This MCQ module is based on: Atomic Mass, Atomic Number, Isotopes and Electronic Configuration
TOPIC 29 OF 50
Atomic Mass, Atomic Number, Isotopes and Electronic Configuration
🎓 Class 9
Science
CBSE
Theory
Ch 8 — Journey Inside the Atom
⏱ ~13 min
🌐 Language: [gtranslate]
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Introduction: Two Numbers That Identify an Atom
Once we know that an atom contains protons, neutrons and electrons, we can describe any atom completely with just two whole numbers — its atomic number and its mass number. These two numbers tell us which element the atom belongs to and how heavy it is. They also help us understand isotopes, isobars, and the way electrons arrange themselves in shells, which in turn decides the chemistry of the element.
8.12 Atomic Number (Z)
The atomic number, written as \(Z\), is the number of protons in the nucleus of an atom. Because an atom is electrically neutral, the number of electrons is also equal to \(Z\). The atomic number is what truly defines an element — every atom of carbon has \(Z=6\); every atom of oxygen has \(Z=8\).
8.13 Mass Number (A)
The mass number, written as \(A\), is the sum of the number of protons and neutrons in the nucleus.
Definition. \( A = Z + N \), where \(N\) is the number of neutrons. So the number of neutrons \( = A - Z\).
Standard Atomic Notation
An element X is written as \( {}^{A}_{Z}\text{X} \). The atomic number \(Z\) is the subscript and the mass number \(A\) is the superscript. For example, sodium is written as \( {}^{23}_{11}\text{Na} \), which tells us:
- Number of protons (and electrons) = 11
- Mass number = 23
- Number of neutrons = \(23 - 11 = 12\)
8.14 Isotopes
Isotopes are different forms of the same element. They have the same number of protons (so the same chemistry) but a different number of neutrons (so a different mass). Some common examples:
| Element | Isotopes | Protons | Neutrons | Use |
|---|---|---|---|---|
| Hydrogen | \({}^{1}_{1}\text{H}\), \({}^{2}_{1}\text{H}\), \({}^{3}_{1}\text{H}\) | 1, 1, 1 | 0, 1, 2 | Tritium in nuclear research |
| Carbon | \({}^{12}_{6}\text{C}\), \({}^{13}_{6}\text{C}\), \({}^{14}_{6}\text{C}\) | 6, 6, 6 | 6, 7, 8 | \({}^{14}\)C used for radio-carbon dating of fossils |
| Chlorine | \({}^{35}_{17}\text{Cl}\), \({}^{37}_{17}\text{Cl}\) | 17, 17 | 18, 20 | Average mass of chlorine ≈ 35.5 u |
| Uranium | \({}^{235}_{92}\text{U}\), \({}^{238}_{92}\text{U}\) | 92, 92 | 143, 146 | \({}^{235}\)U fuel in nuclear reactors |
Why Average Atomic Mass is a Fraction
Because most elements occur as a mixture of isotopes in fixed natural ratios, the average atomic mass is usually a fractional number. Chlorine, for example, exists as 75% \({}^{35}\)Cl and 25% \({}^{37}\)Cl. Hence:
Average mass of Cl \(= \dfrac{75 \times 35 + 25 \times 37}{100} = \dfrac{2625 + 925}{100} = 35.5 \text{ u}\).
Some Important Uses of Isotopes
- Medical: The isotope \({}^{60}\)Co (cobalt-60) is used to treat cancer; \({}^{131}\)I (iodine-131) is used to treat goitre.
- Nuclear power: \({}^{235}\)U is used as fuel inside a nuclear reactor.
- Dating: \({}^{14}\)C dating helps archaeologists determine the age of ancient organic remains.
8.15 Isobars
Isobars are atoms of different elements that happen to have the same mass number. Since they belong to different elements they have different chemical properties.
Example: \({}^{40}_{18}\text{Ar}\) and \({}^{40}_{20}\text{Ca}\). Both have A = 40, but Z is 18 and 20 respectively.
| Feature | Isotopes | Isobars |
|---|---|---|
| Atomic number Z | Same | Different |
| Mass number A | Different | Same |
| Element | Same element | Different elements |
| Chemical properties | Identical | Different |
8.16 Distribution of Electrons in Shells
Niels Bohr together with Bury proposed simple rules for distributing electrons among the K, L, M and N shells. The rules are easy to remember and let us draw the electronic configuration of the first 20 elements without difficulty.
