Volume 1
1.2. WHAT IS MATTER MADE OF? CHEMICAL ELEMENTS

Chemical Elements Are Different Types of Atoms • Metals, Nonmetals, and Metalloids • Chemical Formulas • Exercises

1.2.1. Chemical Elements Are Different Types of Atoms. Substances are made up of molecules. Molecules are made up of atoms. A molecule is a group of atoms bonded and held together in a fixed arrangement.

Think of atoms as building blocks of a construction set (like Lego) and of molecules as various models that can be built with those blocks. A substance would then represent a multitude of such identical models (identical molecules) built from the original blocks (atoms) and packed together in a particular volume.

I reemphasize that a substance consists of identical molecules. Any matter comprising molecules of different types is called a mixture of substances, not a substance.

The number of atoms in a molecule can vary in a broad range from just two to several billion. A molecule of water is made up of 3 atoms, a molecule of sugar of 45 atoms, and a molecule of DNA of up to billions of atoms. A limited number of substances are composed of just single atoms that are called monoatomic molecules. For example, helium gas (which is used to inflate balloons) is often said to consist of monoatomic helium molecules or, in other words, helium atoms.

Molecules and atoms are microscopic in size. Richard Feynman (1918-1988), an outstanding American physicist and a Nobel Prize winner, once made the following comparison.

"If an apple was magnified to the size of the Earth, then the atoms in the apple would be approximately the size of the original apple."

Just like there are different building blocks in a construction set, there are different types of atoms. These types are called chemical elements or just elements. There are over a hundred elements currently known, of which 90 occur in nature. The elements are organized in the periodic table, the original version of which was developed in the 1860s by the Russian chemistry genius Dmitry Mendeleev (Figure 1-1).
Figure 1-1. Dmitry Ivanovich Mendeleev (1834 – 1907) (source).


There are many designs of the periodic table. One of them is presented in Figure 1-2. Interactive periodic tables are also available, such as this one, which I particularly like.
Figure 1-2. Periodic table of chemical elements (source).


Each chemical element occupies a separate cell in the periodic table and has a name, a symbol, and a number. For instance, the symbol of the element hydrogen is H and its number in the periodic table is 1. Element number 6 is carbon, whose symbol is C. Oxygen is element number 8 and has the symbol O. The element iron's symbol is Fe and its number is 26. Most versions of the periodic table also display the mass of each element in atomic mass units (a.m.u.), which we will discuss later in our course. As can be seen from the periodic table in Figure 1-2, the atomic masses of the elements hydrogen (H), carbon (C), oxygen (O), and iron (Fe) are 1.008, 12.011, 15.999, and 55.845 a.m.u., respectively.
Digression. You may find it strange that, while symbols of many chemical elements and their English names make sense, those of some other elements are peculiar. Such symbols as C for carbon, O for oxygen, He for helium, and Fr for francium seem sensible, but why is the symbol for iron Fe, for silver Ag, and for tungsten W? To get an answer to this question, we need to know the origin of chemical element names. Many of the elements known to the Romans were assigned symbols based on their Latin names, such as Fe (iron) from ferrum, Au (gold) from aurum, Pb (lead) from plumbum, Sn (tin) from stannum, etc.

Names and symbols of some other elements have really bizarre origins. For example, the symbol of tungsten, W, is from the German Wolfrahm, literally "wolf's soot" or "wolf's froth" or"wolf's cream". Here is what H. C. Stark writes in his article on tungsten and wolframite, a tungsten mineral.

"The history of tungsten goes back to the end of the Middle Ages. The miners in the Erz Mountains of Saxony noticed that certain ores disturbed the reduction of cassiterite [the tin mineral used for tin smelting - my comment] and induced slagging. "They tear away the tin and devour it like a wolf devours a sheep", a contemporary wrote in the symbolic language of the Middle Ages. The miners gave this annoying ore German nicknames like "wolfart," "wolframite" or"wolfrahm"."

Etymologies of names and symbols for all chemical chemical elements can be found here.
1.2.2. Metals, Nonmetals, and Metalloids. There are three major types of chemical elements: metals, nonmetals, and metalloids.

A body of atoms of a metal element held together by forces called the metallic bond (Volume 2) represents a metal substance. All metal substanes exhibit a characteristic shine, which is silvery in most cases. However, while being lustrous, copper (Cu) is red, gold (Au) is yellow, and cesium (Cs) is pale yellow (Figure 1-3).
Figure 1-3. Left to right: samples of copper (Cu; source), gold (Au; source), and cesium (Cs; source) metals.


