Atomic structure

• the nucleus contains protons (positively charged) and neutrons (neutral)

• electrons (negatively charged) are found in the electron shell and move around the nucleus 

• the positive charge (of the protons) and the negative charge (of the electrons) balance each other out  -> atoms are electrically neutral

Atomic nucleus

• protons and neutrons are located in the atomic nucleus, making it positively charged 

• it is significantly smaller than the electron shell

• 99.9 % of the atom’s total mass is concentrated in the nucleus 

mass of a proton: 1.673*10^-27 kg ≈ 1 u

mass of a neutron: 1.675*10^-27 kg ≈ 1u

mass of an electron: 9.109*10^-31 kg ≈ 0.005 u

• the nucleon number (total number of nucleons, i. e., protons and neutrons, in an atomic nucleus)  corresponds to approximate atomic mass in u (since the mass of the electrons is very small)

Electron shell

• electrons are located in the electron shell 

• atoms strive to have a full outer electron shell -> makes them more stable 

• therefore they gain, lose or share electrons 

Energy Level- and Shell Model

• both models provide an energy based and structural representation of the electron shell

• every electron is assigned to a specific energy state (shell)-> each shell corresponds to a certain energy state

• the 1st energy-level (K-shell) can hold up to two electrons

• the 2nd energy-level (L-shell) can hold up to eight electrons 

• the shell are named using letters, following the KLMN… notation 

• the maximum number of electrons is determined by the formula 2n^2 (n=shell)

• once a shell is fully occupied, the next begins to be filled 

• the arrangements of electrons in these shells is called electron configuration and follows the aufbau principle -> electrons fill lower energy levels first (the closer an electron is to the atomic nucleus, the stronger the attraction)