Ionic bonds, covalent bonds and the shapes molecules take. Learn the language of chemical bonding and molecular geometry.
These terms were extracted from Chemistry by OpenStax, eCampusOntario (© 2016 Rice University; adapted by BCcampus) which is licensed under a Creative Commons Attribution 4.0 International License.
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tendency of heavy atoms to form ions in which their valence s electrons are not lost
The formation of these 1+, 2+, and 3+ cations is ascribed to the inert pair effect, which reflects the relatively low energy of the valence s-electron pair for atoms of the heavy elements of groups 13, 14, and 15.
strong electrostatic force of attraction between cations and anions in an ionic compound
The properties of ionic compounds shed some light on the nature of ionic bonds.
distance between the nuclei of two bonded atoms at which the lowest potential energy is achieved
The bond length is determined by the distance at which the lowest potential energy is achieved.
bond formed when electrons are shared between atoms
Compounds that contain covalent bonds exhibit different physical properties than ionic compounds.
tendency of an atom to attract electrons in a bond to itself
Whether a bond is nonpolar or polar covalent is determined by a property of the bonding atoms called electronegativity.
covalent bond between atoms of different electronegativities; a covalent bond with a positive end and a negative end
This unequal distribution of electrons is known as a polar covalent bond, characterized by a partial positive charge on one atom and a partial negative charge on the other.
(also, nonpolar covalent bond) covalent bond between atoms of identical electronegativities
Electrons shared in pure covalent bonds have an equal probability of being near each nucleus.
covalent bond in which two pairs of electrons are shared between two atoms
A double bond forms when two pairs of electrons are shared between a pair of atoms, as between the carbon and oxygen atoms in CH₂O (formaldehyde) and between the two carbon atoms in C₂H₄ (ethylene):
molecule that contains an odd number of electrons
We call molecules that contain an odd number of electrons free radicals.
molecule containing at least one main group element that has more than eight electrons in its valence shell
Hypervalent molecules have a central atom that has more electrons than needed for a noble gas configuration.
diagram showing lone pairs and bonding pairs of electrons in a molecule or an ion
In this section, we will explore the typical method for depicting valence shell electrons and chemical bonds, namely Lewis symbols and Lewis structures.
symbol for an element or monatomic ion that uses a dot to represent each valence electron in the element or ion
In this section, we will explore the typical method for depicting valence shell electrons and chemical bonds, namely Lewis symbols and Lewis structures.
two (a pair of) valence electrons that are not used to form a covalent bond
The Lewis structure indicates that each Cl atom has three pairs of electrons that are not used in bonding (called lone pairs) and one shared pair of electrons (written between the atoms).
guideline that states main group atoms will form structures in which eight valence electrons interact with each nucleus, counting bonding electrons as interacting with both atoms connected by the bond
The tendency of main group atoms to form enough bonds to obtain eight valence electrons is known as the octet rule.
bond in which a single pair of electrons is shared between two atoms
A single shared pair of electrons is called a single bond.
bond in which three pairs of electrons are shared between two atoms
A triple bond forms when three electron pairs are shared by a pair of atoms, as in carbon monoxide (CO) and the cyanide ion (CN⁻):
charge that would result on an atom by taking the number of valence electrons on the neutral atom and subtracting the nonbonding electrons and the number of bonds (one-half of the bonding electrons)
We can use the concept of formal charges to help us predict the most appropriate Lewis structure when more than one is reasonable.
arrangement of atoms in a molecule or ion
Carbon soot has been known to man since prehistoric times, but it was not until fairly recently that the molecular structure of the main component of soot was discovered.
situation in which one Lewis structure is insufficient to describe the bonding in a molecule and the average of multiple structures is observed
He developed many of the theories and concepts that are foundational to our current understanding of chemistry, including electronegativity and resonance structures.
two or more Lewis structures that have the same arrangement of atoms but different arrangements of electrons
We call the individual Lewis structures resonance forms.
average of the resonance forms shown by the individual Lewis structures
The actual electronic structure of the molecule (the average of the resonance forms) is called a resonance hybrid of the individual resonance forms.