Most metals are solids at room temperature (around 23 oC), except mercury (Hg) that is a liquid. Gallium (Ga) melts at 29.8 oC, below the temperature of the human body (36-37 oC), and therefore one can melt gallium in one's hand, as shown in this video, which is fun to watch. Gallium can be purchased from various online suppliers and is safe to touch, so you may try melting Ga in the palm of your hand if you wish. Just do not forget to wash your hands with soap afterwards. Cesium (Cs) melts at an even slightly lower temperature (28.5 oC), but do not consider doing experiments with this very expensive and highly dangerous metal. Cesium is pyrophoric (ignites spontaneously in air) and explodes on contact with water. That is why the cesium metal shown in Figure 1-3 is sealed in a glass tube, apparently either under vacuum or in an atmosphere of a gas that is chemically inert (inactive) toward cesium. Nitrogen and argon are two such gases.

It is very true, however, that "Not all that glitters is gold". (There is an interesting story of one lady who thought otherwise.) One has to be extremely careful when judging only by appearance whether an element is a metal or not. Take a look at the two pictures presented in Figure 1-4 below. The one on the left shows boron (B) and the one on the right is silicon (Si). Both are lustrous and silvery in color, yet boron and silicon are not metals.
Figure 1-4. Samples of boron, B (left; source) and silicon, Si (right; source).


Metals conduct heat and electricity and are ductile and malleable. Although boron is silvery in color, it is brittle and is a very poor conductor of electricity at room temperature. Silicon is also brittle and is a semiconductor. A semiconductor conducts electricity better than insulators such as polyethylene, wood, or glass, but worse than metals. Elements like boron and silicon are often called metalloids. Metalloids are intermediate between metals and nonmetals. Nonmetals are usually poor conductors and lack the characteristic silvery shine. At room temperature and atmospheric pressure, some nonmetals are gases (hydrogen, oxygen, nitrogen), some are solids (carbon, sulfur, phosphorus), and one is a liquid (bromine). Figure 1-5 shows the location of metal, metalloid, and nonmetal elements in the periodic table.
Figure 1-5. Positions of metals, nonmetals, and metalloids in the periodic table (source).


1.2.3. Chemical Formulas. Atoms can bond to one another to form molecules. For example, two hydrogen atoms (H) and one oxygen atom (O) form a molecule of water, H-O-H, or H2O. On coming together, one carbon atom (C) and two oxygen atoms (O) give rise to a molecule of carbon dioxide, CO2. The simplest and most conventional way to present a chemical formula for a molecule is to write the symbol of the first element comprising the molecule, specify the number of these atoms in the molecule as a subscript, and then do the same with the symbols of all of the other elements in the molecule. If there is only one atom of an element in a molecule, the subscript "1" is omitted for simplicity. Thus, the aforementioned formulas of water and carbon dioxide are written as H2O and CO2, respectively, not H2O1 and C1O2. Figure 1-6 illustrates this rule with the formula of water as an example.
Figure 1-6. Writing the chemical formula of water (source).


Atoms of the same type (of the same element) can also bond together to form molecules. For example, a molecule of hydrogen consists of two hydrogen atoms, H-H, or H2. Similarly, a molecule of oxygen, O2, is produced from two oxygen atoms.

How are chemical formulas read and pronounced? That is easy. Just read the formula letter by letter and number by number. For instance, H2 is pronounced H-two. The formula of sulfuric acid, H2SO4, is pronounced H-two-S-O-four. For the formula of baking soda NaHCO3, the pronunciation is N-A-H-C-O-three.

1.2.4. Exercises.

1. Find the following elements in the periodic table: H, C, N, O, Na, P, S, Cl, K, Ca, Cu, Si. What are the names and numbers of these elements?

2. What are the symbols of the following elements: hydrogen, nitrogen, magnesium, chlorine, iron, potassium, copper, sulfur, oxygen, phosphorus?

3. Are there more metals or nonmetals among chemical elements? [Hint: use Figure 1-5]

4. Which of the following elements are metals, nonmetals, or metalloids: Si, Na, F, Cl, Ca, B, Br, As, Cu, Fe, P, Mg, B, Ba. [Hint: use Figure 1-5]

5. By replacing the subscripts x, y, and z with numbers, write chemical formulas for the following molecules.

(A) Methane (the major component of natural gas), comprising 1 carbon atom and 4 hydrogen atoms, CxHy. Pronounce the resultant formula. [Answer: CH4; C-H-four]

(B) Isopropanol (rubbing alcohol), consisting of 4 carbon atoms, 8 hydrogen atoms, and 1 oxygen atom, CxHyOz. Pronounce the resultant formula. [Answer: C4H8O; C-four-H-eight-O]

(C) Sucrose (sugar), consisting of 12 carbon atoms, 22 hydrogen atoms, and 11 oxygen atoms, CxHyOz. Pronounce the resultant formula. [Answer: C12H22O11; C-twelve-H-twenty-two-O-eleven]

(D) Iron oxide (rust), consisting of 2 iron atoms and 3 oxygen atoms, FexOy. Pronounce the resultant formula. [Answer: Fe2O3; F-E-two-O-three]

(E) A molecule of oxygen comprising two oxygen atoms Ox and ozone comprising three oxygen atoms, Oy. Pronounce the resultant formulas. [Answer: O2 and O3; O-two and O-three]