Structure of the Atom

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CLASS IX Science ~5–6 marks/year Ch 4 of 15
Structure of the Atom

Class 9 · Science · NCERT chapter notes · Akanksha Classes

Snapshot
  • Subatomic particles: electrons (negative), protons (positive) and neutrons (neutral) make up every atom.
  • Three atomic models — Thomson's plum-pudding (1904), Rutherford's nuclear model (1911), Bohr's orbit model (1913) — each improved on the last.
  • Bohr's shells K, L, M, N hold a maximum of 2n² electrons (n = shell number).
  • Electronic configuration tells us how electrons fill the shells; the outermost-shell electrons are the valence electrons that decide chemical behaviour.
  • Atomic number (Z) = number of protons; Mass number (A) = protons + neutrons.
  • Isotopes — same Z, different A. Isobars — same A, different Z.
  • Board weightage: ~5–6 marks/year — typically one short-answer on models, one on electronic configuration, and one on isotopes/isobars.
Detailed notes

1. Discovery of Charged Particles Inside the Atom

By the late 1800s scientists had shown that atoms are not indivisible — they contain smaller charged bits. Three key discoveries set the stage.

1a. Electron — J. J. Thomson (1897)

Thomson used a cathode-ray tube (a sealed glass tube with metal electrodes, most air pumped out). When a high voltage was applied, a beam of rays shot from the cathode (negative plate) toward the anode (positive plate). These rays:

  • Travelled in straight lines and cast sharp shadows — proving they are particles.
  • Were deflected toward the positive plate — so they carried negative charge.
  • Were the same regardless of the cathode metal or the gas inside — proving they are a universal component of all matter.

Thomson called these particles electrons. Their charge-to-mass ratio was measured as 1.76 × 10¹¹ C kg&sup-1;. Millikan's oil-drop experiment later gave charge e = 1.6 × 10&sup-1;&sup9; C, so mass = 9.1 × 10&sup-3;¹ kg.

1b. Proton — Goldstein (canal rays, 1886)

Eugen Goldstein perforated the cathode of his discharge tube and observed rays travelling through the holes in the opposite direction to cathode rays. These canal rays (anode rays):

  • Deflected toward the negative plate — they carry positive charge.
  • When the gas was hydrogen, the positive particle had the smallest mass (1836 × mass of electron) — this particle was named the proton.

Proton: charge = +1.6 × 10&sup-1;&sup9; C; mass = 1.67 × 10&sup-2;&sup7; kg.

1c. Neutron — James Chadwick (1932)

Chadwick bombarded beryllium with alpha particles and detected a new radiation that was not deflected by electric or magnetic fields — proving it was electrically neutral. He named the particle the neutron. Mass of neutron ≈ mass of proton = 1.67 × 10&sup-2;&sup7; kg. Neutrons are present in all nuclei except ordinary hydrogen.

Summary — subatomic particles
Electron (e&sup-): relative charge −1, relative mass ~0, location outside nucleus.
Proton (p+): relative charge +1, relative mass 1 u, location in nucleus.
Neutron (n°): relative charge 0, relative mass 1 u, location in nucleus.

2. Thomson's Plum-Pudding Model (1904)

After discovering the electron, J. J. Thomson proposed the first structural model of the atom. He pictured the atom as a uniform sphere of positive charge (like a pudding) with electrons embedded in it like plums in a pudding or seeds in a watermelon.

  • The atom is a solid positive sphere of radius ~10&sup-¹° m.
  • Electrons are distributed at fixed points inside the sphere.
  • Total positive charge = total negative charge ↠ atom is electrically neutral.
Thomson model in one line: atom = positive ball with electrons stuck inside it like raisins in a cake.

Limitation

Thomson's model could not explain Rutherford's alpha-scattering results (1911) — if positive charge were spread uniformly, no alpha particle could bounce straight back. The model was replaced by Rutherford's nuclear model.

3. Rutherford's Alpha-Scattering Experiment (1911)

Setup

Rutherford (with Geiger and Marsden) directed a beam of fast-moving alpha particles (helium nuclei: charge +2, mass ~4 u) from a radioactive source at a very thin gold foil (~100 nm thick). A circular zinc-sulphide screen surrounding the foil flashed wherever an alpha particle struck it, allowing every deflection angle to be measured.

