 
There are two main types of chemical bonds in chemistry: covalent bonds and ionic bonds. Chemical bonds hold atoms together in a molecule, and occur when two or more atoms either share or donate electrons.Electrons are negatively charged subatomic particles that orbit the nucleus of an atom. The nucleus itself doesn't play a part in chemical bonds; its components don't change or move. The electrons in an atom are what is responsible for forming bonds. The bonding electrons are called the valence electrons; they're the electrons that are found in the outermost shell of the atom's electron cloud. The difference between covalent and ionic bonds is how the electrons are used. - In covalent bonding, two or more atoms share electrons. - In ionic bonding, two or more atoms, as ions, donate electrons to one another. Covalent Bonds Non-metal elements on the periodic table when combined with other non-metals, make covalent compounds. Electronegativity is a value that measures an atom's affinity for electrons. Non-metals that are missing one or two electrons to complete their outer shell have high electronegativity, such as chlorine and fluorine. On the periodic table, electronegativity increases as you move up in a group and to the right across the periods. In covalent bonds, the atoms have similar electronegativity; they share electrons so that both have a complete outer shell. Elements can have different numbers of valence electrons; this can be determined from the periodic table by identifying the element's group. For example, elements in the boron group (3A) have three valence electrons. In a covalent bond, the electrons are drawn to their nuclei by electrostatic attraction. Atoms share electrons to attain a stable electronic configuration. Examples of covalent bonds include the compounds water (H2O), carbon dioxide (CO2), ammonia (NH3), ozone (O3), glucose ( C6H12O6), carbon monoxide (CO), methane (CH4), phosphorus trichloride (PCl3), fructose, and chlorine gas (Cl2). Ionic Bonds Ionic compounds are formed when metals and non-metals combine. Which elements are which can also be determined from the periodic table. These bonds are formed from the electrostatic attraction between oppositely charged ions in a chemical compound. Ionic bonds occur when two atoms have very different electronegativity. The bond forms when the valence (outermost) electrons of one atom are transferred permanently to another atom ... one atom donates electrons to the other. The more electronegative atom takes electron(s) from the less electronegative atom. The atom that loses the electron(s) becomes a positively charged ion (cation), while the one that gains them becomes a negatively charged ion (anion). The ions are arranged so that the positive and negative charges balance one another, so the overall charge of the entire molecule is zero. The size of the electrostatic force in ionic compounds, particularly crystals, is very large. This makes the substances hard and non-volatile (they don't easily evaporate). Salt, for example, has very little smell. Examples of ionic bonds include the compounds sodium chloride (NaCl) , magnesium sulfate (MgSO4), cesium fluoride (CeF), and potassium cyanide (KCN).  A Comparison of Ionic and Covalent Bonds: Generally:
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