Why compounds form

For example, a mixture of fruits in a salad can be separated back into groups of different kinds of fruit. Salt and water can be combined in a mixture, but water is still water, and salt is still salt. To separate the two components, the water can be evaporated so that the salt can be collected. Sand and water can be separated by using a filter. The ocean, rocks, blood, and even the air we breathe are mixtures rather than compounds. On the other hand, the components in a compound cannot be separated by physical means.

Learn more about compounds and mixtures. Physical changes do not break down compounds. Physical changes affect the size, shape, or state of the substance, but not the chemical properties. You can change the state of matter , but the compound does not change. If you leave an ice cube out in the sun it will melt into liquid water, but in either state it is still made of water molecules.

You can apply a physical force to a solid glass and break it, but the molecules that make up glass will remain. Chemical changes in compounds happen when chemical bonds are created or destroyed.

Then the molecular structure changes; new molecules form and a new substance is created. Often heat is used to begin a chemical change, as when baking a cake.

Another example of a chemical reaction is the rusting of a metal trash can. The rusting happens because the iron Fe in the metal combines with oxygen O 2 in the air.

Chemical bonds are created and destroyed to eventually make iron oxide Fe 2 O 3 , which we call rust. It is not easy to break chemical bonds, but it can be done in chemical reactions using energy to break the bonds. For example, an electric current passed through water can cause a chemical change that breaks water down into hydrogen and oxygen. When a chemist mixes different compounds in a chemical reaction, the compounds may join together to make one compound or change into several new compounds.

Some of the signs of a chemical reaction are a change in temperature, the formation of a gas, or a color change. Scientists have a specific way of naming compounds. There are some complex rules, but let's focus on the simple ones. For molecules with two elements, the compound name has two words: the name of the first element, and the name of the second element changing its ending to "ide.

If one of the elements has more than one atom, you add a prefix to the beginning of the name of the element, depending on the number of atoms. If there are two atoms, you add "di" at the beginning. If there are three, you add "tri" at the beginning. If there are four, you add "tetra. The compound of one atom of sodium and one atom of chlorine is named sodium chloride.

The compound of one atom of magnesium and one atom of sulfur MgS is named magnesium sulfide. The compound of one atom of carbon and two atoms of oxygen is named carbon dioxide. The compound of one atom of carbon and four atoms of chlorine is named carbon tetrachloride. Compounds When two or more atoms join together, we call it a molecule. Note that the arrows in the diagram always point in the direction where the electrons are more strongly attracted.

In chemistry, an ionic compound is a chemical compound comprised ions held together by electrostatic forces, termed ionic bonding. The compound is neutral overall, but consists of positively charged cations and negatively charged anions. Individual ions within an ionic compound usually have multiple nearest neighbours, so are not considered to be part of molecules, but instead part of a continuous three-dimensional network, usually in a crystalline structure. A The crystal structure of sodium chloride, NaCl, a typical ionic compound.

B Halite, the mineral form of sodium chloride, forms when salty water evaportates leaving the ions behind. Source: A Benjah-bmm27 B Lavisky, R. All other ionic compounds without these ions are known as salts. Ionic compounds typically have high melting and boiling points, and are hard and brittle.

As solids, they are most often electrically insulating, but when melted or dissolved they become highly conductive, because the ions are mobilized. Unlike ionic compounds, with their extended crystal lattices, covalent molecules are discrete units with specific three-dimensional shapes. The shape of a molecule is determined by the fact that covalent bonds, which are composed of shared negatively charged electrons, tend to repel one another.

The result is a linear molecule:. The molecules BeCl 2 and BF 3 actually violate the octet rule; however, such exceptions are rare and will not be discussed in this text. The four covalent bonds in CCl 4 arrange themselves three dimensionally, pointing toward the corner of a tetrahedron and making bond angles of So, how can this theory of electron repulsion be used in a simple way to predict the shape of a molecule? First, it is necessary to understand how many electron pairs are involved and whether or not those electron pairs are in bonded relationships between two atoms Bonded Pairs or whether they are Lone Pairs.

