📖 In-Depth Theory
Atomic Number and Mass Number
ATOMIC NUMBER (Z) = number of PROTONS in the nucleus.
Defines the element — all atoms of the same element have the same atomic number.
In a neutral atom: atomic number = number of electrons.
MASS NUMBER (A) = total number of PROTONS + NEUTRONS.
Also called nucleon number.
Number of neutrons = mass number − atomic number = A − Z
NUCLEAR NOTATION:
₍Z₎A X
Where X = element symbol, A = mass number (top), Z = atomic number (bottom).
EXAMPLES:
¹²₆C — carbon: 6 protons, 6 neutrons, 6 electrons
²³₁₁Na — sodium: 11 protons, 12 neutrons, 11 electrons
²³⁵₉₂U — uranium: 92 protons, 143 neutrons, 92 electrons
Isotopes
ISOTOPES are atoms of the same element with the SAME atomic number but DIFFERENT mass numbers.
Same element → same number of protons.
Different mass numbers → different numbers of NEUTRONS.
EXAMPLES:
Carbon isotopes:
¹²C — 6 protons, 6 neutrons (most common, stable)
¹³C — 6 protons, 7 neutrons (stable, rare)
¹⁴C — 6 protons, 8 neutrons (radioactive — used in carbon dating)
Hydrogen isotopes:
¹H — 1 proton, 0 neutrons (protium — most common)
²H — 1 proton, 1 neutron (deuterium)
³H — 1 proton, 2 neutrons (tritium — radioactive)
CHEMICAL PROPERTIES:
Isotopes of the same element have the SAME chemical properties — same electron arrangement.
Physical properties differ slightly (different mass → different density, melting point etc.).
SOME ISOTOPES ARE RADIOACTIVE (unstable nucleus) — they undergo radioactive decay.
Ions
When atoms GAIN or LOSE electrons they become IONS.
POSITIVE ION (CATION): loses electrons → more protons than electrons → positive charge.
NEGATIVE ION (ANION): gains electrons → more electrons than protons → negative charge.
Example:
Sodium atom ²³₁₁Na: 11 protons, 11 electrons — neutral.
Sodium ion Na⁺: 11 protons, 10 electrons — lost 1 electron → charge +1.
Fluorine atom ¹⁹₉F: 9 protons, 9 electrons — neutral.
Fluoride ion F⁻: 9 protons, 10 electrons — gained 1 electron → charge −1.
Note: NUCLEAR NOTATION and mass number / atomic number are NOT changed by ion formation — only the electron count changes.
⚠️ Common Mistake
Mass number is protons + NEUTRONS — not protons alone. To find neutrons: subtract atomic number FROM mass number. Isotopes have the same atomic number (same element) but different mass numbers (different neutron counts).