(11) Environmental Science

"You are what you do, not what you say you wi`ll do"

C.G. Jung


Understanding Atoms, elements, and molecules


What Is Matter?

Concepts 2-2 Matter consists of elements and compounds, which are in turn made up of atoms, ions, or molecules.

Matter Consists of Elements and Compounds

We start at the most basic level, looking at the stuff that makes up life and its environment-matter. Matter is anything that has mass and takes up space. It is found intwo chemical forms. One is elements: the distinctive building blocks of matter that make up every material substance. For example, gold is an element that cannot be broken down into any other substance.

 Some matter consists of one element, but most matter consists of compounds: combinations of two or more different elements held together in fixed proportions. For example, water is a compound made of the elements hydrogen and oxygen, which have chemically combined with one another.

To simplify things, chemists represent each element by a one- or two-letter symbol. Just four elements - oxygen, carbon, hydrogen, and nitrogen-make up about 96% of your body weight and that of most other living things.

Atoms, Ions, and Molecules Are the Building Blocks of Matter

The most basic building block of matter is an atom: the smallest unit of matter into which an element can be divided and still retain its chemical properties. The ideathat all elements are made up of atoms is called the atomic theory. It is the most widely accepted scientific theory in chemistry.

Atoms are incredibly small. In fact, more than 3 million hydrogen atoms could sit side by side on the period at the end of this sentence. If you could view them with a super microscope, you would find that each different type of atom contains a certain number of subatomic particles.

There are three types of these atomic building blocks: positively charged protons (p), uncharged neutrons (n), and negatively charged electrons (e). Each atom consists of an extremely small and dense center called its nucleus, which contains one or more protons and, in most cases, one or more neutrons, and one or more electrons moving rapidly around the nucleus. Each atom has equal numbers of positively charged protons and negatively charged electrons. Because these electrical charges cancel one another, the atom as a whole has no net electrical charge.

Each element has a unique atomic number, equal to the number of protons in the nucleus of its atom. The simplest element, hydrogen (H), has only 1 proton in its nucleus, so its atomic number is 1. Carbon (C), with 6 protons, has an atomic number of 6, whereas uranium (U), a much larger atom, has 92 protons and an atomic number of 92.

Because electrons have so little mass compared with the masses of protons or neutrons, most of an atom’s mass is concentrated in its nucleus. The mass of an atom is described by its mass number: the total number of neutrons and protons in its nucleus. For example, a carbon atom with 6 protons and 6 neutrons in its nucleus has a mass number of 12, and a uranium atom with 92 protons and 143 neutrons in its nucleus has a mass number of 235 (92 +143 = 35).

All atoms of an element have the same number of protons in their nuclei. But the nuclei may contain different numbers of neutrons and therefore have different mass numbers. Forms of an element having the same atomic number but different mass numbers are called isotopes of that element. Scientists identify isotopes by attaching their mass numbers to the name or symbol of the element. For example, the three most common isotopes of carbon are carbon-12 (with six protons and six neutrons), carbon-13 (with six protons and seven neutrons), and carbon-14 (with six protons and eight neutrons). Carbon-12 makes up about 98.9% of all naturally occurring carbon. A second building block of matter is an ion-an atom or groups of atoms with one or more net positive or negative electrical charges. An ion forms when an atom gains or loses one or more electrons. An atom that loses one or more of its electrons has a positive electrical charge because the number of positively charged protons in its nucleus is now greater than the number of negatively charged electrons outside its nucleus.

Similarly, when an atom gains one or more electrons, it becomes an ion with one or more negative electrical charges because the number of negatively charged electrons is greater than the number of positively charged protons.

The number of positive or negative charges on an ion is shown as a superscript after the symbol for an atom or a group of atoms. Examples encountered include positive hydrogen ions (H+) and negative chloride ions (Cl-). One example of the importance of ions in environmental science is the nitrate ion (NO3-), a nutrient essential for plant growth. Chemical analysis of the water flowing through the dams of the cleared forest area showed a 60-fold rise in the concentration of nitrate ions (NO3-) compared to water running off of the unclear forest area. So many nitrates were lost from the experimental (cleared) valley that the stream below this valley became covered with algae whose populations soared as a result of an excess of nitrate plant nutrients.

After a few years, however, vegetation began growing back on the cleared valley and nitrate levels in its runoff returned to normal levels. Ions are also important for measuring a substance’s acidity in a water solution, a chemical characteristic that helps determine how a substance dissolved in water will interact with and affect its environment. Scientists use pH as a measure of the acidity based on the amount of hydrogen ions (H+) and hydroxide ions (OH-) contained in a particular volume of a solution.

Pure water (not tap water or rainwater) has an equal number of H+ and OH- ions. It is called a neutral solution and has a pH of 7. An acidic solution has more hydrogen ions than hydroxide ions and has a pH less than 7. A basic solution has more hydroxide ions than hydrogen ions and has a pH greater than 7.

The third building block of matter is a molecule: a combination of two or more atoms of the same or different elements held together by chemical bonds. Molecules are the basic units of some compounds (called molecular compounds).

Chemists use a chemical formula to show the number of each type of atom or ion in a compound. This shorthand contains the symbol for each element present and uses subscripts to represent the number of atoms or ions of each element in the compound’s basic structural unit. Examples of compounds and their formulas encountered in this book are sodium chloride (NaCl) and water (H2O, read as “H-two-O”).

Table sugar, vitamins, plastics, aspirin, penicillin, and most of the chemicals in your body are organic compounds, which contain at least two carbon atoms combined with atoms of one or more other elements, such as hydrogen, oxygen, nitrogen, sulfur, phosphorus, chlorine, and fluorine. One exception, methane (CH4), has only one carbon atom. All other compounds are called inorganic compounds.

Elements Important to the Study of Environmental Science

Element - Symbol

Hydrogen - H

Carbon - C

Oxygen - O

Nitrogen - N

Phosphorus - P

Sulfur - S

Chlorine - Cl

Fluorine - F

Bromine – Br

Sodium – Na

Calcium – Ca

Lead – Pb

Mercury – Hg

Arsenic - As

Uranium - U

Ions Important to the Study of Environmental Science

Positive Ion - Symbol

Hydrogen ion H+

sodium ion Na+

calcium ion Ca2+

aluminum ion Al3+

ammonium ion NH4

Negative Ion Symbol

Chloride ion Cl-

Hydroxide ion OH-

Nitrate ion NO3-

Sulfate ion SO4 (2- )

Phosphate ion - PO

Compounds Important to the Study of Environmental Science

Compound - Formula

Sodium chloride - NaCl

Carbon monoxide - CO

Carbon dioxide - CO2

Nitric oxide - NO

Nitrogen dioxide - NO2

Nitrous oxide - N2O

Nitric acid - HNO3

Compound Formula


Mmethane - CH4

Glucose - C6H12O6

Water - H2O

Hydrogen sulfide - H2S

Sulfur dioxide - SO2

Sulfuric acid - H2SO4

Ammonia - NH3

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