Atoms
Atoms are the smallest indivisible particles of matter contains proton, neutron and electron. Molecules are two or more atoms chemically combined together. All matter is composed of element that cannot broken down to smaller substances. Proton, electron and neutron are the component of atoms. Protons are located in the center of an atoms, each atom must at least contain one proton and proton contain a positive charge +1. Neutron is located in the nucleus of an atom, a neutron have charge. The atomic number is the number of proton in a stable atom. The atomic mass in an atom is the number the of proton plus neutron. Atoms with the same number proton , but different number neutron are called isotopes. For example, Hydrogen have different isotopes; Hydrogen-1 (1 proton,0 neutron), Hydrogen-2 (1 proton,1 neutron), Hydrogen-3 (1 proton,2 neutron) are three different isotopes of Hydrogen. Isotope of any element are only differ in the neutron but not proton.
Electrons are extremely small particles located outside the nucleus of an atom. Bohr came up with the Quantum Theory that explain the existence of electron in an atom. The theory stated that electron have a quantity of energy that keeps them out of the nucleus. It also explain how electron exist in different energy levels surrounding in the nucleus. There are seven main energy level called shells and with in each shell there are various sub-levels called orbitals which are s, p, d, f. There are some aspects to the Quantum Theory; 1) The Aufbau principle stated that electron fill in energy levels from lowest to highest energy levels. If an electron absorbs energy it can jump to a high energy level, but it will always fall back to the lowest empty energy level. 2) Pauli Exclusion principle stated that no more then two electrons in a sub-level. If there are two electrons in a sub-level they are spin in opposite directions. 3) Hund's Rule stated that before electrons are paired in an orbital, one electron fill in each sub-level of the orbital. 4) Heisenberg's Uncertainty principle stated that electron speed and position cannot be determined exactly. The area of highest probability can be determine.
Electron also play the main role in bonding of atoms. Atoms bonding in two form which are Ionic Bond and Covalent. Ions are atoms with a charge or an atom that has lost or gained electrons. A positive ions are called cation and a negative ions are called anion. Atoms become ions because they are more stable with a full s and p orbitals in their outer shell. Bonding fallow the Octet rule which stated that atoms are most stable with 8 electrons in their outer shell. They will lose, gain or share electrons with other atoms to get 8 electrons in their outer shell. When an atoms loses or gains electrons it is trying to get a noble gas configuration. Group one element, Alkali Metals, has one outer shell electron. They will react with other element and loses that one element. All group one one follow the same pattern, all loses one electron and become +1. They are the most reactive metals. Group seventeen elements, Halogens, have seven outer shell electrons. It easier for them to gain one electron and all the Halogens gain one electron when react. Halogen are the most reactive non-metals and become -1. Ions are more stable then their parent atom because they have a noble gas configuration. Electrostatic Attraction are attraction between the opposite charge and that what cause ionic bonds.
When the Alkali Metals react with the Halogens they always formed salts for