Observations

  1. Most alpha particles passed straight through the foil without deflection.
  2. Some particles were deflected by small angles.
  3. A very few (≈1 in 12,000) bounced back at angles greater than 90°, some even at 180°.

Rutherford famously said this was as astonishing as firing artillery shells at tissue paper and having some bounce back!

Conclusions — Rutherford's Nuclear Model

  • Most of the atom is empty space (explains why most particles go through).
  • All positive charge and nearly all mass are concentrated in an extremely small, dense region called the nucleus.
  • Nucleus radius ≈ 10&sup-1;&sup5; m; atom radius ≈ 10&sup-1;° m ↠ nucleus is 100,000 times smaller than the atom.
  • Electrons revolve around the nucleus in circular orbits in the large empty space, held by electrostatic attraction.
  • Number of protons in nucleus = number of orbiting electrons (atom is neutral).
NCERT Numerical — Volume ratio

If the radius of the nucleus is 10&sup-1;&sup5; m and the radius of the atom is 10&sup-1;° m, find the ratio of volumes.

Volume of a sphere ∝ r³. Ratio = (10&sup-1;°)³ / (10&sup-1;&sup5;)³ = (10&sup5;)³ = 10¹&sup5;. The atom is 10¹&sup5; times larger in volume than its nucleus.

Limitations of Rutherford's Model

  1. Stability: Classical physics says a revolving charged particle must radiate energy continuously. Electrons should lose energy, spiral inward, and crash into the nucleus in ~10&sup-&sup8; s. But atoms are stable — Rutherford's model cannot explain this.
  2. Atomic spectra: A spiralling electron would emit all frequencies continuously, giving a continuous spectrum. But elements show discrete line spectra — Rutherford's model gives no explanation.

4. Bohr's Model of the Atom (1913)

Niels Bohr modified Rutherford's model using Planck's quantum theory to resolve both its failures.

Postulates of Bohr's model

  1. Electrons revolve around the nucleus only in certain fixed circular orbits called shells or energy levels. While in these orbits they do not radiate energy — they are in stationary states.
  2. Each shell has a fixed, definite energy. Inner shells have lower energy; energy increases as we go outward from the nucleus.
  3. An electron can move to a higher shell by absorbing a specific amount of energy, and drops to a lower shell by emitting a photon of exactly that energy. This explains the line spectrum of hydrogen.

Shells — names and electron capacities

Shells are labelled by the principal quantum number n (n = 1, 2, 3, 4, …) or by letters:

n = 1 → K shell
n = 2 → L shell
n = 3 → M shell
n = 4 → N shell

Maximum electrons in shell n:

Maximum electrons = 2n²
K (n=1): 2 × 1² = 2
L (n=2): 2 × 2² = 8
M (n=3): 2 × 3² = 18
N (n=4): 2 × 4² = 32

NCERT filling rules

  • Shells fill in order: K first, then L, M, N.
  • The outermost (valence) shell can hold at most 8 electrons, even if 2n² allows more (the octet rule).
  • The penultimate (second-last) shell can hold at most 18 electrons.

Limitations of Bohr's model

  • Works well only for hydrogen (one electron); fails for multi-electron atoms.
  • Cannot explain the fine structure (splitting) of spectral lines.
  • Does not account for the wave nature of electrons (wave mechanics).

5. Electronic Configuration of Elements

The electronic configuration is the distribution of electrons among the shells. It is written as the electron count in each shell from K outward, separated by commas.

Steps to write electronic configuration

  1. Fill K shell first (max 2).
  2. Fill L shell (max 8).
  3. Fill M shell (max 8 for elements Z = 1–18 in NCERT; max 18 for heavier elements).
  4. Never place more than 8 in the outermost shell.
NCERT Table — Electronic configurations, first 20 elements

H (Z=1, A=1): K=1. Config: 1. Valence e&sup-;: 1. Valency: 1.

He (Z=2, A=4): K=2. Config: 2. Valence e&sup-;: 2. Valency: 0. (K full, noble gas.)

Li (Z=3, A=7): K=2, L=1. Config: 2,1. Valence e&sup-;: 1. Valency: 1.

Be (Z=4, A=9): K=2, L=2. Config: 2,2. Valence e&sup-;: 2. Valency: 2.

B (Z=5, A=11): K=2, L=3. Config: 2,3. Valence e&sup-;: 3. Valency: 3.

