Atomic Structure – Definition, Theory & FAQs

Atoms – The Building Blocks of Matter

• Elements are made up of only one kind of particles called atoms.
• No matter where an element is found, its atoms always show the same chemical properties.
• Around 500 BC, Indian saint Maharishi Kanad said that matter could be divided into smaller particles until we get indivisible particles called paramanus.
• These paramanus were later called atoms.

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Atomic Structure Class 7th

ExtraOrdinary Minds: Democritus

• Who was he?
  Democritus was a Greek philosopher and scientist (around 460–370 BC).
• Main Idea
  He proposed that everything in the universe is made up of tiny particles called atoms.
• Key Points
  • Atoms are indivisible (cannot be broken further).
  • Atoms are eternal and always in motion.
  • Atoms combine in different ways to form matter.
• Origin of the Word “Atom”
  The word ‘atom’ comes from the Greek word atomos, which means “indivisible”.

Dalton’s Atomic Theory (1803)

• All matter is made up of tiny particles called atoms.
• Atoms are indivisible (they cannot be broken down further).
• All atoms of the same element are identical but differ from atoms of other elements.
• An atom is the smallest particle that shows all the properties of that element.
• Atoms of different elements can combine to form compounds.

FAQs

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Modern Atomic Theory

• Later studies found that atoms can be broken into smaller particles called subatomic particles:

Subatomic Particles

• Protons:
  • Have a positive charge (+1).
  • Nearly equal in mass to a hydrogen atom.
  • The number of protons defines the element.
  • For example, hydrogen has 1 proton, helium has 2, carbon has 6.
• Neutrons:
  • Neutral charge (0).
  • Mass is slightly more than a proton.
  • Neutrons and protons are of almost equal size.
• Electrons:
  • Have a negative charge (-1).
  • The smallest subatomic particle.
  • The number of electrons in a neutral atom equals the number of protons.
  • Mass is 1/1837th the mass of a proton.

Structure of an Atom

1. An atom has two main parts – the nucleus and the extra-nuclear part.
2. The nucleus is the central part of the atom which contains protons and neutrons.
3. The extra-nuclear part consists of electrons that revolve in orbits around the nucleus.
4. Electrons move around the nucleus in a manner similar to planets revolving around the Sun.
5. The properties of subatomic particles (protons, neutrons, and electrons) remain the same for all elements, but the number of these particles differs from one atom to another, which makes each element unique.

Table – Charge, Relative Mass, and Mass of Subatomic Particles

Particle	Charge	Relative Mass	Mass (kg)
Electron	-1	Negligible	9.110 × 10⁻³¹ kg
Proton	+1	1	1.672 × 10⁻²⁷ kg
Neutron	Neutral	1	1.675 × 10⁻²⁷ kg

Representing an Atom

• An atom is represented by two numbers:
  • Atomic Number (Z) = Number of protons (also equals number of electrons in neutral atom)
  • Mass Number (A) = Number of protons + neutrons
• Nuclide Symbol:
  • Written as: ZA​X
  • Where:
    • X = Element symbol
    • A = Mass number
    • Z = Atomic number

Remember

• The entire mass of an atom is concentrated in the nucleus.
• Electrons have negligible mass and revolve outside the nucleus.

Wonderment

• A 70 kg human body contains approximately 7 × 10²⁷ atoms.
• This is also called “seven billion billion billion.”
• Hydrogen, oxygen, and carbon make up 99% of the body’s atoms.

Atomic Composition of Carbon

• The nuclide symbol of carbon is written as ₆¹²C.
• This shows that the atomic number of carbon is 6 and its mass number is 12.

The number of neutrons =Mass Number – Atomic Number

FAQS

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MOLECULES

• Noble gases are very stable elements and can exist independently as single atoms.
• However, most other elements cannot exist alone. They need to combine with other atoms to form molecules.
• Molecules are the smallest units of matter that can exist independently.
• Two or more atoms combine in simple whole-number ratios to form molecules.
• Depending on the type of atoms present, molecules can be classified into:
  • Molecules of elements
  • Molecules of compounds

Molecules of Elements

• Atoms of the same element combine to form molecules of elements.
• For example:
  • Oxygen gas (O₂) is made up of two atoms of oxygen.
  • Hydrogen gas (H₂) is made of two hydrogen atoms.
  • Chlorine gas (Cl₂) consists of two chlorine atoms.
• In these cases, same type of atoms are combined to form one molecule of an element.

