Element vs. Compound
Atoms of the same atomic number are called an element. They are chemically not separable and have an atomic (e.g. noble gases, atoms exist individual), molecular (e.g. nitrogen, oxygen, atoms of the same element will connect with each other) or polymeric (atoms of the same element will make multiple/ branched, chain-like compounds) form.
In chemistry, three basic types of bonds can be distinguished:
- Atomic bond
- Ionic bond
- Metal bond
A compound is a pure substance from different element atoms that are in a fixed ratio to one another. They can be separated into elements. At this, a distinction can be made due to several manifestations:
- Molecular form
- Polymeric form
- Ionogenic form
The knotting of bonds between the atoms, whereby the valence electrons, which are the outer electrons, enter the crucial interactions, forms the compounds. Which type of bond is preferred, can be easily determined using the electronegativity.
This is the energy that attracts electrons from the nucleus and stands in the periodic table next to each element (=EN). The EN of the involved elements should be considered and the difference needs to be calculated. If he is 0 a metal bond, between 0 and 2 an atomic bond and above 2 an ionic bond is present.
The Ionic Bond
In an ionic bond, ions (=charged atoms) are arranged three-dimensionally in a lattice. Cations are referred to as positively charged particles; the negatively charged particles are called anions. They are caused by the complete delivery of the valance electron of an element (metal) to another (non-metal). Both achieve by this process the desired, stable noble gas configuration. The bond is based on electrostatic attractive forces, which are non-directional. This means, that the attraction acts spatially in all directions.
Since it comes with increasing proximity of the two particles to repulsion between the like-charged atomic nuclei and electron shells, they reverse a certain, stable distance at which repulsive and attractive forces are equal. This is also referred to as the equilibrium distance.
Ionic bonds form crystals and exist in salts and salt-like compounds. Since the electrostatic forces are very strong, their boiling and melting temperatures are in high areas. In their melts and aqueous solutions, the cations and anions are free or no longer bound in the ionic lattice, so that they can receive useful properties of electrical conductivity.
The charged ions become charge carriers. Salts are also very brittle. This is due to the arrangement of the ions: if a force postpones them to another position in the lattice, they stand against the same charged particles and therefore repel each other. An example of an ionic bond is the simple common salt, NaCl.
The Atomic Bond
An atomic bond or covalent bond is based on the formation of a common pair of electrons between two atoms. Thereby, each binding partner contributes one electron. The atomic bond, other than the ionic bond, is a directional type of bond. The attractive forces act only in one direction.
The meaning of the electron “sharing” is to achieve a full outer shell and thus a steady state configuration. By overlapping of a molecular orbital, the more energetically favorable out of two atomic orbitals is chosen.
If the partners of an atomic bond have widely differing EN values, so-called charge centers arise (= δ). In the molecule of water, for example, one O and two H atoms utilize in total two electron pairs together. The electronegativity of hydrogen is 2.2. Oxygen has an EN of 3.44.
The electron pairs are thus more attracted to the oxygen, so there is a negative charge center (δ-) and the hydrogen has a positive charge center (δ+). Such atomic bonds are called polar atomic bonds.
Between individual molecules with polar covalent bond, hydrogen bonds can be built. These are stronger intermolecular attraction forces (no further type of bonds!) compared to the Van-der-Whaals forces, which represent weaker intermolecular attractive forces. The valency or valance is the number of the possible bonds from an atom.
A coordinative atomic bond is a special form, in which the pair of electrons (both electrons) originates from a single binding partner. This one must have a free electron pair that means he needs to have an occupied atomic orbital with two electrons, while the accepting partner needs to present an electron gap, therefore a blank atomic orbital. The coordinative atomic bond is equal to the covalent atomic bond.
The Metal Bond
In a metal bond, the metal cations also possess solid lattice sites. They have submitted their valence electrons, which are free to move in the lattice. They are called electron gas. Non-directed, electrostatic attractive forces between cations and electrons are the basis of this bond type. Unlike the ionic bond, the materials are easily malleable and ductile, as mechanical action and the resulting displacement of the atoms do not change the relationship to their neighbors. The bond is very stable and shows high melting and boiling temperatures.
Due to the freely moving electrons, which act as charge carriers, metals are very good electrical conductors. Upon heat supply, which means power supply, the cations start to oscillate. They encounter each other and pass warmth via the oscillations. Metals are also good conductors of heat. The most known elements are metals.
The Covalent Bond
Covalent bonding occurs as a result of sharing off electrons between atoms. The atoms involved in this type of bonding are of the same element or have close proximity to each other in the periodic table. Primarily, this type bonding occurs between non-metals although it has also been observed between non-metals and metals but to a lesser extent.
If atoms are observed to bear similarities in regards to their electronegativities (affinity for electrons), then covalent bonds are likely to be formed. This implies that since both atoms have similar affinity for electrons, none would readily lose their electrons thus they share their electrons to attain stability. Another argument for the formation of covalent bonds is the fact that the electron affinity of the atom may be too small while the ionization energy required is too large for the formation of ionic bonds. For instance, carbon rarely forms ionic bonds since it possess four valence electrons. Such a scenario would require a carbon atom to either lose all the four valence electrons or gain an additional four electrons which is highly unfavorable.
Accordingly, carbon shares its valence electrons through double single and triple bonds to achieve the octet electron configuration. Moreover, covalent bonds involves the interactions of pi and sigma orbitals.