The Periodic Table as a tool

Combining Atoms Through Chemical Bonding Chemical bonding is the joining of atoms to form new substances. An interaction that holds two atoms together is called a chemical bond. When chemical bonds form, electrons are shared, gained, or lost.

Electron Number and Organization The number of electrons in an atom can be determined from the atomic number of the element. Electrons in an atom are organized in energy levels. The next slide shows a model of the arrangement of electron in an atom. This model and models like it are useful for counting electrons, but do not show the true structure of an atom.

Electron Number and Organization Outer-Level Electrons and Bonding Most atoms form bond using only its valence electrons, the electrons in an atom’s outermost energy level.

Electron Number and Organization Valence Electrons and the Periodic Table You can use the periodic table to determine the number of valence electrons for atoms of some elements, as shown on the next slide.

To Bond or Not to Bond The number of electrons in the outermost energy level of an atom determine whether an atom will form bonds. Atoms that have 8 electrons in their outermost energy level do not usually form bonds. The outermost energy level is considered to be full if it contains 8 electrons.

To Bond or Not to Bond Filling the Outermost Level An atom that has fewer than 8 valence electrons is more likely to form bonds than at atom that has 8 valence electrons is. Atoms bond by gaining, losing, or sharing electrons to have a filled outermost energy level.

To Bond or Not to Bond Is Two Electrons a Full Set? Not all atoms need 8 valence electrons to have a filled outermost energy level. Helium atoms need only 2 valence electrons because the outermost level is the first energy level. Atoms of hydrogen and lithium form bonds by gaining, losing, or sharing electrons to achieve 2 electrons in the first energy level.

Electronegativity Defined as how strongly an atom is able to tug on bonding electrons Range of electronegativity runs from 0,7 to 4,0 Increased from left to right The greater an atom`s electronegativity the greater its ability to pull electrons toward itself when bonded

Electronegativity The Pauling scale is the most commonly used. Fluorine (the most electronegative element) is assigned a value of 4.0, and values range down to Cesium and Francium which are the least electronegative at 0.7

Electronegativity Noble gases are not included as electronegative elements, even when some of them are in the upper right side of the Periodic Table (rarely participate in chemical bonding)

Electronegativity Trends in electronegativity across a period As you go across a period the electronegativity increases

Electronegativity Trends in electronegativity down a group As you go down a group, electronegativity decreases. (If it increases up to fluorine, it must decrease as you go down).

Electronegativity The attraction that a bonding pair of electrons feels for a particular nucleus depends on: the number of protons in the nucleus; the distance from the nucleus; the amount of screening by inner electrons (energy required to remove the most loosely held electron).

Electronegativity Electronegativity increases across a period because the number of charges on the nucleus increases. That attracts the bonding pair of electrons more strongly (more protons). As you go down a group, electronegativity decreases because the bonding pair of electrons is increasingly distant from the attraction of the nucleus (the electrons are in levels that are far away from the nucleus)