C (Z=6, A=12): K=2, L=4. Config: 2,4. Valence e&sup-;: 4. Valency: 4.

N (Z=7, A=14): K=2, L=5. Config: 2,5. Valence e&sup-;: 5. Valency: 3.

O (Z=8, A=16): K=2, L=6. Config: 2,6. Valence e&sup-;: 6. Valency: 2.

F (Z=9, A=19): K=2, L=7. Config: 2,7. Valence e&sup-;: 7. Valency: 1.

Ne (Z=10, A=20): K=2, L=8. Config: 2,8. Valence e&sup-;: 8. Valency: 0. (L full, noble gas.)

Na (Z=11, A=23): K=2, L=8, M=1. Config: 2,8,1. Valence e&sup-;: 1. Valency: 1.

Mg (Z=12, A=24): K=2, L=8, M=2. Config: 2,8,2. Valence e&sup-;: 2. Valency: 2.

Al (Z=13, A=27): K=2, L=8, M=3. Config: 2,8,3. Valence e&sup-;: 3. Valency: 3.

Si (Z=14, A=28): K=2, L=8, M=4. Config: 2,8,4. Valence e&sup-;: 4. Valency: 4.

P (Z=15, A=31): K=2, L=8, M=5. Config: 2,8,5. Valence e&sup-;: 5. Valency: 3.

S (Z=16, A=32): K=2, L=8, M=6. Config: 2,8,6. Valence e&sup-;: 6. Valency: 2.

Cl (Z=17, A=35): K=2, L=8, M=7. Config: 2,8,7. Valence e&sup-;: 7. Valency: 1.

Ar (Z=18, A=40): K=2, L=8, M=8. Config: 2,8,8. Valence e&sup-;: 8. Valency: 0. (Noble gas.)

K (Z=19, A=39): K=2, L=8, M=8, N=1. Config: 2,8,8,1. Valence e&sup-;: 1. Valency: 1.

Ca (Z=20, A=40): K=2, L=8, M=8, N=2. Config: 2,8,8,2. Valence e&sup-;: 2. Valency: 2.

NCERT Example — Draw the structure of sodium atom

Na: Z=11 (11 protons in nucleus), A=23, so neutrons = 23 − 11 = 12. Electrons = 11 distributed as K=2, L=8, M=1. Draw three concentric circles around the nucleus (labelled K, L, M); place 2 electrons on K, 8 on L, 1 on M.

6. Valence Electrons and Valency

The electrons in the outermost shell of an atom are its valence electrons. They determine the atom's chemical reactivity, bonding ability, and valency.

Rules for finding valency (NCERT)

Valence electrons 1, 2, or 3 → Valency = valence electrons (tend to lose).
Valence electrons = 4 → Valency = 4 (share electrons; e.g. carbon).
Valence electrons 5, 6, or 7 → Valency = 8 − (valence electrons) (tend to gain).
Valence electrons = 8 → Valency = 0 (stable noble gas; does not react).
NCERT examples of valency

Na: 1 valence e&sup-; → valency 1 (loses 1 e&sup-; to form Na+).
Mg: 2 valence e&sup-; → valency 2.
Al: 3 valence e&sup-; → valency 3.
C: 4 valence e&sup-; → valency 4 (forms 4 covalent bonds).
N: 5 valence e&sup-; → valency 3 (8−5).
O: 6 valence e&sup-; → valency 2 (8−6).
Cl: 7 valence e&sup-; → valency 1 (8−7).
Ar: 8 valence e&sup-; → valency 0 (noble gas).

Key point: Elements in the same group of the periodic table have the same number of valence electrons and the same valency — this is why they have similar chemical properties.

7. Atomic Number (Z) and Mass Number (A)

Atomic Number (Z)

The atomic number of an element is the number of protons in its nucleus.

Z = number of protons
In a neutral atom: Z = protons = electrons.
Z uniquely identifies every element.

Mass Number (A)

The mass number is the total count of protons and neutrons (nucleons) in the nucleus.

A = protons + neutrons = Z + neutrons
Therefore: Neutrons = A − Z

Electrons have negligible mass, so they do NOT contribute to the mass number.

NCERT Example — Neutrons in Chlorine

Chlorine: Z = 17. Isotope Cl-35: A = 35, neutrons = 35 − 17 = 18. Isotope Cl-37: A = 37, neutrons = 37 − 17 = 20.

