Atomic Structure

Atoms are composed of three type of particles: protons, neutrons, and electrons.

Protons and neutrons are responsible for most of the atomic mass.
The mass of an electron is very small (9.108 X 10^-28 grams).

1 - Now let's start with Neutral Atoms:

Both the protons and neutrons reside in the nucleus. Protons have a postive (+) charge, neutrons have no charge, ie they are neutral. Electrons reside in orbitals around the nucleus. They have a negative charge (-).

The number of protons determines the atomic number, e.g., H = 1. The number of protons in an element is constant (e.g., H=1, Ur=92) but the neutron number may vary, so the mass number (protons + neutrons) may vary.

Helium Atom

 2 - Ions:

- In an ion, number of electrons =/ number of protons, but protons = atomic number.
- Electrons are either lost or gained, making the ion either positive or negative.






3 - Isotopes:

The same element may contain varying numbers of neutrons; these forms of an element are called isotopes. The chemical properties of isotopes are the same, although the physical properties of some isotopes may be different. Some isotopes are radioactive-meaning they "radiate" energy as they decay to a more stable form, perhaps another element half-life: time required for half of the atoms of an element to decay into stable form. Another example is oxygen, with atomic number of 8 can have 8, 9, or 10 neutrons. 

To sum this up:

- Isotopes are atoms with same number of electrons and protons, but different numbers of neutrons.
- Different numbers of neutrons means different atomic masses.
- Different amounts of isotopes exist for different elements.








The mass number of elements can be calculated by getting the averages of the atomic masses of isotopes. For example:

potassium has three main isotopes; potassium-39 (93.26%), potassium-40 (0.0117%), potassium-41 (6.73%), so to get the average you multiply the percentages by their atomic masses, then add them together.

(39*0.9326) + (40*0.000117) + (41*0.0673) = 39.13538g/mol

*Always; # neutrons = mass # - atomic #, electron's mass = 1, neutron mass = 1837, proton mass = 1836