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Atomic Structure
Protons, Neutrons, and Electrons
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Every atom consists of a dense nucleus (protons + neutrons) surrounded by electrons in energy levels called shells or orbitals. The number of protons - the atomic number - uniquely identifies the element and never changes for a given element. The number of neutrons can vary, giving rise to isotopes. The number of electrons in a neutral atom equals the number of protons, but electrons can be gained or lost to form ions.
Electrons do not orbit the nucleus like planets. They occupy regions of probability called orbitals, organized into shells (n = 1, 2, 3...) and subshells (s, p, d, f). The first shell holds 2 electrons, the second holds 8, and the pattern continues. The arrangement of electrons in the outermost shell - the valence shell - determines how an element bonds and reacts.
Key Facts
- Atomic number
- Mass number
- Isotopes: same number of protons, different number of neutrons - same element, different mass.
- Ion: atom that has gained or lost electrons. Positive ion (cation): lost electrons. Negative ion (anion): gained electrons.
- First shell: max 2 electrons. Second shell: max 8 electrons. Third shell: max 18 electrons.
- Valence electrons: electrons in the outermost occupied shell; determine bonding behavior.
Vocabulary
- Proton
- Positively charged particle in the nucleus; defines the element's atomic number.
- Neutron
- Neutral particle in the nucleus; contributes to atomic mass and distinguishes isotopes.
- Electron
- Negatively charged particle outside the nucleus; determines chemical behavior.
- Isotope
- Atoms of the same element with different numbers of neutrons (and therefore different mass numbers).
- Electron shell
- A region around the nucleus where electrons of a given energy level are found.
Common Mistakes to Avoid
- Confusing atomic number with atomic mass. Atomic number (whole number) counts protons; atomic mass (often a decimal) is the weighted average across isotopes.
- Thinking the mass number is always a whole number for a specific element. Individual atoms have integer mass numbers, but the periodic table shows average atomic mass across all natural isotopes.
- Forgetting that in an ion, the number of electrons changes but protons don't. Fe³⁺ still has 26 protons but only 23 electrons.
- Placing more than 8 electrons in the second shell in simple Bohr models. The Bohr model caps shells at 2, 8, 8, 18 - but the third shell holds more than 8 only after the 4s fills.
Practice Questions
- 1 An atom has 17 protons, 18 neutrons, and 17 electrons. Identify the element, write its atomic symbol, and state its mass number.
- 2 Carbon-12 and Carbon-14 are isotopes. How do they differ in atomic structure?
- 3 An ion has 11 protons and 10 electrons. What element is it, and what is the charge on the ion?