NCERT Example — Phosphorus

P: Z = 15, A = 31. Neutrons = 31 − 15 = 16. Config: 2, 8, 5.

NCERT Example — Calcium

Ca: Z = 20, A = 40. Neutrons = 40 − 20 = 20. Config: 2, 8, 8, 2.

8. Isotopes

Isotopes are atoms of the same element that have the same atomic number (Z) but different mass numbers (A). They differ in the number of neutrons. Isotopes have identical chemical properties (same valence electrons) but different physical properties (different masses).

Isotopes of Hydrogen (most important NCERT example)

Protium (ordinary hydrogen, ¹H): Z=1, A=1, neutrons=0.
Deuterium (²H or D): Z=1, A=2, neutrons=1. Also called heavy hydrogen.
Tritium (³H or T): Z=1, A=3, neutrons=2. Radioactive.

Isotopes of Carbon

Carbon-12 (¹²C): Z=6, A=12, neutrons=6. Most abundant (98.9%).
Carbon-13 (¹³C): Z=6, A=13, neutrons=7. Stable (1.1%).
Carbon-14 (¹&sup4;C): Z=6, A=14, neutrons=8. Radioactive — used in radiocarbon dating.

Isotopes of Chlorine

Chlorine exists as Cl-35 (~75%) and Cl-37 (~25%). The atomic mass of chlorine is the weighted average ≈ 35.5 u — this explains why it is a non-integer value.

Uses of radioactive isotopes (NCERT)

  • Uranium-235 — fuel in nuclear reactors and atomic bombs.
  • Cobalt-60 — treatment of cancer (radiotherapy).
  • Iodine-131 — treatment of goitre (thyroid disorders).
  • Carbon-14 — radiocarbon dating of archaeological remains.
Why isotopes have the same chemistry: they have the same number of protons and electrons, so the same electronic configuration and valence electrons → identical chemical behaviour.

9. Isobars

Isobars are atoms of different elements (different atomic numbers) that have the same mass number. They differ in the number of protons (and electrons), so they have completely different chemical properties.

NCERT examples of isobars

Calcium (Ca): Z=20, A=40, config: 2,8,8,2.
Argon (Ar): Z=18, A=40, config: 2,8,8.
Both have A=40 but are completely different elements with different properties.
Carbon-14 (¹&sup4;C): Z=6, A=14.
Nitrogen-14 (¹&sup4;N): Z=7, A=14.
Same mass number, different elements.

Isotopes vs Isobars — comparison table

Isotopes: same Z, different A, different number of neutrons. Same element. Same chemical properties. Different physical properties.

Isobars: different Z, same A, different elements. Different chemical properties. Examples: Ca and Ar (A=40).

10. NCERT Exercises — Fully Solved

Ex 1 — What are canal rays?

Canal rays (anode rays) are positively charged particles discovered by Goldstein using a perforated cathode. They travel through the holes toward the cathode, opposite to cathode rays. When hydrogen gas is used, the canal ray particle is the proton — the smallest positive ion.

Ex 2 — If an element has no neutrons in its nucleus, what are its atomic number and mass number?

The only naturally occurring atom with no neutrons is ordinary hydrogen (protium). Atomic number Z = 1 (one proton). Mass number A = protons + neutrons = 1 + 0 = 1.

Ex 3 — Helium has 2 protons and 2 neutrons. Write its electronic configuration.

Z = 2, so 2 electrons. K shell (max 2) accommodates both. Configuration: 2. K is completely filled. He is a noble gas with valency 0.

Ex 4 — How many electrons, protons, and neutrons are in an atom with Z=3, A=7?

Element: Lithium. Protons = 3. Electrons = 3 (neutral atom). Neutrons = A − Z = 7 − 3 = 4. Electronic configuration: 2, 1. Valence electrons = 1, valency = 1.

Ex 5 — Write the electronic configuration and find neutrons for Na (Z=11, A=23)

Protons = 11. Electrons = 11. Neutrons = 23 − 11 = 12. Config: K=2, L=8, M=1 (written 2,8,1). Valence electrons = 1. Valency = 1. Bohr diagram: nucleus with 11p + 12n; three concentric rings with 2, 8, 1 electrons.

