MCAT Chemistry > Atomic Structure
Protons (H) form part of the nucleus
- Fundamental charge = +1 (1.6 x 10-19 C)
- Massproton = 1 AMU
See Atomic Number
Atomic Number = number of protons.
Neutrons (n) form part of the nucleus; amount may vary (isotopes)
- Fundamental charge = 0
- Massneutron = 1 AMU
See Atomic Mass
Atomic Mass = number of protons + neutrons in an atom
- 1 Atomic Mass Unit (AMU) = 1/12 of a C12 atom = 1.66 x 10-19g
Electrons (e-) move in discrete pathways around the nucleus called orbitals. Electrons closest to nucleus have lower energy and electrons furthest away have the highest energy
- Fundamental charge = -1 (-1.6 x 10-19 C)
- Masselectron = 1/1836 that of a proton.
See Atomic Weight
Atomic Weight (Molar Mass) (g/mol) is the mass of one mole of atoms
1 mole = Avogadro's number = 6.022 x 1023 particles = 22.4 L
Bohr's Atomic Model states that there is a dense core nucleus of one proton around which, a single electron travels in a discrete circular orbital. The electrons energy increases as it moves further away from nucleus into surrounding orbitals.
Transferring an amount of energy equal to the difference between pathways results in "jumping" between pathways and moving from the ground to an excited state or vice versa. Electrons are only affected by radiation and high temperature
E = hf
Angular Momentum = L = (nh)/2π
Energy of the electron = E = -RH/n2
Atomic Emission & Absorption Spectra
Atoms at room temperature are in the ground state (n = 1), however electrons can become excited by absorbing heat and energy (+ΔE/endothermic) to jump to higher orbital levels. The excitation of electrons results in energy absorption at a specific λ. As the electrons rapidly return to the ground state from this excited state, energy emission in the form of discrete amounts of photons (light) occurs at a specific frequency on the visible light spectrum (−ΔE/exothermic). We see color not absorbed by the compound. Each element possesses unique characteristic absorption spectra. The λ of absorption corresponds to λ of emission (energy level difference unchanged).
E = (hc)/λ
Balmer Series represents a pattern of photon emission from hydrogen electrons falling from n > 2 to n = 2 energy level (4 wavelengths)
Lyman Series represents a pattern of transitions from n > 1 to n = 1. This causes larger energy transitions and a shorter photon wavelength. Molecules usually appear in the UV region.
E = -RH(1/nf2) – (1/ni2)
Heisenberg's Uncertainty Principle
Heisenberg's Uncertainty Principle states that we can only determine either momentum or location of an electron but not both at a given time
See Quantum Numbers
Click here to move onto periodic table trends.