Molecules of Compounds

• Molecules of compounds are made up of two or more different types of atoms.
• For example:
  • Ammonia (NH₃) contains 1 atom of nitrogen and 3 atoms of hydrogen.
  • Water (H₂O) contains 2 atoms of hydrogen and 1 atom of oxygen.
  • Carbon dioxide (CO₂) contains 1 atom of carbon and 2 atoms of oxygen.
  • Sulphur dioxide (SO₂) and Calcium oxide (CaO) are also compounds.
• In compounds, different atoms combine chemically in fixed ratios.
• A molecule of a compound is the smallest independent unit that shows all the properties of that compound.

FAQs

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ATOMICITY

• Atomicity is the number of atoms present in a molecule of an element.
• Based on atomicity, molecules of elements are classified as:

1. Monoatomic Molecules

1. Monoatomic molecules consist of only a single atom.
2. The atomicity of monoatomic molecules is 1.
3. All noble gases, such as Helium (He), Neon (Ne), Argon (Ar), Krypton (Kr), Xenon (Xe), and Radon (Rn), exist naturally as monoatomic molecules.

Easy to Remember: Noble gases are always monoatomic and stable.

2. Diatomic Molecules

1. The word ‘di’ comes from Greek and means two.
2. Diatomic molecules consist of two atoms joined together.
3. Their atomicity is 2.
4. If the two atoms are the same, they form elemental molecules such as Hydrogen (H₂), Nitrogen (N₂), Oxygen (O₂), Fluorine (F₂), and Chlorine (Cl₂).
5. If the two atoms are different, they form compound molecules such as Hydrogen chloride (HCl) and Carbon monoxide (CO).

Exam Tip:

• Same atoms → Elemental diatomic molecules (H₂, O₂, N₂).
• Different atoms → Compound diatomic molecules (HCl, CO).

3. Triatomic Molecules

1. Triatomic molecules consist of three atoms joined together.
2. Their atomicity is 3.
3. If the three atoms are the same, they form elemental molecules such as ozone (O₃).
4. If the three atoms are different, they form compound molecules such as carbon dioxide (CO₂) and water (H₂O).

Exam Tip:

• Same atoms → Elemental triatomic molecule (O₃).
• Different atoms → Compound triatomic molecules (CO₂, H₂O).

4. Polyatomic Molecules

1. Polyatomic molecules are those molecules which contain more than three atoms.
2. They are usually made up of multiple atoms of the same element joined together.
3. Examples: Phosphorus (P₄) and Sulphur (S₈) are common polyatomic molecules.
4. Phosphorus (P₄) is a tetratomic molecule (contains four atoms).
5. Sulphur (S₈) is an octatomic molecule (contains eight atoms).

Exam Tip:

• Tetratomic = 4 atoms → P₄
• Octatomic = 8 atoms → S₈
• In general, polyatomic = more than 3 atoms.

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Valency

1. Valency is the combining capacity of an element.
2. It tells us the number of atoms of other elements with which one atom of a given element can combine.
3. The valency of an element can be understood as the number of hydrogen atoms that one atom of the element can combine with or replace.

Examples of Valency

• In ammonia (NH₃), one nitrogen atom combines with three hydrogen atoms → valency of nitrogen = 3.
• In water (H₂O), one oxygen atom combines with two hydrogen atoms → valency of oxygen = 2.
• In hydrogen chloride (HCl), one chlorine atom combines with one hydrogen atom → valency of chlorine = 1.

Variable Valency

1. Some elements can show more than one valency; these are called variable valencies.
2. Iron (Fe) →
   • Valency 2 in FeO (ferrous oxide).
   • Valency 3 in Fe₂O₃ (ferric oxide).
3. The suffix “-ous” is used for the lower valency (ferrous = Fe²⁺) and “-ic” for the higher valency (ferric = Fe³⁺).
4. Other elements with variable valency include copper (Cu¹⁺, Cu²⁺), tin (Sn²⁺, Sn⁴⁺), lead (Pb²⁺, Pb⁴⁺), and mercury (Hg¹⁺, Hg²⁺).

Exam Tip:

• Valency = combining power.
• Fixed valency elements: H (1), O (2), N (3), C (4).
• Variable valency elements: Fe (2,3), Cu (1,2), Sn (2,4), Pb (2,4), Hg (1,2).

