Ions

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An electrostatic potential map of the nitrate ion (NO−3). The 3-dimensional shell represents a single arbitrary isopotential.

LOOKING AT IONS

We've talked about ions before. Now it's time to get down to basics. Ions are atoms with either extra electrons or missing electrons. A normal atom is called a neutral atom. That term describes an atom with a number of electrons equal to the atomic number.

What do you do if you are a sodium (Na) atom? You have eleven electrons, one too many to have your shell filled. You need to find another element who will take that electron away from you. Bring in chlorine (Cl). Chlorine (Cl) will take that electron away and leave you with 10 electrons inside of two filled shells. You are a happy atom. Now you are also an ion and missing one electron. You are a sodium ion (Na+). You have one less electron than your atomic number.

ION CHARACTERISTICS

So now you've become a sodium ion (Na+). Now you have ten electrons. That's the same number as neon (Ne). But you aren't neon (Ne). Since you're missing an electron you aren't really a complete sodium (Na) atom either. You are now something completely new. An ion. Your whole goal as an atom was to become a "happy atom" with completely filled electron orbitals. Now you have those filled shells. You are stable. What do you do that's so special now? Now that you have given up the electron, you are quite electrically attractive. Other electrically charged atoms (ions) are now looking at you and seeing a good partner to bond with. That's where chlorine comes in.

ELECTROVALENCE

Don't get worried about the big word. Electrovalence is just another word for something that has given up its electron and become an ion. If you look at the periodic table, you might notice that elements on the left side usually become positively charged ions and elements on the right side get a negative charge. That trend means the left side has a positive valence and the right side has a negative valence. Valence is a measure of how much an atom wants to bond with other atoms.

There are two main types of bonding, covalent and electrovalent. Scientists also call ionic bonds electrovalent bonds. Ionic bonds are just groups of charged ions held together by electric forces. Scientists call these groups ionic agglomerates. When in the presence of other ions, the electrovalent bonds are weaker because of outside electrical forces and attractions.

Look at sodium chloride (table salt) as an example. Salt is a very strong bond when it is sitting on your table. It would be nearly impossible to break those ionic bonds. However, if you put that salt into some water the bonds break very quickly. It happens easily because of the electrical attraction of the water. Now you have sodium (Na+) and chloride (Cl-) ions. Remember that ionic bonds are normally strong but very weak in water. 

Formation

Formation of monatomic ions

Monatomic ions are formed by the addition of electrons to the valence shell of the atom, which is the outer-most electron shell in an atom, or the losing of electrons from this shell. The inner shells of an atom are filled with electrons that are tightly bound to the positively charged atomic nucleus, and so do not participate in this kind of chemical interaction. The process of gaining or losing electrons from a neutral atom or molecule is called ionization.

Atoms can be ionized by bombardment with radiation, but the more usual process of ionization encountered in chemistry is the transfer of electrons between atoms or molecules. This transfer is usually driven by the attaining of stable ("closed shell") electronic configurations. Atoms will gain or lose electrons depending on which action takes the least energy.

For example, a sodium atom, Na, has a single electron in its valence shell, surrounding 2 stable, filled inner shells of 2 and 8 electrons. Since these filled shells are very stable, a sodium atom tends to lose its extra electron and attain this stable configuration, becoming a sodium cation in the process

Na → Na⁺ + e⁻

On the other hand, a chlorine atom, Cl, has 7 electrons in its valence shell, which is one short of the stable, filled shell with 8 electrons. Thus, a chlorine atom tends to gain an extra electron and attain a stable 8-electron configuration, becoming a chloride anion in the process:

Cl + e⁻ → Cl⁻

This driving force is what causes sodium and chlorine to undergo a chemical reaction, where the "extra" electron is transferred from sodium to chlorine, forming sodium cations and chloride anions. Being oppositely charged, these cations and anions form ionic bonds and combine together to form sodium chloride, NaCl, more commonly known as rock salt.

Na⁺ + Cl⁻ → NaCl

Formation of polyatomic and molecular ions

Polyatomic and molecular ions are often formed by the gaining or losing of elemental ions such as H⁺ in neutral molecules. For example, when ammonia, NH₃, accepts a proton, H⁺, it forms the ammonium ion, NH+4. Ammonia and ammonium have the same number of electrons in essentially the same electronic configuration, but ammonium has an extra proton that gives it a net positive charge.

Ammonia can also lose an electron to gain a positive charge, forming the ion ·NH+3. However, this ion is unstable, because it has an incomplete valence shell around the nitrogen atom, making it a very reactive radical ion.

Due to the instability of radical ions, polyatomic and molecular ions are usually formed by gaining or losing elemental ions such as H⁺, rather than gaining or losing electrons. This allows the molecule to preserve its stable electronic configuration while acquiring an electrical charge.

Ionization potential

Main article: Ionization potential

The energy required to detach an electron in its lowest energy state from an atom or molecule of a gas with less net electric charge is called the ionization potential, or ionization energy. The nth ionization energy of an atom is the energy required to detach its nth electron after the first n − 1 electrons have already been detached.

Each successive ionization energy is markedly greater than the last. Particularly great increases occur after any given block of atomic orbitals is exhausted of electrons. For this reason, ions tend to form in ways that leave them with full orbital blocks. For example, sodium has one valence electron in its outermost shell, so in ionized form it is commonly found with one lost electron, as Na⁺. On the other side of the periodic table, chlorine has seven valence electrons, so in ionized form it is commonly found with one gained electron, as Cl⁻. Caesium has the lowest measured ionization energy of all the elements and helium has the greatest.^[1] The ionization energy of metals is generally much lower than the ionization energy of nonmetals, which is why metals will generally lose electrons to form positively charged ions while nonmetals will generally gain electrons to form negatively charged ions.

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