An atom is electrically neutral because it contains equal numbers of protons (positively charged particles in the nucleus) and electrons (negatively charged particles outside the nucleus). Each proton carries a charge of +1e and each electron carries a charge of −1e. Since the number of protons equals the number of electrons, the positive and negative charges exactly cancel each other, giving the atom a net charge of zero.
An atom is electrically neutral because the number of protons equals the number of electrons.
Proton charge = +1; Electron charge = −1; Neutron charge = 0.
Net charge of neutral atom = 0 (positive and negative charges cancel).
For a neutral atom: atomic number = number of protons = number of electrons.
When an atom loses electrons, it becomes a positive ion (cation).
When an atom gains electrons, it becomes a negative ion (anion).
The number of protons never changes — it defines the element (atomic number).
Only electrons are lost or gained when ions are formed.
Explanation:
Charge of subatomic particles: • Proton: charge = +1 (or +1.6 × 10⁻¹⁹ C) • Electron: charge = −1 (or −1.6 × 10⁻¹⁹ C) • Neutron: charge = 0 (electrically neutral)
In a neutral atom: • Number of protons = Number of electrons • Total positive charge = Total negative charge • Net charge = (+1 × number of protons) + (−1 × number of electrons) = 0
Example — Carbon (C): • Atomic number = 6 • Protons = 6 (charge = +6) • Electrons = 6 (charge = −6) • Neutrons = 6 (charge = 0) • Net charge = +6 + (−6) = 0 ✓
Example — Sodium (Na): • Atomic number = 11 • Protons = 11, Electrons = 11 • Net charge = 0 ✓
When an atom gains or loses electrons, it is no longer neutral — it becomes an ion:
Cation (positive ion): • Atom loses one or more electrons • Protons > Electrons → net positive charge • Example: Na → Na⁺ + e⁻ Na: 11p, 11e → neutral Na⁺: 11p, 10e → charge = +1
Anion (negative ion): • Atom gains one or more electrons • Electrons > Protons → net negative charge • Example: Cl + e⁻ → Cl⁻ Cl: 17p, 17e → neutral Cl⁻: 17p, 18e → charge = −1
Key point: The number of protons (atomic number) NEVER changes — it defines the element. Only the number of electrons changes when ions form.
Discovery of atomic neutrality:
Thomson's Plum Pudding Model (1904): • Proposed that electrons are embedded in a positive sphere • Atom is neutral because positive and negative charges balance
Rutherford's Nuclear Model (1911): • Discovered the nucleus — all positive charge is concentrated in the nucleus • Electrons orbit the nucleus at a distance • Atom is neutral: nuclear charge = electron charge
Bohr's Model (1913): • Electrons in fixed circular orbits (shells) • For element with atomic number Z: Protons = Z, Electrons = Z (neutral atom)
Key rule: For a neutral atom: Atomic number (Z) = Number of protons = Number of electrons If the atom is an ion with charge n: • Number of electrons = Z − n (for n-positive ion) • Number of electrons = Z + n (for n-negative ion)
An atom is electrically neutral because the number of protons (positive charge) in the nucleus equals the number of electrons (negative charge) outside the nucleus. Since each proton has charge +1 and each electron has charge −1, the total positive charge equals the total negative charge, giving a net charge of zero.
Proton: charge = +1 (positive). Electron: charge = −1 (negative). Neutron: charge = 0 (neutral). The nucleus contains protons and neutrons; electrons are outside the nucleus in shells.
When an atom loses electrons, it has more protons than electrons — the net charge becomes positive. This forms a cation (positive ion). For example, Na → Na⁺ + e⁻. Na has 11 protons and 10 electrons, so net charge = +1.
When an atom gains electrons, it has more electrons than protons — the net charge becomes negative. This forms an anion (negative ion). For example, Cl + e⁻ → Cl⁻. Cl⁻ has 17 protons and 18 electrons, so net charge = −1.
In a neutral atom, the atomic number (Z) equals both the number of protons and the number of electrons. Since Z protons give charge +Z and Z electrons give charge −Z, the net charge is zero. Atomic number uniquely identifies each element.
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