How can a chemist predict how an element can combine with other elements?
The forming of compound elements has been described by Dr. Frederick Senese of Frostburg State University, Maryland, as a game of tug of war ("What is Electronegativity?"). Atoms are made up of electrons, and those electrons have a negative electric charge. Electrons also spin, and as they spin, they create a magnetic field. That magnetic field can attract the positively charged nucleus of an atom and, hence, create a tug of war between electrons and nuclei. Whether or not a compound element will form as a result of this tug of war depends on the strength of the electric charge of the atom's nucleus. Chemists can predict which atom will win that tug of war and what compound will result by knowing the strength of the atom's nuclear charge.
While the nucleus of an atom has no charge itself, it is made up of neutrons and protons. Neutrons have no electric charge, while protons have a positive electric charge; therefore, due to the protons, the nucleus of an atom always has a positive electric charge. But, more importantly, those electric charges have different strengths. Scientist Linus Pauling (1901 - 1994) was able to devise a system to rank an atom's likelihood of bonding with electrons of other atoms. He surmised, according to Dr. Senese, "The more energy it takes to pull off the outer electron of an atom ..., the less likely [the atom] is to allow another atom to take those electrons" and, thus, the less likely the atom is to form a compound ("What is Electronegativity?"). Likewise, as Dr. Senese states, "The more energy the atom releases when it gains an electron, the more likely [the atom] is to take electrons from another atom in bonding" ("What is Electronegativity?"). Pauling named these levels of energy electronegativity and developed a numerical score; these scores help scientists predict if an atom is likely or unlikely to bond with other electrons and form a compound. The numerical value of electronegativity for each element has been added to the Periodic Table of Elements and appears as the small value in the right-hand corner, such as the value 2.1 for hydrogen (H).
We also know that atoms are at their greatest level of stability when they have eight valence electrons, which are the electrons in the outer shell of an atom. We can determine the number of valence electrons for any atom using the Periodic Table of Elements. The Periodic Table has been broken down into 18 groups; each group has a known number of valence electrons. For example, hydrogen is group 1; therefore, we know hydrogen has only one valence electron. As we move into higher group numbers across the Periodic Table, the ones place of the group number gives us the number of valence electrons. For example, carbon in group 14 would have 4 valence electrons.
Chemists can also use known numbers of valence electrons to predict if atoms will bond and what the compound would be.