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Table – Some Elements with Variable Valencies

Element	Valency	Name	Common Compounds / Formulae
Copper (Cu)	1	Cuprous (Cu⁺)	CuCl (Copper(I) chloride) Cu₂O (Copper(I) oxide)
	2	Cupric (Cu²⁺)	CuCl₂ (Copper(II) chloride)  CuO (Copper(II) oxide) CuSO₄ (Copper(II) sulphate)
Tin (Sn)	2	Stannous (Sn²⁺)	SnCl₂ (Tin(II) chloride) SnO (Tin(II) oxide)
	4	Stannic (Sn⁴⁺)	SnCl₄ (Tin(IV) chloride) SnO₂ (Tin(IV) oxide)
Iron (Fe)	2	Ferrous (Fe²⁺)	FeCl₂ (Iron(II) chloride) FeO (Iron(II) oxide) FeSO₄ (Iron(II) sulphate)
	3	Ferric (Fe³⁺)	FeCl₃ (Iron(III) chloride) Fe₂O₃ (Iron(III) oxide) Fe₂(SO₄)₃ (Iron(III) sulphate)
Lead (Pb)	2	Plumbous (Pb²⁺)	PbCl₂ (Lead(II) chloride) PbO (Lead(II) oxide)
	4	Plumbic (Pb⁴⁺)	PbCl₄ (Lead(IV) chloride) PbO₂ (Lead(IV) oxide)
Mercury (Hg)	1	Mercurous (Hg₂²⁺)	Hg₂Cl₂ (Mercurous chloride) Hg₂O (Mercurous oxide)
	2	Mercuric (Hg²⁺)	HgCl₂ (Mercuric chloride) HgO (Mercuric oxide)

Key Points

1. Valency 1 = lower state → name ends with -ous.
2. Valency 2 (or higher state) → name ends with -ic.
3. Mercury ke case me Hg₂²⁺ ion (dimer) hota hai jab valency 1 hoti hai → ise Mercurous kehte hain.

FAQs

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Radicals and Ions

Radicals

• A radical is an atom or a group of atoms that carries a specific charge.
• Radicals are highly reactive because of the presence of charge.
• The valency of a radical = the magnitude of its charge.

Ions

Ions are charged particles formed when atoms or groups of atoms gain or lose electrons.
They are commonly found in compounds of metals and non-metals.

Types of Ions

1. Cations (Positive ions)
   1. Formed when an atom loses electrons.
   1. Always carry a positive charge.
   1. Examples:
      1. Sodium ion → Na⁺
      1. Calcium ion → Ca²⁺
2. Anions (Negative ions)
   1. Formed when an atom gains electrons.
   1. Always carry a negative charge.
   1. Examples:
      1. Chloride ion → Cl⁻
      1. Sulphate ion → SO₄²⁻

Exam Tip

• Cation = Loss of electrons → Positive charge
• Anion = Gain of electrons → Negative charge

Examples of Electron Transfer and Valency

• Na → Na⁺ + 1e⁻ → Sodium loses 1 electron, so valency is 1.
• Mg → Mg²⁺ + 2e⁻ → Magnesium loses 2 electrons, so valency is 2.
• Cl + 1e⁻ → Cl⁻ → Chlorine gains 1 electron, so valency is 1.
• O + 2e⁻ → O²⁻ → Oxygen gains 2 electrons, so valency is 2.

Compounds with Ions

• Sodium chloride (NaCl) is a compound that contains a positively charged sodium ion (Na⁺) and a negatively charged chloride ion (Cl⁻).
• Baking soda (sodium bicarbonate, NaHCO₃) contains a positively charged sodium ion (Na⁺) and a negatively charged bicarbonate ion (HCO₃⁻).
• Calcium sulphate consists of a calcium ion (Ca²⁺) and a sulphate ion (SO₄²⁻).

FAQs

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Monoatomic and Polyatomic Ions

• Monoatomic ions are ions made up of a single charged atom.
• Examples of monoatomic ions include hydrogen ion (H⁺), calcium ion (Ca²⁺), chloride ion (Cl⁻), and oxide ion (O²⁻).
• Monoatomic ions are formed from a single atom by losing or gaining electrons.
• The charge on a monoatomic ion depends on the number of electrons lost or gained.
• Polyatomic ions are compound ions made up of a group of atoms carrying an overall charge.
• For example, hydroxide ion (OH⁻) is a polyatomic ion made of hydrogen and oxygen with a negative charge.
• Sulphate ion (SO₄²⁻) is another polyatomic ion formed from sulphur and oxygen atoms with an overall negative charge.

