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Atoms, Molecules, and Compounds

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The content and activity in this topic will work towards building an understanding of the structure of atoms and how elements are organized on the periodic table.

Chemical Structures

The properties of elements and compounds are determined by their structures. The simplest structural unit of an element is an atom. Atoms are very small. A hundred million (100,000,000) hydrogen atoms put side-by-side is only as long as one centimeter!



Fig. 2.8. Elements can be made of one atom, like He, or be elemental molecules, such as hydrogen (H2), oxygen (O2), chlorine (Cl2), ozone (O3), and sulfur (S8). Atoms are not drawn to scale.

Image by Bryon Inuoye

Some elements are monatomic, meaning they are made of a single (mon-) atom (-atomic) in their molecular form. Helium (He, see Fig. 2.8) is an example of a monatomic element. Other elements contain two or more atoms in their molecular form (Fig. 2.8). Hydrogen (H2), oxygen (O2), and chlorine (Cl2) molecules, for example, each contains two atoms. Another form of oxygen, ozone (O3), has three atoms, and sulfur (S8) has eight atoms. All elemental molecules are made of atoms of a single element.



Fig. 2.9. Compounds are made of two or more atoms of different elements, such as water (H2O) and methane (CH4). Atoms are not drawn to scale.

Image by Bryon Inuoye

Molecules of compounds have atoms of two or more different elements. For example, water (H2O) has three atoms, two hydrogen (H) atoms and one oxygen (O) atom. Methane (CH4), a common greenhouse gas, has five atoms, one of carbon (C) and four of hydrogen (H, see Fig. 2.9).


Electrostatic Forces


Fig. 2.10. Electrostatic forces can raise your hair!

Image courtesy of Chris Darling from Wikipedia

Electrostatic forces hold atoms in molecules. The electrostatic forces that hold atoms together in molecules are the same type of forces that cause static electricity. Common examples of static electricity are when someone gets a shock when reaching for a doorknob or when a child’s hair is raised when going down a plastic slide (Fig. 2.10).


Parts of Atoms

The particles that make up an atom are called subatomic particles (sub- means “smaller size”). These particles are the

  • proton (p+), which is positively (+) charged;
  • electron (e), which is negatively (–) charged; and
  • neutron (n0), which has no charge, it is neutral (0).



Fig. 2.11. Model of an atom (not to scale)

Image by Byron Inouye

Protons and neutrons occupy the nucleus, or center, of the atom. Electrons exist in regions called shells outside of the atom’s nucleus (Fig. 2.11).


Electrostatic forces hold atoms together in molecules—like the two hydrogen atoms held together in H2 gas. Electrostatic forces also hold electrons and protons together in the atom. The attraction between negatively charged electrons and positively charged protons in an atom give the atom its structure. The strong force holds neutrons and protons together in the nucleus. This force got its name because it is strong enough to overcome the force of the positively charged protons repelling each other. The number of electrons and protons in an atom determines its chemical properties. Chemical properties include the specific ways that atoms and molecules react and the energy that they release or use in these reactions.


Size of Subatomic Particles

One hundred million (100,000,000) hydrogen atoms put side-by-side equals about a centimeter. Protons and neutrons are both about one-thousandth (1/1000) the diameter of a hydrogen atom. This means it would take about one hundred billion (100,000,000,000) protons or neutrons put side-by-side to equal a centimeter. Electrons are about one-thousandth (1/1000) the diameter of a proton or neutron. This means that it would take one hundred trillion (100,000,000,000,000) electrons put side-by-side to equal a centimeter!


Neutral Atoms

The subatomic particles in an atom determine the properties of the atom. Some atoms exist naturally as neutral, or uncharged, atoms. A single uncharged atom has an equal number of protons (+) and electrons (–). An uncharged atom is electrically neutral because electrons and protons have opposite charges of equal sizes. When the number of protons and electrons in an atom are same, the charges cancel out, or counteract each other.


