2.1 The Elements

Learning Objectives

  1. Define a chemical element and give examples of the abundance of different elements.
  2. Represent a chemical element with a chemical symbol.

An element, as defined in Chapter 1 "Chemistry, Matter, and Measurement", is a substance that cannot be broken down into simpler chemical substances. There are about 90 naturally occurring elements known on Earth. Using technology, scientists have been able to create nearly 30 additional elements that do not occur in nature. Today, chemistry recognizes 118 elements—some of which were created an atom at a time. Figure 2.1 "Samples of Elements" shows some of the chemical elements.

Abundance

The elements vary widely in abundance. In the universe as a whole, the most common element is hydrogen (about 90% of atoms), followed by helium (most of the remaining 10%). All other elements are present in relatively minuscule amounts, as far as we can detect.

On the planet Earth, however, the situation is rather different. Oxygen makes up 46.1% of the mass of Earth’s crust (the relatively thin layer of rock forming Earth’s surface), mostly in combination with other elements, while silicon makes up 28.5%. Hydrogen, the most abundant element in the universe, makes up only 0.14% of Earth’s crust. Table 2.1 "Elemental Composition of Earth" lists the relative abundances of elements on Earth as a whole and in Earth’s crust. Table 2.2 "Elemental Composition of a Human Body" lists the relative abundances of elements in the human body. If you compare Table 2.1 "Elemental Composition of Earth" and Table 2.2 "Elemental Composition of a Human Body", you will find disparities between the percentage of each element in the human body and on Earth. Oxygen has the highest percentage in both cases, but carbon, the element with the second highest percentage in the body, is relatively rare on Earth and does not even appear as a separate entry in Table 2.1 "Elemental Composition of Earth"; carbon is part of the 0.174% representing “other” elements. How does the human body concentrate so many apparently rare elements?

Table 2.1 Elemental Composition of Earth

Earth’s Crust Earth (overall)
Element Percentage Element Percentage
oxygen 46.1 iron 34.6
silicon 28.2 oxygen 29.5
aluminum 8.23 silicon 15.2
iron 5.53 magnesium 12.7
calcium 4.15 nickel 2.4
sodium 2.36 sulfur 1.9
magnesium 2.33 all others 3.7
potassium 2.09
titanium 0.565
hydrogen 0.14
phosphorus 0.105
all others 0.174

Source: D. R. Lide, ed. CRC Handbook of Chemistry and Physics, 89th ed. (Boca Raton, FL: CRC Press, 2008–9), 14–17.

Table 2.2 Elemental Composition of a Human Body

Element Percentage by Mass
oxygen 61
carbon 23
hydrogen 10
nitrogen 2.6
calcium 1.4
phosphorus 1.1
sulfur 0.20
potassium 0.20
sodium 0.14
chlorine 0.12
magnesium 0.027
silicon 0.026
iron 0.006
fluorine 0.0037
zinc 0.0033
all others 0.174

Source: D. R. Lide, ed. CRC Handbook of Chemistry and Physics, 89th ed. (Boca Raton, FL: CRC Press, 2008–9), 7–24.

The relative amounts of elements in the body have less to do with their abundances on Earth than with their availability in a form we can assimilate. We obtain oxygen from the air we breathe and the water we drink. We also obtain hydrogen from water. On the other hand, although carbon is present in the atmosphere as carbon dioxide, and about 80% of the atmosphere is nitrogen, we obtain those two elements from the food we eat, not the air we breathe.

Looking Closer: Phosphorus, the Chemical Bottleneck

There is an element that we need more of in our bodies than is proportionately present in Earth’s crust, and this element is not easily accessible. Phosphorus makes up 1.1% of the human body but only 0.105% of Earth’s crust. We need phosphorus for our bones and teeth, and it is a crucial component of all living cells. Unlike carbon, which can be obtained from carbon dioxide, there is no phosphorus compound present in our surroundings that can serve as a convenient source. Phosphorus, then, is nature’s bottleneck. Its availability limits the amount of life our planet can sustain.

Higher forms of life, such as humans, can obtain phosphorus by selecting a proper diet (plenty of protein); but lower forms of life, such as algae, must absorb it from the environment. When phosphorus-containing detergents were introduced in the 1950s, wastewater from normal household activities greatly increased the amount of phosphorus available to algae and other plant life. Lakes receiving this wastewater experienced sudden increases in growth of algae. When the algae died, concentrations of bacteria that ate the dead algae increased. Because of the large bacterial concentrations, the oxygen content of the water dropped, causing fish to die in large numbers. This process, called eutrophication, is considered a negative environmental impact.

Today, many detergents are made without phosphorus so the detrimental effects of eutrophication are minimized. You may even see statements to that effect on detergent boxes. It can be sobering to realize how much impact a single element can have on life—or the ease with which human activity can affect the environment.

