MCAT Study Guide/Atoms
John Dalton's Theories on the Atom
a. An atom is the smallest representative unit of an element that retains all of the characteristics of that element. [Law of Conservation of Matter] b. Atoms of different elements have different properties. c. During a (non-nuclear) chemical reactions are neither created nor destroyed. d. Atoms of more than one element may react to form compounds; the number of atoms of each element is constant in a pure compound.
Components of the Atom
- Nucleus: center of the atom
- Neutrons have no charge; have a mass of ~1 dalton; number of neutrons determines isotope
- Protons have a relative charge of +1; have a mass of ~1 dalton; number of protons determines atomic number and element identity
- Electron Orbitals: surround the nucleus
- Electrons have a relative charge of -1; have a mass of ~0.0005 dalton; number of electrons is typically equal to the number of protons in the nucleus, except in ions
Energy Levels of Electron Orbitals
Quantum numbers determine placement of the electrons:
- Principal quantum number (n) determines shell; start with 1, in increasing succession (1, 2, 3, 4, ...), in increasing distance from the nucleus and increasing energy. The K shell has a principal quantum number of 1. The total number of electrons in a shell is 2n2
- Angular momentum quantum number (l) determines the subshell; starts with 0, in increasing succession (0, 1, 2, ..., n-1).
- Magnetic quantum number (m1) determines the orbital; starts with l, in decreasing and increasing succession [-l, (1-l), ..., 0, ..., (l-1), l].
- Spin quantum number (m2) determines the electron spin; value is always either -1/2 or +1/2.
Pauli's exclusion principle explains that no two electrons in an atom have exactly identical sets of quantum numbers.
Orbitals are filled from lower to higher energy levels.