Even though in the first group of the periodic table, hydrogen is considered a nonmetal because of its tendency to form covalent compounds. It doesn't have a lot of chemical properties of its own, but is very important in the chemistry of other compounds. Hydrogen can also form ionic compounds, called metal hydrides, in which the \(H^-\) is present. These hydrides are unstable compounds, which easily react with substances like water or oxygen.
In the 13th group of the periodic table, boron is the only element considered a nonmetal. Because of its small number of electrons (\(ns^2np^1\)) it forms compounds with unique covalent structures. It's mainly found as boron trihalides (\(\ce{BX3}\)) and boron hydrides (boranes) such as \(BH_3\) or \(B_2H_6\)
Group 14 nonmetals (carbon and silicium) are tetravalent elements with the outer shell structure \(ns^2np^2\). Because of their high valency, they have a well-known tendency to catenate - that is, form long chains held together by covalent bonds. That's why carbon is the base of the very well known organic compounds, while silicium forms the lesser known complex silicates which make up close to 90% of the Earth's minerals.
In the Earth's crust, carbon is commonly found in the form of carbonates, ionic compounds containing the \(CO^{2-}_3\) anion. Upon heating, carbonates decompose into metals oxides and carbon dioxide. For example, the decomposition of sodium carbonate is:
\[\ce{Na2CO3 \rightarrow Na2O + CO2}\]
Because of its tetravalency, carbon can be found in a number of different allotropes - the most common being graphite (paralel sheets with the carbon atoms arranged in a hexagonal pattern) and diamond (all carbon atoms have other carbons arranged tetrahedrically around them). Graphite is one of the softest known materials, while diamond is one of the hardest ones.
Group 15 nonmetals (nitrogen, phosphorus and arsenic) have an \(ns^2np^3\) outer shell structure and are generally trivalent, but found very often in pentavalent state.
The nitrogen molecule \(N_2\) makes up 80% of the Earth's atmosphere. It's a very stable molecule due to the strong triple bond between the atoms. Nitrogen is also found as nitrates (with the \(NO_3^-\) anion) and nitrites (with the \(NO_2^-\) anion).
Nitrogen is also known to form various oxides. These oxides are:
Phosphorus mainly forms phosphates, containing \(PO_4^{3-}\). In the elemental form it's found as different allotropes, such as \(P_4\) and it can form two oxides - \(P_4O_6\) and \(P_4O_{10}\)
Group 16 nonmetals (oxygen, sulfur, selenium and tellurium) are generally divalent, with an \(ns^2np^4\) outer shell structures. The heavier ones (sulfur, selenium and tellurium) can also exhibit tetravalent or hexavalent states.
Oxygen is one of the most common elements, being found in a lot of compounds. However, it is not the main element in most compounds and just accompanies the central atom. Compounds with relevance to oxygen chemistry are the two allotropes (dioxygen \(O_2\) and ozone \(O_3\)) and the different oxides it can form:
Sulfur has a pretty high tendency to catenate, with its elemental form being an \(S_8\) molecule. Because of its tendency to catenate, we also see S-S bonds in compounds more than other X-X bonds.
Group 17 nonmetals (fluorine, chlorine, bromine, iodine and astatine), also called halogens, are monovalent elements, with the outer shell structure \(ns^2np^5\). Apart from fluorine, halogens can also exhibit trivalent, pentavalent and heptavalent states.
They are mostly found as \(X^-\) anions, rather than oxoanions, but they can form \(XO_n^-\) anions (n=1, 2, 3, 4).
Group 18 nonmetals (helium, neon, argon, krypton, xenon and radon), also known as noble gases, are generally unreactive due to their stable \(ns^2np^6\) outer shell structure. However, compounds containing heavier noble gases have been synthesized.
Nonmetals can be arranged in a reactivity series similar to that for the metals. Their reactivity is generally governed by electronegativty, with more electrongetaive nonmetals being able to displace less electronegative nonmetals from their compounds. For example:
\[\ce{KI + \frac{1}{2}Cl2 \rightarrow KCl + \frac{1}{2}I2}\]
Written by Alex Jicu