The 2n² Rule
The maximum number of electrons that the nth shell can hold is given by:
Maximum electrons in shell n \(= 2n^{2}\)
| Shell | n | Maximum Electrons (2n²) |
|---|---|---|
| K | 1 | 2 × 1² = 2 |
| L | 2 | 2 × 2² = 8 |
| M | 3 | 2 × 3² = 18 |
| N | 4 | 2 × 4² = 32 |
Additional Rules
- Electrons are filled from the innermost shell outward — first K, then L, then M, then N.
- The outermost shell of any atom can hold a maximum of 8 electrons, even if its 2n² capacity is larger.
- The next-to-outermost shell cannot have more than 18 electrons.
- A new shell starts only when the inner shell is filled to the limit allowed by the above two rules.
Examples for the First 20 Elements
| Element | Z | K | L | M | N | Configuration |
|---|---|---|---|---|---|---|
| Hydrogen (H) | 1 | 1 | 1 | |||
| Helium (He) | 2 | 2 | 2 | |||
| Lithium (Li) | 3 | 2 | 1 | 2,1 | ||
| Carbon (C) | 6 | 2 | 4 | 2,4 | ||
| Nitrogen (N) | 7 | 2 | 5 | 2,5 | ||
| Oxygen (O) | 8 | 2 | 6 | 2,6 | ||
| Neon (Ne) | 10 | 2 | 8 | 2,8 | ||
| Sodium (Na) | 11 | 2 | 8 | 1 | 2,8,1 | |
| Magnesium (Mg) | 12 | 2 | 8 | 2 | 2,8,2 | |
| Aluminium (Al) | 13 | 2 | 8 | 3 | 2,8,3 | |
| Silicon (Si) | 14 | 2 | 8 | 4 | 2,8,4 | |
| Phosphorus (P) | 15 | 2 | 8 | 5 | 2,8,5 | |
| Sulphur (S) | 16 | 2 | 8 | 6 | 2,8,6 | |
| Chlorine (Cl) | 17 | 2 | 8 | 7 | 2,8,7 | |
| Argon (Ar) | 18 | 2 | 8 | 8 | 2,8,8 | |
| Potassium (K) | 19 | 2 | 8 | 8 | 1 | 2,8,8,1 |
| Calcium (Ca) | 20 | 2 | 8 | 8 | 2 | 2,8,8,2 |
Note: For potassium and calcium the M-shell stops at 8 (not 18) because of the "outermost shell ≤ 8" rule. Once the outer N-shell starts filling, the M-shell can later expand to 18.
🔋 Shell Walk-Through — Click each shell of sodium L1 Remember
Sodium (Z = 11) has 11 electrons spread across three shells. Click the nucleus and each shell — K, L, M — to recall its capacity (2n²) and how many electrons sit there in Na.
Click the nucleus or any shell (K · L · M) above to recall what sits there and why.
8.17 Valence Electrons, Octet Rule and Valency
The electrons in the outermost shell of an atom are called valence electrons. They are the only electrons that take part in chemical reactions, so they decide the way an element bonds with others.
The Octet Rule
Atoms are most stable when their outermost shell contains 8 electrons (or 2 in the case of K-shell only). This stable arrangement is called an octet. Atoms react with other atoms in order to gain, lose or share electrons until they achieve an octet.
The noble gases (He, Ne, Ar, Kr) already possess this configuration and so are chemically inert.
Valency
The valency of an element is the combining capacity of its atom. The rule is:
- If the outermost shell has 1, 2 or 3 electrons, valency = number of valence electrons (these atoms lose them).
- If the outermost shell has 5, 6 or 7 electrons, valency = \(8 - \text{number of valence electrons}\) (these atoms gain the missing electrons).
- If the outermost shell has 4 electrons, valency = 4 (the atom usually shares electrons).
