Ion

An ion is defined as an electrically charged atom or group of atoms formed by the loss or gain of one or more electrons. A positive ion is created by electron loss and a negative ion is created by an electron gain.

Example: Air conditioners, fluorescent lamps and printers, etc., are generators of positive ion or cation, which is very harmful for health.

On the other hand negative ions are in abundance in forests. The natural negative ions neutralize the harmful positive ions in the atmosphere as they attract dust particles and other pollutants.

Positively charged ion is called cation and a negative ion called anion.

Some common, simple and polyatomic ions

NEET Foundation Chemistry Notes For Chapter 3 Atoms And Molecules Writing Chemical Formulae

The chemical formula of a compound represents its chemical composition. A chemical formula essentially indicates 2 things:

• The elements which constitutes that compound and the number of each element in it.
• The number of each constituent.

The atoms of the elements are indicated by the symbols and the number is indicated as a subscript with the symbol.

Example: In CO₂, C and O are the symbols of carbon and oxygen respectively while 2 indicates that 2 atoms of oxygen have been used in the compound.

There are three fundamental rules of writing chemical formula:

• The valencies of the elements must balance.
• When a compound contains both metal and non-metal elements the name of metal comes first.Example: Sodium Chloride (NaCl)
• In polyatomic ion compounds, the ion is enclosed in bracket before writing the formula to indicate the ratio. In case the ion is mono-atomic then bracket is not required.

Formula of Simple Compound

For writing a formula for the simple compound the symbol and the valencies of the atoms are written and then the valencies are crossed with the atom to balance the formula.

Illustration 1: Figure explains that the valency of carbon atom is 4 while that of chlorine is 1. To balance the formula the valency is crossed with the symbol to write the correct formula.

Formulation of Ionic Compounds

When writing a formula with ionized atoms it should be kept in mind that the cation and anion should be balanced.

Illustration 2: The figure explains that the Aluminum atom has 3 positive charges while Oxygen atom has 2 negative charges. To balance this, 2 atoms of Al and 3 atoms of O are used to form neutralized Al₂O₃.

NEET Foundation Chemistry Notes For Chapter 3 Atoms And Molecules Molecular Mass and Mole Concept

The molecular mass is the sum of atomic masses of all the atoms present in the molecule. The relative mass of molecule is expressed in atomic mass unit (u). The formula unit mass is the sum of the atomic masses of all atoms in a formula unit of a compound. It is calculated in the way we calculate the molecular mass. We use formula unit for those substances which are made up of ions.

Atomic Mass Unit (u) is defined as a unit of mass used to express atomic and molecular weights, equal to one twelfth of the mass of an atom of carbon-12.

Example: Atomic mass of carbon is 12.0 u while atomic mass of oxygen atom is 16.0 u, so the molecular mass of 1 molecule of CO₂ will be 44.0 u (12.0 u + 16.0 u + 16.0 u).

Mole Concept

It is not necessary that all the substances are formed by joining of atoms. In practice 2 or more molecules can also be mixed to get new substances.

Example: 2 molecules of hydrogen combine with 1 molecule of oxygen to make 2 molecules of water. It is evident from this example that 2 hydrogen and 1 oxygen is the minimum required unit to create molecule of water.

In other words mole is the number of atoms in 12 grams of C-12. This is the basic counting unit for scientists.

1. The number of atoms in 12 gram of C-12 is 6.022 × 10²³. This is called Avogadro’s number.

2. When Avogadro number is divided by mole it becomes a constant known as Avogadro’s constant denoted by N_A ⋅ N_A = 6.02 × 10²³ mol^-1.

Example: Atomic mass of carbon (C) is 12 u.

This means 12 u C has only 1 atom of carbon.

12 g carbon has 1 mole atoms of carbon

Therefore, 12 gram carbon has 6.02 × 10²³ atoms of carbon.

Similarly, atomic mass of He (Helium) is 4 u

4 u He will have only 1 atom of He.

4 gram of He will have 1 mole atoms of He.

Therefore, 4 grams He has 6.02 × 10²³ atoms of He.

Molar Mass

Mass of 1 mole of substance is called molar mass.

Example: Atomic mass of He is 4 u

So 4 u of He = 1 atom of He

So 4 gram of He = 1 mole atom of He

Therefore, molar mass of He is 4 gram.