To make this determination, it is useful to draw the Lewis Structure for the molecule and show all of the bonding groups and lone pair electrons. Note that in VSEPR theory that a double or triple bond are treated as a single bonding group, because all of the electrons involved in the bond are shared with only a single atom. For example, if you have a molecule of NH 3 :. We can see that there are three atoms of hydrogen bonded to the central nitrogen atom.

We can also see that the central nitrogen has one lone pair of electrons extending from the top of the atom. We derive two important pieces of information from this. With the steric number and AXE formula calculated, we can now use Table 4. In Table 4.

In this case, the second selection is correct: AX 3 E 1. So we can see from this table that the shape of NH 3 is trigonal pyramidal or it looks like a pyramid with three corners with a hydrogen at each one. Notice that a lone pair electrons on the central atom affect the shape by their presence by pushing the hydrogens below the central plain of the molecule, but that it is not included in the overall shape of the molecule Figure 4.

The lone pair density in NH 3 contributes to the overall shape of the molecule by pushing the hydrogens below the plain of the nitrogen central atom. However, they are not visible in the final molecular geometry, which is trigonal pyramidal. Using Table 4. How about the shapes of molecules with multiple bonds? They are determined by treating the multiple bonds as one bonding group.

In addition to learning about the bond characteristics and shapes of molecules, it is also very important to learn about how molecules interact with other molecules around them. This type of interaction, known as an intermolecular interaction , is important for determining broader characteristics of the molecule including reactivity and function. Intermolecular interactions between molecules are dependent on the phase that the molecule exists.

A phase is a certain form of matter that includes a specific set of physical properties. That is, the atoms, the molecules, or the ions that make up the phase do so in a consistent manner throughout the phase. As mentioned in Chapter 1, science recognizes three stable phases: the solid phase , in which individual particles can be thought of as in contact and held in place defined volume and shape ; the liquid phase , in which individual particles are in contact but moving with respect to each other defined volume but, shape of the container ; and the gas phase no defined shape or volume , in which individual particles are separated from each other by relatively large distances.

Not all substances will readily exhibit all phases on the Earth. For example, carbon dioxide does not exhibit a liquid or solid phase on Earth unless the pressure is greater than about six times normal atmospheric pressure. Other substances, especially complex organic molecules, may decompose or breakdown at higher temperatures, rather than becoming a liquid or a gas. For example, think about roasting a marshmallow. If it gets too close to the flames it will become charred and blackened, breaking down the sugar molecules inside.

The sugar is not converted into the liquid or gaseous phase. Thus, water is very unique in its ability to exist on the Earth in all three phase states solid ice — liquid water — water vapor.

Which phase a substance adopts depends on the pressure and the temperature it experiences. Of these two conditions, temperature variations are more obviously related to the phase of a substance.

When it is very cold, H 2 O exists in the solid form as ice. When it is warmer, the liquid phase of H 2 O is present. At even higher temperatures, H 2 O boils and becomes steam gaseous phase. Pressure changes can also affect the presence of a particular phase as we indicated for carbon dioxide , but its effects are less obvious most of the time. We will mostly focus on the temperature effects on phases, mentioning pressure effects only when they are important.

Most chemical substances follow the same pattern of phases when going from a low temperature to a high temperature: the solid phase, then the liquid phase, and then the gas phase. However, the temperatures at which these phases are present differ for all substances and can be rather extreme.

Table 4. As you can see, there is extreme variability in the temperature ranges. Still, most of these elements are found in mixtures. When two distinct elements are chemically combined—i. Most elements on Earth bond with other elements to form chemical compounds, such as sodium Na and Chloride Cl , which combine to form table salt NaCl.

Water is another example of a chemical compound. The two or more component elements of a compound can be separated through chemical reactions. Chemical compounds have a unique and defined structure, which consists of a fixed ratio of atoms held together in a defined spatial arrangement by chemical bonds. Chemical compounds can be:. Pure chemical elements are not considered chemical compounds, even if they consist of diatomic or polyatomic molecules molecules that contain only multiple atoms of a single element, such as H 2 or S 8.

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Elements and Compounds. Learning Objective Differentiate between elements and compounds and explore separation techniques.