Protons and Neutrons

Every atom of a particular element has the same number of protons. The atomic number is equal to the number of protons in an element. On the periodic table, the atomic number is usually given as the whole number above the symbol for the element (see Fig. 2.13). For example, hydrogen (H) has an atomic number of one (1). This means a hydrogen atom has one proton. If a hydrogen atom is neutral, it must also have one electron. An oxygen atom (O) has an atomic number of eight (8). This means a neutral oxygen atom has eight protons and eight electrons. The element Actium (Ac) has an atomic number of 89, so it has 89 protons and 89 electrons in a neutral atom. Table 2.2 shows the atomic number, atomic symbol, atomic structure, and number of protons, neutrons, and electrons for the first three elements.

Table 2.2. The first three elements in the periodic table showing atomic numbers, atomic symbols, number of protons, number of electrons, number of neutrons, and atomic structure.
  Hydrogen Helium Lithium
Atomic Number 1 2 3
Atomic Symbol H He Li
Number of Protons 1 2 3
Number of Electrons 1 2 3
Number of Neutrons 0 2 4
Atomic Structure
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Image by Byron Inouye

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Image by Byron Inouye

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Image by Byron Inouye

Neutrons affect the mass of an atom and play a role in the stability of atoms. Unlike protons, the numbers of neutrons in elements varies. For example, most hydrogen atoms have no neutrons, but a few have one neutron, and some rare hydrogen atoms have two neutrons. Most helium atoms have two neutrons, but some have three neutrons.


Periods, Groups, and the Periodic Table

The periodic table (Fig. 2.12) is a commonly used method of organizing the elements that provides useful information about the elements and their behavior. In Fig. 2.12, elements in blue are metals and elements in yellow are nonmetals. In Figure 2.13, the entry for hydrogen highlights the placement of the atomic number, element symbol, element name, and atomic weight.


Image caption

Fig. 2.12. The periodic table of the elements (2014). This periodic table shows metal elements in blue and nonmetal elements in yellow.

Image copyright and source

Image by Byron Inouye


The periodic table has three prominent features. First, the periodic table is arranged in horizontal rows, which are called periods. There are seven periods. In Period 1 there are two elements, hydrogen (H) and helium (He). The second and third periods both contain eight elements, the fourth and fifth periods contain 18 elements, and the sixth and seventh periods contain 32 elements.



Fig. 2.13. The listing for hydrogen in the periodic table

Image by Byron Inuoye


Second, all of the elements are listed sequentially according to their atomic numbers. The atomic number corresponds to the number of protons and is found above the elements’ symbol. For example, in Figure 2.13, the atomic number of hydrogen is 1, found over the H.


Third, the periodic table is arranged in columns of elements that react similarly. These columns are called groups. The group number is found at the top of the column. Groups 1–12 contain only metals, Groups 13–16 contain both metals and nonmetals, and Groups 17 and 18 contain only nonmetals. One exception is hydrogen. Although technically a nonmetal, hydrogen has properties of both metals and nonmetals and is often placed in Group 1. The two long rows that are at the bottom of the periodic table are exceptions. The elements in each of these rows behave similarly, so are considered groups. These two groups are arranged in rows rather than columns.


Metals and Nonmetals

Metals are elements that conduct heat and electricity. Metals are usually malleable, they can be bent or molded without breaking, and lustrous, or shiny. Most metals are silvery in color (Fig. 2.14 A–C), although some are not, like copper (Cu, Fig. 2.14 D). Most metals are solid at room temperature. One exception is mercury (Hg), which is a liquid at room temperature (Fig. 2.14 A). The elements in Group 1, including lithium (Li), sodium (Na, Fig. 2.14 B), potassium (K, Fig. 2.14 C), and rubidium (Rb), are all metals. These metallic Group 1 elements have similar reactive properties. In Fig 2.12, the metals are shown in blue.


