atom:
  Smallest (about 10^-10 m) unit of matter, forming a specific element. It
  consists of:

  - nucleus: Small (about 10000 smaller than atom) but very dense (almost all
    atom weight) area containing nucleons, which are:

    - protons: positively charged particles
    - neutrons: particles without electric charge

    these are held together by nuclear force that overcomes electric force that
    otherwise pushes protons away from each other.

  - electron cloud: Contains electrons (negatively charged particles), that are
    attached to protons via electromagnetic force. The cloud is subdivided into
    shells (layers), which are subdivided into subshells:

    shell    subshells   max electrons                ^ closer to nucleus
    --------------------------------------------      | lower energy
    1 (K)    1s          2    > 2 = 2                 |
                                                      
    2 (L)    2s          2    \ 2 + 6 = 8
             2p          6    /   

    3 (M)    3s          2    \
             3p          6     > 2 + 6 + 10 = 18
             3d          10   /

    4 (N)    4s          2    \
             4p          6     \ ... = 32
             4d          10    /
             4f          14   /

    5 (O)    5s          2    \
             5p          6     \
             5d          10     > ... = 50
             5f          14    /
             5g          18   /
    ...                                               |
                                                      | higher energy
                                                      v further from nucleus

    The last (furthest) shell occupied by electrons is called valence shell and
    mostly determines chemical properties such as conductivity. Shells are
    filled from the innermost, but not generally -- with the higher ones the
    rule is more complex (so there can be non-filled non-valence shell).

    Each subshell can further take a form of one of possible orbitals, i.e.
    probability "shape" of where the electron can be found. Some subshells, e.g.
    1s or 2s have only one possible orbital (shape) but others, e.g. 2p, have
    multiple (2px, 2py, 2pz).

  Atom has the same number of protons and electrons and so has a net electric
  charge 0, but can have different number of neutrons, forming different
  isotopes.

electron configuration: Says the specific placement of electrons in (sub)shells.
  The lowest energy configuration is called a ground state, e.g. 1s^2 2s^2 2p^6
  3s^1 for sodium, others are called excited states, e.g. 1s^2 2s^2 2p^6 3p^1.
  Transition between configurations consumes or emits energy (photons).

isotope: Variant of the same atom, differing in number of neutrons (and weight),
  e.g. deuterium (helium with 2 neutrons, so called heavy hydrogen).

ion: Is formed from an atom or molecule that has different number of electrons
  than protons, giving it either a positive or negative electric charge (e.g.
  after a collision with subatomic particle, radiation etc.).  

molecule: Electrically neutral group of multiple atoms held together by chemical
  bond. E.g. O2, H2O etc.

chemical bond: Lasting bond that holds atoms together. Some types of bonds are:

  - covalent: Atoms share electrons, mostly electron pairs. Atoms want to fill
    the valent shell fully, so e.g. H2O is formed by O wanting 2 electrons
    (it has 8 electrons total, 2 in first shell, 6 out of 8 possible in the
    second valent shell, missing 2) and each O wants 1 (having 1 of 2 possible
    in the first shell), so they join to satisfy this.


  - 


mole: SI unit of substance amount, means exactly 6.02214076 * 10^23 particles.

Elements are denoted as:

  Z   <--- atomic number, the number of protons, e.g. 1 for hydrogen
   E  <--- element name, e.g. H for hydrogen
  M   <--- atomic mass (in grams/mole), e.g. 1.00794 for hydrogen


periodic table of elements:
  Graphical ordering of elements in 2D table, it consists of:

  - groups: columns, there are 18 groups
  - periods: rows, there are 7 periods

  The table shows periodic trends (caused by number of electrons needed to fill
  the s, p, d and f subshells, which mostly determines chemical properties),
  i.e. repeating patterns, e.g.:

  - metalicity: towards bottom-left corner elements are more metalic
  - atomic radius: towards top-right corner elements have bigger atomic radius
  - ionization energy: towards top-right elements have more ionization energy