FAQs

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Periodic Table (Introduction & Groups)

1. It is not possible for any chemist to remember the properties of all the 118 elements.
2. To make the study of chemistry easier, scientists prepared a chart in which elements with similar properties were placed together.
3. This chart, consisting of all elements arranged in increasing order of their atomic numbers, is called the Periodic Table.
4. The Periodic Table consists of seven horizontal rows called periods and eighteen vertical columns called groups.
5. Similar types of elements are placed in the same group of the periodic table.
6. The group number of the elements gives an idea of the valency of the elements.
7. The valency of elements in the same group is always equal.
8. Lithium (Li), Sodium (Na), and Potassium (K) lie in Group 1 and all of them have valency 1.
9. Beryllium (Be), Magnesium (Mg), and Calcium (Ca) lie in Group 2 and all of them have valency 2.

Exam Tip:

• Group → valency is same.
• Period → number of shells is same.

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Periodic Table

1. The Periodic Table is a systematic arrangement of all known elements in such a way that elements with similar properties fall in the same groups (columns).
2. The modern periodic table is based on the atomic number of elements (not atomic mass).
3. It was first given by Dmitri Mendeleev (1869) using atomic mass, but later corrected and improved by Henry Moseley (1913) on the basis of atomic number.
4. The modern periodic law states:
   • “The physical and chemical properties of elements are the periodic function of their atomic numbers.”

Structure of the Modern Periodic Table

• There are 7 periods (horizontal rows).
• There are 18 groups (vertical columns).
• Elements are arranged in increasing order of atomic number (Z).
• Metals are found on the left side, non-metals on the right side, and metalloids lie between them.

FAQs – Atomic structure

Q3. Who discovered the electron?
J. J. Thomson.

Q4. What decides the identity of an element?
Its atomic number (Z), i.e., the number of protons.

Q5. What is a molecule?
A molecule is a group of one or more atoms bonded together and acting as a single unit.

Q6. What is atomicity? Give examples.
Atomicity = number of atoms in one molecule.
Monoatomic: He; Diatomic: O₂, N₂; Triatomic: O₃; Polyatomic (3 or more atoms): NH₃, H₂SO₄.

Q7. Is helium monoatomic?
Yes, helium (He) exists as single atoms (monoatomic).

Q8. What is valency?
Valency is the combining capacity of an atom—equal to the number of electrons lost, gained, or shared to reach a stable configuration.

Q9. What is the valency of aluminium?
3. (It loses three electrons to form Al³⁺.)

Q10. What are ions?
Ions are charged particles formed when atoms or groups of atoms lose or gain electrons.
Cations (+): e.g., Na⁺, Ca²⁺; Anions (–): e.g., Cl⁻, O²⁻.

Q11. What are monoatomic ions? Give two of each type.
Ions formed from single atoms.
Cations: Na⁺, Ca²⁺; Anions: Cl⁻, O²⁻.

Q12. What are polyatomic ions? Give examples.
Charged groups of atoms acting as a single unit.
Cations: NH₄⁺ (ammonium), H₃O⁺ (hydronium); Anions: SO₄²⁻ (sulfate), CO₃²⁻ (carbonate).

Q13. What are radicals?
Atoms or groups of atoms carrying a net charge (essentially ions that behave as one unit in reactions).

Q14. In a neutral atom, how are protons and electrons related?
Number of electrons = number of protons.

Q15. What is the difference between atomic number and mass number?
Atomic number Z = protons.
Mass number A = protons + neutrons.

Q16. Differentiate: molecule of an element vs molecule of a compound.
Element molecule: same kind of atoms (e.g., O₂, H₂).
Compound molecule: different kinds of atoms chemically combined (e.g., H₂O, CO₂).

Q17. What force holds Na⁺ and Cl⁻ together in NaCl?
Electrostatic attraction between oppositely charged ions (ionic bond).

Q18. Do atoms of the same element always have the same neutrons?
Not necessarily—isotopes have the same protons but different neutrons; chemical properties remain essentially the same.

Q19. Give two quick facts about each subatomic particle.
Electron: negative charge (–1), very small mass; moves in shells.
Proton: positive charge (+1), ~1 u mass; defines Z.
Neutron: no charge, ~1 u mass; adds to A and nuclear stability.

Q20. What is the atomicity of nitrogen?
Nitrogen gas N₂ is diatomic → atomicity 2.