Atomic Concepts and the Periodic Table
This domain covers atomic models, subatomic particles, isotopes, electron configurations, and the periodic trends that let you predict radius, ionization energy, and electronegativity without memorizing every element.
Atomic structure
- Protons (+) and neutrons (0) sit in the nucleus; electrons (−) occupy the space outside it.
- Atomic number = protons. Mass number = protons + neutrons. A neutral atom has electrons equal to protons.
- Isotopes share an atomic number but differ in mass number. The atomic mass on the periodic table is the weighted average of isotope masses, so it is rarely a whole number.
Electron arrangement
- Electrons fill the lowest available sublevels: 1s²2s²2p⁶3s²3p⁶4s²3d¹⁰.
- Valence electrons lie in the highest principal energy level and control chemical behavior; for main-group elements the group number gives the valence count.
- An excited-state configuration shows an electron promoted to a higher sublevel, such as 1s²2s²2p⁶3s¹3p¹ for magnesium. The electron drops back and emits light at fixed frequencies, producing the bright-line spectrum that proves energy levels are quantized.
Periodic trends
Across a period, atomic radius decreases while ionization energy, electronegativity, and nonmetallic character increase. Down a group, radius increases while ionization energy and electronegativity decrease. Metals on the left lose electrons to form cations; nonmetals on the upper right gain electrons; metalloids border the staircase.
Worked example
Which has the larger atomic radius, Na or Mg? Both are in period 3, but nuclear charge rises from Na to Mg while shielding stays similar, pulling electrons inward, so Na is larger. Which has the higher first ionization energy, Mg or Ca? Ca's valence electron is in a higher energy level, farther from the nucleus and better shielded, so Mg has the higher value.
Traps
- Confusing mass number with atomic mass.
- Giving noble gases high electronegativity values; they are generally not assigned.
- Forgetting that a cation has fewer electrons than its neutral atom.
- Writing an excited-state configuration that still follows the normal filling order.