Names and Symbols

Each element has a name. Some of these names date from antiquity, while others are quite new. Today, the names for new elements are proposed by their discoverers but must be approved by the International Union of Pure and Applied Chemistry, an international organization that makes recommendations concerning all kinds of chemical terminology.

Note

Today, new elements are usually named after famous scientists.

The names of the elements can be cumbersome to write in full, especially when combined to form the names of compounds. Therefore, each element name is abbreviated as a one- or two-letter chemical symbolA one- or two-letter abbreviation for an element.. By convention, the first letter of a chemical symbol is a capital letter, while the second letter (if there is one) is a lowercase letter. The first letter of the symbol is usually the first letter of the element’s name, while the second letter is some other letter from the name. Some elements have symbols that derive from earlier, mostly Latin names, so the symbols may not contain any letters from the English name. Table 2.3 "Element Names and Symbols" lists the names and symbols of some of the most familiar elements.

Table 2.3 Element Names and Symbols

aluminum Al magnesium Mg
argon Ar manganese Mn
arsenic As mercury Hg*
barium Ba neon Ne
bismuth Bi nickel Ni
boron B nitrogen N
bromine Br oxygen O
calcium Ca phosphorus P
carbon C platinum Pt
chlorine Cl potassium K*
chromium Cr silicon Si
copper Cu* silver Ag*
fluorine F sodium Na*
gold Au* strontium Sr
helium He sulfur S
hydrogen H tin Sn*
iron Fe tungsten W†
iodine I uranium U
lead Pb* zinc Zn
lithium Li zirconium Zr
*The symbol comes from the Latin name of element.
†The symbol for tungsten comes from its German name—wolfram.

Note

Element names in languages other than English are often close to their Latin names. For example, gold is oro in Spanish and or in French (close to the Latin aurum), tin is estaño in Spanish (compare to stannum), lead is plomo in Spanish and plomb in French (compare to plumbum), silver is argent in French (compare to argentum), and iron is fer in French and hierro in Spanish (compare to ferrum). The closeness is even more apparent in pronunciation than in spelling.

Example 1

Write the chemical symbol for each element without consulting Table 2.3 "Element Names and Symbols".

  1. bromine
  2. boron
  3. carbon
  4. calcium
  5. gold

Solution

  1. Br
  2. B
  3. C
  4. Ca
  5. Au

Skill-Building Exercise

    Write the chemical symbol for each element without consulting Table 2.3 "Element Names and Symbols".

  1. manganese

  2. magnesium

  3. neon

  4. nitrogen

  5. silver

Example 2

What element is represented by each chemical symbol?

  1. Na
  2. Hg
  3. P
  4. K
  5. I

Solution

  1. sodium
  2. mercury
  3. phosphorus
  4. potassium
  5. iodine

Skill-Building Exercise

    What element is represented by each chemical symbol?

  1. Pb

  2. Sn

  3. U

  4. O

  5. F

Concept Review Exercises

  1. What is an element?

  2. Give some examples of how the abundance of elements varies.

  3. Why are chemical symbols so useful? What is the source of the letter(s) for a chemical symbol?

Answers

  1. An element is the basic chemical building block of matter; it is the simplest chemical substance.

  2. Elements vary from being a small percentage to more than 30% of the atoms around us.

  3. Chemical symbols are useful to concisely represent the elements present in a substance. The letters usually come from the name of the element.

Key Takeaways

  • All matter is composed of elements.
  • Chemical elements are represented by a one- or two-letter symbol.

Exercises

  1. Which of the following substances are elements?

    1. sodium
    2. milk
    3. gold
    4. water
    5. air
    6. liquefied nitrogen

  2. Which of the following substances are elements?

    1. paper
    2. electricity
    3. neon
    4. carbon
    5. wood
    6. concrete

  3. Write the chemical symbol for each element.

    1. silver
    2. sulfur
    3. nitrogen
    4. neon

  4. Write the chemical symbol for each element.

    1. bromine
    2. oxygen
    3. lithium
    4. boron

  5. Explain why it is improper to write CO as the chemical symbol for cobalt.

  6. Explain why it is improper to write NO as the chemical symbol for nobelium.

  7. Complete the following table.

    Element Symbol Element Name
    F
    Fe
    I
    Cr
    C
    P

  8. Complete the following table.

    Element Symbol Element Name
    Mg
    Mn
    Ca
    Cl
    K
    Pt

Answers

    1. element
    2. not an element
    3. element
    4. not an element
    5. not an element
    6. element

    1. Ag
    2. S
    3. N
    4. Ne

  5. By convention, the second letter in an element’s symbol is always lowercase.

  7. Element Symbol Element Name
    F fluorine
    Fe iron
    I iodine
    Cr chromium
    C carbon
    P phosphorus