Ex 6 — Complete the particle table (NCERT Exercise 4.4 type)

Carbon: Z=6, A=12, p=6, n=6, e=6, config=2,4.
Aluminium: Z=13, A=27, p=13, n=14, e=13, config=2,8,3.
Chlorine-35: Z=17, A=35, p=17, n=18, e=17, config=2,8,7.
Calcium: Z=20, A=40, p=20, n=20, e=20, config=2,8,8,2.

Ex 7 — Name two isotopes of hydrogen and state how they differ

Deuterium (D, A=2) and Tritium (T, A=3). Both have Z=1 (one proton and one electron). Deuterium has 1 neutron, tritium has 2 neutrons. Both have the same electronic configuration (1 electron in K shell) so they are chemically almost identical, but tritium is radioactive.

Ex 8 — Which is heavier: nucleus or the whole atom? By how much?

The nucleus contains virtually all the mass (protons and neutrons). Electrons contribute only about 1/1836 of a proton's mass each. For example, in carbon-12 (6 protons, 6 neutrons, 6 electrons), electrons contribute 6 × (1/1836) ≈ 0.003 u out of 12 u. The nucleus accounts for more than 99.9% of the atom's mass.

11. Common Mistakes to Avoid

  • Confusing isotopes (same Z, different A) with isobars (same A, different Z). Memory trick: isotoPes → same number of Protons.
  • Using 2n² for the outermost shell — it is capped at 8 in NCERT; only inner shells use the full 2n² formula.
  • Saying atomic number = mass number — they are equal only if there are no neutrons (ordinary H only).
  • Saying electrons contribute to mass number — they do not (mass negligible).
  • In Rutherford's experiment, saying most particles bounce back — only a very few do; most pass straight through.
  • Proton was discovered by Goldstein (canal rays); neutron by Chadwick. Do not mix them up.
  • Thomson's model was disproved by Rutherford's alpha-scattering experiment, not by Goldstein.
  • Calling valence electrons of noble gases as 8 and saying valency is 8 — noble gas valency is 0 (complete octet, does not react).
Practice MCQs
1. Who discovered the electron?
  1. Goldstein
  2. Chadwick
  3. J. J. Thomson
  4. Rutherford
Answer: (C) J. J. Thomson discovered the electron in 1897 using cathode-ray tube experiments.
2. In Rutherford's alpha-scattering experiment, most alpha particles:
  1. bounced straight back at 180°
  2. were absorbed by the gold foil
  3. passed straight through without deflection
  4. were deflected at exactly 90°
Answer: (C) Most passed straight through — proving the atom is mostly empty space.
3. The maximum number of electrons in the M shell (n=3) by the 2n² formula is:
  1. 8
  2. 16
  3. 18
  4. 32
Answer: (C) 2n² = 2 × 3² = 18. (For NCERT purposes the outermost M shell is capped at 8, but the shell capacity is 18.)
4. The electronic configuration of Chlorine (Z=17) is:
  1. 2, 8, 8
  2. 2, 8, 6
  3. 2, 8, 7
  4. 2, 7, 8
Answer: (C) 2, 8, 7 — K=2, L=8, M=7. Valence electrons = 7, valency = 1 (needs one more to complete octet).
5. Isotopes of an element have the same:
  1. mass number
  2. number of neutrons
  3. atomic number
  4. number of nucleons
Answer: (C) Isotopes have the same atomic number (same Z = same protons) but different mass numbers.
6. Calcium (Z=20, A=40) and Argon (Z=18, A=40) are an example of:
  1. Isotopes
  2. Isobars
  3. Isotones
  4. Allotropes
Answer: (B) Isobars — same mass number (40) but different atomic numbers (20 and 18). They are different elements.
7. The neutron was discovered by:
  1. Thomson
  2. Rutherford
  3. Goldstein
  4. Chadwick
Answer: (D) James Chadwick discovered the neutron in 1932 by bombarding beryllium with alpha particles.
8. An atom has electronic configuration 2, 8, 2. Its valency is:
  1. 8
  2. 6
  3. 4
  4. 2
Answer: (D) 2 valence electrons → valency = 2. This element is Magnesium (Z=12).
9. Which of the following is the main limitation of Rutherford's atomic model?
  1. It could not explain cathode rays
  2. It said electrons are embedded in positive charge
  3. It could not explain the stability of atoms
  4. It could not explain the existence of protons
