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Part 1 of 1
The atom consists of a dense, positively charged nucleus containing protons and neutrons, surrounded by electrons in quantized orbitals. The atomic number (Z) defines the identity of an element by its proton count, while the mass number (A) represents the total sum of protons and neutrons (A = Z + N). Isotopes are atoms of the same chemical element that possess the identical atomic number Z (and identical electron count, conferring identical chemical properties) but differ in neutron count N and mass number A. For example, standard Carbon-12 has 6 protons and 6 neutrons, whereas the radioisotope Carbon-14 has 6 protons and 8 neutrons (14 - 6 = 8).
Electrons occupy discrete energy levels governed by quantum mechanics. The Aufbau principle dictates that electrons fill lower-energy subshells before higher ones (1s < 2s < 2p < 3s < 3p < 4s < 3d). The Pauli exclusion principle mandates that an orbital holds at most two electrons with opposite spins. Hund's rule requires that degenerate orbitals within a subshell are each occupied by one electron before pairing begins.
The ground-state electron configuration of neutral carbon (Z = 6) is 1s^2 2s^2 2p^2. When main-group elements form ions to achieve a stable noble-gas valence octet, they gain or lose valence electrons: a neutral sodium atom (Z = 11, 1s^2 2s^2 2p^6 3s^1) loses its single 3s valence electron to form the sodium cation (Na^+), yielding the stable neon-isoelectronic configuration 1s^2 2s^2 2p^6.
Worked example
Step 1: Determine the subatomic composition and ground-state electron configuration of a neutral chlorine atom (Z = 17, A = 35) and its chloride anion (Cl^-). Step 2: Protons = Z = 17; Neutrons = A - Z = 35 - 17 = 18; Electrons in neutral atom = 17. Step 3: Fill subshells according to Aufbau: 1s^2 2s^2 2p^6 3s^2 3p^5. The valence shell is n = 3, holding 2 + 5 = 7 valence electrons. Step 4: Formation of Cl^-: Chlorine gains 1 electron to complete its valence shell, producing Cl^- with 18 electrons and configuration 1s^2 2s^2 2p^6 3s^2 3p^6, matching noble gas argon.
For the exam
1.Isotopes of an element share the same atomic number Z but have different mass numbers A.
2.The ground-state electron configuration of a neutral carbon atom (Z = 6) is 1s^2 2s^1 2p^3.
3.A sodium ion Na^+ has the electron configuration 1s^2 2s^2 2p^6.
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