- If the outermost shell has 8 electrons (or 2 for He), valency = 0 — the atom is inert.
| Element | Configuration | Valence Electrons | Valency |
|---|---|---|---|
| Hydrogen | 1 | 1 | 1 |
| Sodium | 2,8,1 | 1 | 1 |
| Magnesium | 2,8,2 | 2 | 2 |
| Aluminium | 2,8,3 | 3 | 3 |
| Carbon | 2,4 | 4 | 4 |
| Nitrogen | 2,5 | 5 | 3 (8 − 5) |
| Oxygen | 2,6 | 6 | 2 (8 − 6) |
| Chlorine | 2,8,7 | 7 | 1 (8 − 7) |
| Neon | 2,8 | 8 | 0 (inert) |
8.18 Activity — Building Configurations
Activity 8.3 — Filling Up Shells with CountersL3 Apply
Predict first: For an atom of phosphorus (Z = 15), how many shells will be occupied and how many electrons will sit in the outermost shell?
- Draw three concentric circles on a sheet of paper to represent the K, L and M shells.
- Take 15 small counters (beans or paper bits) — one for each electron of phosphorus.
- Place 2 counters on the K-shell (its maximum).
- Place 8 counters on the L-shell (its maximum).
- Place the remaining 5 counters on the M-shell.
- Write the configuration as 2, 8, 5. Identify the valence electrons and predict the valency.
Observation: The K-shell holds 2, the L-shell holds 8 and the M-shell takes the remaining 5. Configuration: 2, 8, 5. Valence electrons = 5; valency = \(8 - 5 = 3\). Phosphorus therefore combines with 3 hydrogen atoms to form PH₃.
Conclusion: The 2n² rule together with the outermost-shell limit gives a clear, mechanical method to write the electronic configuration of any of the first 20 elements.
Conclusion: The 2n² rule together with the outermost-shell limit gives a clear, mechanical method to write the electronic configuration of any of the first 20 elements.
Quick Recap
- \(Z\) = number of protons = number of electrons (in a neutral atom).
- \(A\) = number of protons + number of neutrons. Number of neutrons = \(A - Z\).
- Notation: \( {}^{A}_{Z}\text{X} \).
- Isotopes: same Z, different A. Isobars: different Z, same A.
- Maximum electrons in shell \(n = 2n^{2}\); outermost shell ≤ 8.
- Valency depends on the number of valence electrons.
Competency-Based Questions
A sample contains three different atoms — atom X with 17 protons and 18 neutrons, atom Y with 17 protons and 20 neutrons, and atom Z with 18 protons and 17 neutrons.
Q1. Which two atoms in the sample are isotopes of each other? L2
(a) X and Y both have Z = 17 (chlorine) but different mass numbers (35 and 37) — classic isotopes.
Q2. Identify the isobar pair in the sample and explain. L3
X has A = 17 + 18 = 35, Z has A = 18 + 17 = 35. Both have A = 35 but different Z (17 and 18). Hence X and Z are isobars.
Q3. Write the electronic configuration of the chloride ion Cl⁻ and explain why it has the same configuration as argon. L3
Cl has Z = 17 and configuration 2,8,7. By gaining 1 electron it becomes Cl⁻ with 18 electrons, configuration 2,8,8 — the same as argon (Z = 18). Both have a stable octet, hence Cl⁻ is unreactive.
Q4. State whether True or False: "Isotopes of an element have the same chemical properties but different physical properties." L1
True. Chemistry is decided by electrons, which are the same in all isotopes. Mass-dependent physical properties (density, boiling point) differ slightly because of the different number of neutrons.
Q5. Element Q has Z = 12. Predict (a) its electronic configuration, (b) its valency, (c) the formula of its oxide. L4
(a) 2, 8, 2 — Q is magnesium. (b) Valence electrons = 2 ⇒ valency = 2. (c) Mg combines with O (valency 2) in 1:1 ratio to form MgO.
Assertion–Reason Questions
Options: (A) Both A and R are true and R is the correct explanation of A. (B) Both true but R is not the correct explanation. (C) A true, R false. (D) A false, R true.
A: Isotopes of the same element have identical chemical behaviour.
R: Chemical behaviour depends on the number of valence electrons, which is the same for all isotopes of an element.
(A) Both true and R correctly explains A.
A: Noble gases are chemically inert.
R: Their outermost shell is completely filled with 8 electrons (2 for helium).
(A) Both true and R correctly explains A. With a stable octet, they have no tendency to lose, gain or share electrons.
A: The M-shell of a potassium atom holds 8 electrons even though its 2n² capacity is 18.
R: The outermost shell of an atom can never contain more than 8 electrons.
(A) Both true and R correctly explains A. Since the M-shell is potassium's outermost shell when it has 8, the next electron must start a new (N) shell.
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