What is inside an atom, how electrons are arranged, and how atoms join by giving, taking or sharing electrons.
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An atom has a tiny dense nucleus holding protons (charge +1, mass 1 u) and neutrons (charge 0, mass 1 u), with electrons (charge −1, mass about 1/1836 u) moving in shells around it. The atomic number Z is the number of protons (equal to the electrons in a neutral atom); the mass number A is protons plus neutrons, so neutrons = A − Z. Isotopes are atoms of the same element with the same Z but different A (chlorine-35 and chlorine-37; hydrogen-1, -2, -3). Isobars have the same A but different Z.
Find the number of protons, electrons and neutrons in sodium (Z = 11, A = 23).
Protons 11, electrons 11, neutrons 23 − 11 = 12.
Chlorine has isotopes Cl-35 (75%) and Cl-37 (25%). Find its average atomic mass.
0.75 × 35 + 0.25 × 37 = 35.5.
Why is an atom electrically neutral?
The number of protons (positive) equals the number of electrons (negative).
Electrons occupy shells K, L, M, N (n = 1, 2, 3, 4) with a maximum of 2n² electrons: 2, 8, 18, 32. The outermost shell holds at most 8 (2 for the first shell) and the electrons there are valence electrons. For the first 20 elements fill K with 2, L with 8, M with 8 and then N. Sodium (Z = 11) is 2,8,1; chlorine (Z = 17) is 2,8,7; argon (Z = 18) is 2,8,8 and has a full outer shell, so it is inert. Atoms bond to reach this stable octet (duplet for hydrogen and helium).
Write the electronic configuration of Z = 11, 17, 18 and 20.
Na 11: 2,8,1. Cl 17: 2,8,7. Ar 18: 2,8,8. Ca 20: 2,8,8,2.
What is the valency of chlorine and why?
Chlorine is 2,8,7. It needs one electron to complete its octet, so its valency is 1 (it forms Cl⁻).
What is the maximum number of electrons in the third shell?
2 × 3² = 18.
An ionic (electrovalent) bond forms by transfer of electrons from a metal to a non-metal: Na (2,8,1) gives one electron to Cl (2,8,7), forming Na⁺ and Cl⁻ held by electrostatic attraction. A covalent bond forms by sharing electron pairs between non-metals: H₂ (one pair, single bond), O₂ (two pairs, double bond), N₂ (three pairs, triple bond), H₂O, NH₃, CH₄, CO₂. Ionic compounds are hard crystalline solids with high melting points that conduct when molten or dissolved. Covalent compounds are usually gases, liquids or soft solids with low melting points and do not conduct.
Show how MgCl₂ forms by electron transfer.
Mg (2,8,2) loses two electrons to become Mg²⁺; each of two Cl atoms (2,8,7) gains one to become Cl⁻. Result: Mg²⁺ and 2Cl⁻.
How many shared pairs are in a nitrogen molecule N₂?
Three pairs (triple bond), since each N has 5 valence electrons and needs 3 more.
Why does sodium chloride conduct electricity only when molten or in solution?
Only then are its ions free to move; in the solid they are fixed in the crystal.
1. Name the three fundamental particles of an atom with their charges.
2. Define atomic number and mass number.
3. Find the number of neutrons in Cl-35.
4. What are isotopes? Give an example.
5. Write the electronic configuration of oxygen (Z = 8) and name its valency.
6. State the maximum electrons in the K, L, M and N shells.
7. Differentiate ionic and covalent bonds.
8. Name the bond in (a) NaCl, (b) H₂O.
9. Why is argon inert?
10. Give two properties of ionic compounds.