Elements, compounds, and mixtures

Elements, Compounds, and Mixtures

Elements Atoms Compounds Characteristics of Compounds
Determining Ionic Vs. Covalent Molecules Mixtures Vs. Compunds


Any substance that contains only one kind of an atom is known as an element. Because atoms cannot be created or destroyed in a chemical reaction, elements such as phosphorus (P4) or sulfur (S8) cannot be broken down into simpler substances by these reactions.

Example: Water decomposes into a mixture of hydrogen and oxygen when an electric current is passed through the liquid. Hydrogen and oxygen, on the other hand, cannot be decomposed into simpler substances. They are therefore the elementary, or simplest, chemical substances - elements.

Each element is represented by a unique symbol. The notation for each element can be found on the periodic table of elements.

The elements can be divided into three categories that have characteristic properties: metals, nonmetals, and semimetals. Most elements are metals, which are found on the left and toward the bottom of the periodic table. A handful of nonmetals are clustered in the upper right corner of the periodic table. The semimetals can be found along the dividing line between the metals and the nonmetals.

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Elements are made up of atoms, the smallest particle that has any of the properties of the element.John Dalton, in 1803, proposed a modern theory of the atom based on the following assumptions.

1. Matter is made up of atoms that are indivisible and indestructible.

2. All atoms of an element are identical.

3. Atoms of different elements have different weights and different chemical properties.

4. Atoms of different elements combine in simple whole numbers to form compounds.

5. Atoms cannot be created or destroyed. When a compound decomposes, the atoms are recovered unchanged.

Go To Atoms Vs. Ions

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Elements combine to form chemical compounds that are often divided into two categories.

Metals often react with nonmetals to form ionic compounds. These compounds are composed of positive and negative ions formed by adding or subtracting electrons from neutral atoms and molecules.


Nonmetals combine with each other to form covalent compounds, which exist as neutral molecules.


The shorthand notation for a compound describes the number of atoms of each element, which is indicated by a subscript written after the symbol for the element. By convention, no subscript is written when a molecule contains only one atom of an element. Thus, water is H2O and carbon dioxide is CO2.

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Characteristics of Ionic and Covalent Compounds

Ionic Compounds

Covalent Compounds

Contain positive and negative ions (Na+Cl-)

Exist as neutral molecules (C6H12O2)

Solids suchs as table salt (NaCl(s))

Solids, liquids,or gases (C6H12O6(s), H2O(l), CO2(g))

High melting and boiling points

Lower melting and boiling points (i.e., often exist as a liquid or gas at room temperature)

Strong force of attraction between particles

Relatively weak force of attraction between molecules

Separate into charged particles in water to give a solution that conducts electricity


Remain as same molecule in water and will not conduct electricity


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Determining if a Compound is Ionic or Covalent

Calculate the difference between the electronegativities of two elements in a compound and the average of their electronegativites, and find the intersection of these values on the figure shown below to help determine if the compound is ionic or covalent, or metallic.


Practice Problem 1:

For each of the following compounds, predict whether you would expect it to be ionic or covalent.  

(a) chromium(III) oxide, Cr2O3

(b) carbon tetrachloride, CCl4

(c) methanol, CH3OH

(d) strontium fluoride, SrF2

Click here to check your answer to Practice Problem 1


Practice Problem 2:

Use the following data to propose a way of distinguishing between ionic and covalent compounds.

Compound Melting Point ( oC) Boiling Point ( oC)
Cr2O3 2266 4000
SrF2 1470 2489
CCl4 -22.9 76.6
CH3OH -97.8 64.7

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Practice Problem 3:

Which of the following compounds should conduct an electric current when dissolved in water?

(a) methanol, CH3OH

(b) strontium fluoride, SrF2

Click here to check your answer to Practice Problem 3

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A molecule is the smallest particle that has any of the properties of a compound. The formula for a molecule must be neutral. When writing the formula for an ionic compound, the charges on the ions must balance, the number of postive charges must equal the number of negative charges.


CaCl2 Balanced formula has 2 positive charges (1 calcium ion with +2 charge) and 2 negative charges (2 chloride ions with a -1 charge)
Al2(SO4)3 Balanced formula has 6 positive charges (2 aluminum ions with a +3 charge) and 6 negative charges (3 sulfate ions with -2 charge)

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Mixtures Vs. Compounds

The law of constant composition states that the ratio by mass of the elements in a chemical compound is always the same, regardless of the source of the compound. The law of constant composition can be used to distinguish between compounds and mixtures of elements: Compounds have a constant composition; mixtures do not. Water is always 88.8% O and 11.2% H by weight regardless of its source. Brass is an example of a mixture of two elements: copper and zinc. It can contain as little as 10%, or as much as 45%, zinc.

Another difference between compounds and mixtures of elements is the ease with which the elements can be separated. Mixtures, such as the atmosphere, contain two or more substances that are relatively easy to separate. The individual components of a mixture can be physically separated from each other.

Chemical compounds are very different from mixtures: The elements in a chemical compound can only be separated by destroying the compound. Some of the differences between chemical compounds and mixtures of elements are illustrated by the following example using raisin bran and "Crispix.".

Raisin bran has the following characteristic properties of a mixture.

Crispix has some of the characteristic properties of a compound.

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