A stable chemical bond

between two atoms is formed when they are at the correct distance apart—a position of minimum en­ergy. When they are too far apart, in a higher energy state, there is little mutual attraction. No bonds are formed. If they approach too close to each other, also in a high energy state, the positive charges on their nuclei repel each other. Again, no stable bond is formed. At the correct dis­tance apart, their atomic or-bitals overlap to form a mo­lecular orbital.

nor absorbs energy. It is said to be in its ground state. When an electron releases or absorbs energy, it jumps to another orbital. It is no longer at its average distance from the nucleus.

What happens inside an atom during such changes is explained by quantum mechanics, a special field of physics. Chemistry is con­cerned with the chemical reactions that hap­pen between atoms. It is these chemical reac­tions that form the molecules and compounds with which chemistry is concerned.

The attraction between electrons and their nucleus is based on their opposite electric charges. Thus, the farther away an electron is from its nucleus, the less tightly it is held. As a result of chemical reactions, these outermost electrons transfer from one atom to another. Or, they become shared between atoms, adopting a single orbital around both nuclei.

For example, two atoms of hydrogen can combine together to form a molecule of hy­drogen. The two electrons (one from each of the hydrogen atoms) then move along a single pathway (the molecular orbital). This molecular orbital treats the two nuclei as a common cen­ter. It provides each nucleus with a share in both electrons and helps hold both nuclei to­gether. Such a molecular combination is more stable than a combination in which each nu­cleus retains its own electron.

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H atom

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H, molecule

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H atom

When two hydrogen atomsget close enough (A), their atomic orbitals overlap and form a molecu­lar orbital. The electrons, one from each hydrogen atom, are shared equally in the resulting covalent bond in a hydrogen molecule. When hydrogen forms a similar bond with an atom of fluorine (B), an irregular pear-shaped molecular or­bital results. This is because the fluorine atom has a larger share of the bonding electrons.

HF molecule

Isomersare pairs of mole­cules containing the same elements but in different spatial arrangements, in some isomers (A), the atoms (one of oxygen, two of car­bon, and six of hydrogen) are joined in a different se­quence. This gives two to­tally different substances. In ethane (B), the two methyl groups (each composed of a carbon atom [black] and three hydrogen atoms [yellow]) can rotate about the carbon-carbon single bond. In ethene (C, top), the carbon-carbon double bond prevents such rotation. If two chlorine atoms (green) replace two hydrogens, cis (same side) and trans (oppo­site side) isomers are formed (C, middle and bot­tom). A carbon atom bonded to four different atoms or groups (D) exists as two optical isomers, which are mirror images of each other.

Date: 2015-12-11; view: 893