Answer: (C) A revolving charged electron should radiate energy continuously and spiral into the nucleus — Rutherford's model gave no mechanism for atomic stability.
10. Iodine-131 (a radioactive isotope) is used in:
  1. Nuclear reactors as fuel
  2. Cancer radiotherapy
  3. Treatment of goitre (thyroid disorders)
  4. Radiocarbon dating
Answer: (C) Iodine-131 is used in the treatment of goitre and thyroid disorders.
11. Canal rays were discovered by:
  1. Chadwick
  2. Thomson
  3. Goldstein
  4. Bohr
Answer: (C) Eugen Goldstein discovered canal rays (anode rays) in 1886 using a perforated cathode.
12. An element X has Z=11 and A=23. The number of neutrons in X is:
  1. 11
  2. 23
  3. 12
  4. 34
Answer: (C) Neutrons = A − Z = 23 − 11 = 12. Element X is Sodium.
Assertion–Reason
A: Isotopes of the same element have identical chemical properties.
R: Isotopes have the same number of valence electrons.
Answer: Both A and R are true, and R is the correct explanation of A. Isotopes differ only in neutrons (in the nucleus). Their electron arrangement — and especially valence electrons — is identical, giving identical chemical behaviour.
A: In Rutherford's experiment, very few alpha particles were deflected at large angles.
R: The nucleus of an atom is very small and densely packed with positive charge.
Answer: Both A and R are true, and R is the correct explanation of A. Only when an alpha particle approaches extremely close to the tiny, dense, positively charged nucleus does it experience a strong enough repulsive force to bounce back.
Previous-year questions
PYQ 1. State the observations and conclusions of Rutherford's alpha-particle scattering experiment. (CBSE, 3 marks)
Observations: (i) Most alpha particles passed straight through the gold foil without deflection. (ii) Some were deflected by small angles. (iii) A very few (≈1 in 12,000) bounced back at angles greater than 90°, some even at 180°.
Conclusions: (i) The atom is mostly empty space. (ii) All positive charge and nearly all the mass are concentrated in a tiny, dense nucleus at the centre. (iii) Electrons revolve around the nucleus in the vast empty space.
PYQ 2. Write the electronic configuration of (a) Oxygen (Z=8), (b) Sodium (Z=11), (c) Calcium (Z=20). State the valence electrons and valency of each. (CBSE, 3 marks)
(a) Oxygen: config 2, 6 — valence electrons = 6, valency = 2 (8−6).
(b) Sodium: config 2, 8, 1 — valence electrons = 1, valency = 1.
(c) Calcium: config 2, 8, 8, 2 — valence electrons = 2, valency = 2.
PYQ 3. What are isotopes? Give two examples and state two uses of radioactive isotopes. (CBSE, 3 marks)
Isotopes are atoms of the same element with the same atomic number (Z) but different mass numbers (A) — they differ in the number of neutrons.
Examples: (i) Hydrogen isotopes: protium (A=1), deuterium (A=2), tritium (A=3), all with Z=1. (ii) Carbon isotopes: C-12 and C-14, both Z=6.
Uses of radioactive isotopes: (i) Cobalt-60 is used in cancer treatment (radiation therapy). (ii) Iodine-131 is used in treatment of goitre/thyroid disorders.
PYQ 4. Differentiate between Thomson's model and Bohr's model of the atom. (CBSE, 2 marks)
Thomson's model: atom is a uniform positively charged sphere; electrons are embedded throughout the sphere; no separate nucleus.
Bohr's model: atom has a central positively charged nucleus; electrons revolve in fixed circular orbits (shells) of definite energy; electrons do not radiate energy while in their orbits; they jump between orbits by absorbing or emitting specific quanta of energy, explaining the line spectra of elements.
PYQ 5. An element has 15 protons and 16 neutrons. Find its atomic number, mass number, and electronic configuration. Also find its valency. (CBSE, 3 marks)
Element: Phosphorus.
Atomic number Z = 15 (number of protons).
Mass number A = protons + neutrons = 15 + 16 = 31.
Electronic configuration: K=2, L=8, M=5 → written as 2, 8, 5.
Valence electrons = 5. Valency = 8 − 5 = 3 (phosphorus gains 3 electrons or shares 3).
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