NEET Foundation Chemistry Notes For Chapter 3 Atoms And Molecules Fill In the Blanks

Question 1. Ozone is an example of __________ molecule.
Answer. Triatomic

Question 2. In water, the ratio of hydrogen and oxygen is __________.
Answer. 2:1

Question 3. The valency of nitrogen in NH3 is __________.
Answer. 3

Question 4. All noble gas molecules are __________ in nature.
Answer. Monoatomic

Question 5. The __________ is the sum of atomic masses of all the atoms present in the molecule.
Answer. Molecular Mass

Question 6. A __________ is a molecule made up of 2 or more different types of atoms.
Answer. Compound

Question 7. The number of atoms needed to make a molecule is called as __________ of that element.
Answer. Atomicity

Question 8. During a chemical reaction matter is neither created nor destroyed, it may change from one form to other, is explained by___________ law.
Answer. Conservation of Mass

Question 9. When there is more number of electrons than protons, the atom is called ________.
Answer. Anion

Question 10. __________ is a tendency in an atom to achieve the state of stability.
Answer. Valency

Question 11. While writing a chemical formula, brackets are required in case of ____________ ions.
Answer. Polyatomic

Question 12. Atoms of the elements are indicated by the ____________ in a chemical formula.
Answer. Symbols

Question 13. 6.022 × 10²³ is the number of atoms in 12 g of ____________.
Answer. Carbon – 12

Question 14. ___________ is the study of the transformation of matter from one form to the other.
Answer. Chemistry

Question 15. Mass of 1 mole of substance is called ____________.
Answer. Molar mass

Question 16. 6.022 × 10²³ is called the ____________.
Answer. Avogadro’s number

Question 17. Law of definite proportion was given by______________.
Answer. Joseph Proust

Question 18. __________ are basic building blocks of matter.
Answer. Atoms

Question 19. The total number of protons present in the nucleus of an atom is called_________.
Answer. Atomic number

Question 20. Electrons present in the outermost orbit are known as_________.
Answer. Valence electrons

Question 21. A positively charged ion is called ________.
Answer. Cation

Question 22. Air conditioners, fluorescent lamps and printers, etc., are generators of __________.
Answer. Ions

Question 23. ___________ refers to the number of atoms present in one molecule of an element or substance.
Answer. Atomicity

Question 24. When a compound contains both metal and non-metal elements the name of ___________ comes first.
Answer. Metal

NEET Foundation Chemistry Notes For Chapter 3 Atoms And Molecules True Or False

Question 1. Law of conservation of mass was given by Antoine Lavoisier and verified by Landolt. (True/False)
Answer. True

Question 2. Protons revolve around the nucleus of an atom (True/ False).
Answer. False

Question 3. The relative mass of molecule is expressed in atomic mass unit (u). (True/False)
Answer. True

NEET Foundation Chemistry Notes For Chapter 3 Atoms And Molecules Match the Column

Question 1. Match the following and choose the correct code:

Select the correct option:

1. A-4, B-3, C-1, D-2
2. A-2, B-3, C-4, D-1
3. A-3, B-1, C-2, D-4
4. A-1, B-4, C-2, D-3

Answer. 3. A-3, B-1, C-2, D-4

Question 2. Match the following and choose the correct code:

Select the correct option:

1. A-4, B-1, C-2, D-3
2. A-3, B-4, C-1, D-2
3. A-2, B-3, C-4, D-1
4. A-4, B-2, C-1, D-3

Answer. 2. A-3, B-4, C-1, D-2

Question 3. Match the following and choose the correct code:

Select the correct option:

1. A-4, B-3, C-1, D-2
2. A-2, B-4, C-1, D-3
3. A-1, B-3, C-2, D-4
4. A-3, B-2, C-4, D-1

Answer. 1. A-4, B-3, C-1, D-2

Question 4. Match the following and choose the correct code:

Select the correct option:

1. A-4, B-3, C-2, D-1
2. A-2, B-1, C-4, D-3
3. A-3, B-2, C-1, D-4
4. A-3, B-4, C-2, D-1

Answer. 4. A-3, B-4, C-2, D-1

Question 5. Match the following and choose the correct code:

Select the correct option:

1. A-3, B-4, C-2, D-1
2. A-2, B-4, C-1, D-3
3. A-1, B-2, C-4, D-3
4. A-4, B-3, C-2, D-1

Answer. 2. A-2, B-4, C-1, D-3

NEET Foundation Chemistry Notes For Chapter 3 Atoms And Molecules Assertion Reasoning

For the following questions the options will remain the following:

1. Both A and R are correct and R is correct explanation of A.
2. Both A and R are correct but R is not a logical explanation of A.
3. A is correct but R is incorrect.
4. R is correct but A is incorrect.