Fig. 2.14. (A) Mercury (Hg) is a metallic element.

Image courtesy of p.Gordon

Fig. 2.14. (B) Sodium (Na) is a metallic element.

Image courtesy of Dennis S.K from Wikipedia


Fig. 2.14. (C) Potassium (K) is a metallic element.

Image courtesy of Dennis S.K from Wikipedia

Fig. 2.14. (D) Copper (Cu) is a metallic element that is not silvery in color like many other metals.

Image courtesy of Jurii from Wikipedia


Nonmetals are poor conductors of heat and electricity; they are not lustrous and exist in nature as solids, liquids, or gases. When solid, non-metals tend to be brittle, such as sulfur, which flakes apart rather than bending like a metal would (Fig. 2.15 A). The elements in Group 17, including fluorine (F2), chlorine (Cl2, Fig. 2.15 B), bromine (Br2, Fig. 2.15 C), and iodine (I2, Fig. 2.15 D), are all nonmetals. The nonmetals in Group 17 are all diatomic (two atoms) in their elemental form and have similar reactive properties. In Fig 2.12, the nonmetals are shown in yellow.


Fig. 2.15. (A) Sulfur (S)

Image courtesy of Ben Mills from Wikipedia

Fig. 2.15. (B) Chlorine (Cl2, yellow-green gas)

Image courtesy of Tomihahndorf from Wikipedia


Fig. 2.15. (C) Bromine (Br2, orange-red gas and liquid)

Image courtesy of Tomihahndorf from Wikipedia


Fig. 2.15. (D) Iodine (I2, dark purple solid and gas)

Image courtesy of Tomihahndorf from Wikipedia


See Table 2.3 for a summary of the properties of metals and nonmetals.

Table 2.3. Properties of metals and nonmetals
  Metals Nonmetals
Physical Properties Good conductor of heat and electricity Poor conductor of heat and electricity
Malleable - can be beaten or deformed without cracking; pliable Brittle
Ductile - can be made into wire Non-ductile
Lustrous Not lustrous, may be opaque or transparent
Solid at room temperature (except Hg and a few other metals that are liquid at or near room temperature) Solid, liquid, or gas at room temperature
Chemical Properties Usually have 1-3 valence electrons Usually have 4-8 valence electrons
Tend to lose valence electrons Tend to gain electrons


Other Organizational Features of the Periodic Table

There are other organizational features of the periodic table. Most periods have the first element of the period in Group 1 and the last element in Group 18. An exception is the first period. In Fig 2.12, hydrogen (H) is in Group 1. Sometimes hydrogen (H) is placed in Group 17, above fluorine (F), because it has similar properties to the nonmetals in that group; for example, in its elemental state hydrogen exists as a diatomic gas, H2. Sometimes hydrogen is placed in both Groups 1 and 17.


Groups of elements have similar properties. The properties of some groups are so unique or important that the groups are referred to by special names. The last group, Group 18, includes helium (He), neon (Ne), argon (Ar), krypton (Kr), xenon (Xe), and radon (Rn). The elements in this group are called the noble gases. Noble gases seldom react with other elements. Noble gases have many uses, for example, they are used in neon signs (Fig 2.16).


Image caption

Fig 2.16. “Neon” signs are composed of different noble gases, only one of which is the element neon. Nobel gases light up with different colors when electricity is passed through them.

Image copyright and source

Images courtesy of Pslawinski from Wikipedia


Group 1 is often referred to as the alkali metals, Group 2 as the alkaline earth metals, and Group 17 as the halogens. The two groups that are pulled out on the bottom of the periodic table in rows are called the lanthanide rare earth series (top row) and the actinide series (bottom row).



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Exploring Our Fluid Earth, a product of the Curriculum Research & Development Group (CRDG), College of Education. University of Hawaii, 2011. This document may be freely reproduced and distributed for non-profit educational purposes.