(also, bond dissociation energy) energy required to break a covalent bond in a gaseous substance
The energy required to break a specific covalent bond in one mole of gaseous molecules is called the bond energy or the bond dissociation energy.
thermochemical cycle relating the various energetic steps involved in the formation of an ionic solid from the relevant elements
The Born-Haber cycle is an application of Hess’s law that breaks down the formation of an ionic solid into a series of individual steps.
energy required to separate one mole of an ionic solid into its component gaseous ions
In this section, you will learn about the bond strength of covalent bonds, and then compare that to the strength of ionic bonds, which is related to the lattice energy of a compound.
location in a trigonal bipyramidal geometry in which there is another atom at a 180° angle and the equatorial positions are at a 90° angle
As shown in Figure 6, the axial position is surrounded by bond angles of 90°, whereas the equatorial position has more space available because of the 120° bond angles.
angle between any two covalent bonds that share a common atom
A bond angle is the angle between any two bonds that include a common atom, usually measured in degrees.
(also, bond length) distance between the nuclei of two bonded atoms
The strong attraction of each shared electron to both nuclei stabilizes the system, and the potential energy decreases as the bond distance decreases.
separation of charge in a bond that depends on the difference in electronegativity and the bond distance represented by partial charges or a vector
This separation of charge gives rise to a bond dipole moment.
property of a molecule that describes the separation of charge determined by the sum of the individual bond moments based on the molecular structure
This separation of charge gives rise to a bond dipole moment.
arrangement around a central atom of all regions of electron density (bonds, lone pairs, or unpaired electrons)
It is important to note that electron-pair geometry around a central atom is not the same thing as its molecular structure.
one of the three positions in a trigonal bipyramidal geometry with 120° angles between them; the axial positions are located at a 90° angle
As shown in Figure 6, the axial position is surrounded by bond angles of 90°, whereas the equatorial position has more space available because of the 120° bond angles.
shape in which two outside groups are placed on opposite sides of a central atom
According to VSEPR theory, the terminal atom locations (Xs in Figure 6) are equivalent within the linear, trigonal planar, and tetrahedral electron-pair geometries (the first three rows of the table).
shape in which six outside groups are placed around a central atom such that a three-dimensional shape is generated with four groups forming a square and the other two forming the apex of two pyramids, one above and one below the square plane
When a central atom has two lone electron pairs and four bonding regions, we have an octahedral electron-pair geometry.
(also, dipole) molecule with an overall dipole moment
If such a charge separation exists, the molecule is said to be a polar molecule (or dipole); otherwise the molecule is said to be nonpolar.
shape in which four outside groups are placed around a central atom such that a three-dimensional shape is generated with four corners and 109.5° angles between each pair and the central atom
On the other hand, the ammonia molecule, NH₃, also has four electron pairs associated with the nitrogen atom, and thus has a tetrahedral electron-pair geometry.
shape in which five outside groups are placed around a central atom such that three form a flat triangle with 120° angles between each pair and the central atom, and the other two form the apex of two pyramids, one above and one below the triangular plane
In a trigonal bipyramidal electron-pair geometry, lone pairs always occupy equatorial positions because these more spacious positions can more easily accommodate the larger lone pairs.
shape in which three outside groups are placed in a flat triangle around a central atom with 120° angles between each pair and the central atom
The basic geometry is trigonal planar with 120° bond angles, but we see that the double bond causes slightly larger angles (121°), and the angle between the single bonds is slightly smaller (118°).
theory used to predict the bond angles in a molecule based on positioning regions of high electron density as far apart as possible to minimize electrostatic repulsion
VSEPR theory predicts the arrangement of electron pairs around each central atom and, usually, the correct arrangement of atoms in a molecule.
quantity having magnitude and direction
This bond moment can be represented as a vector, a quantity having both direction and magnitude (Figure 13).