Question 1. Assertion: The molecular mass expressed in grams is called gram molecular mass.
Reason: The molecular mass is equal to the sum of the masses of all the atoms of a molecule.

Answer. 2. Both A and R are correct but R is not a logical explanation of A.

Question 2. Assertion: Ion is a charged chemical particle.
Reason: An ion that carries a positive charge is called a cation and an ion that carries a negative charge is called an anion.

Answer. 1. Both A and R are correct and R is correct explanation of A.

Question 3. Assertion: Molar volume is the volume occupied by various moles of a gas under any conditions of temperature and pressure.
Reason: It is equal to 22.4 liters.

Answer. Both A and R are correct and R is correct explanation of A.

NEET Foundation Chemistry Notes For Chapter 3 Atoms And Molecules Comprehension Passage

An atom is defined as the smallest particle of an element which may or may not be capable of free existence. Atom of hydrogen, oxygen, nitrogen, etc., are highly reactive and do not exist in the Free State. They exist in the combined state with atoms of the same element one atoms of the other elements. Dalton was the first scientist to suggest specific symbols for different elements.

Symbols used by him also represented the quantity of the element, i.e., one atom of the element. The actual masses of the atoms are so small that it is difficult to determine the actual masses of individual atoms. For example, actual mass of an atom of hydrogen is found to be 1.673 × 10^-24 g which is extremely small.

However, it was found convenient to compare the masses of atoms of different element with some reference atom. The masses thus obtained are called relative atomic masses and the scale on which these masses are expressed is called atomic mass scale.

A molecule is the smallest particle of a compound which can exist freely under ordinary conditions and shows all the properties of that substance. Molecules of some elements are very big in size containing a large number of atoms linked together. For example, graphite and diamond contain an infinite number of carbon atoms linked together.

These elements are simply represented by their atomic symbols. A third crystalline form of carbon discovered recently contains 60 C-atoms linked together. It is represented by C₆₀ and is known as Buckminsterfullerene. Molecules of a compound are two or more atoms of different elements combined together in a definite proportion by mass to form a species that can exist freely.

As molecules are made up of two or more atoms of the same or different elements and each atom has a definite atom mass, therefore, molecular mass of a molecule of a substance can be calculated by adding atomic masses of all the atoms present in one molecule of the substance.

Questions:

Question 1. Molecule of a compound means ________.

1. Atoms are held together by a chemical bond.
2. To attract the shared pair of electrons.
3. Different elements combined together in a definite proportion by mass.
4. Compounds are formed by the combination of two or more non-metal atoms.

Answer. 3. Different elements combined together in a definite proportion by mass.

Question 2. Which of the following is true about Dalton’s symbols of elements?

1. Suggested the specific symbols for different elements.
2. Element is the relative mass of its atoms.
3. Substances made up of the same kind of atoms.
4. Atoms of the same element may have different masses.

Answer. 1. Suggested the specific symbols for different elements.

Question 3. Graphite and diamond contains ________.

1. Fixed number of carbon atoms.
2. Infinite number of carbon atoms.
3. Very small number of carbon atoms.
4. No carbon atoms.

Answer. 2. Infinite number of carbon atoms.

Question 4. How the carbon represents through the structure of Buckminsterfullerene?

1. C-12
2. C_a
3. CI
4. C₆₀

Answer. 4. C₆₀

Question 5. What is relative atomic mass?

1. Masses of atoms of different element with some reference atom.
2. Scale of the masses expressed.
3. Average atomic mass of an element.
4. Short method of representing an element.

Answer. 1. Scale of the masses expressed.

Question 6. Law of equivalent proportion was given by:

1. Ritcher
2. Joseph Proust
3. Gay–Lussac
4. Maharshi Kanad

Answer. 1. Ritcher

Question 7. If one walks to a forest, which type of ion he will find more naturally?

1. Negative
2. Positive
3. Both
4. None

Answer. 1. Negative

Question 8. What does the chemical formula of a compound represent?

1. Physical composition
2. Chemical composition
3. Ions
4. Atomic number

Answer. 2. Chemical composition