Activity 21 a Quick Review of Elements and Compounds
Elements, Compounds, and Mixtures
Elements
Any substance that contains only one kind of an atom is known as an chemical element. Because atoms cannot be created or destroyed in a chemic reaction, elements such as phosphorus (P4) or sulfur (Southward8) cannot exist broken downwards into simpler substances past 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 paw, cannot be decomposed into simpler substances. They are therefore the uncomplicated, or simplest, chemical substances - elements.
Each element is represented past a unique symbol. The notation for each element can be constitute on the periodic tabular array of elements.
The elements can be divided into three categories that have characteristic backdrop: metals, nonmetals, and semimetals. Well-nigh elements are metals, which are found on the left and toward the bottom of the periodic tabular array. A handful of nonmetals are clustered in the upper right corner of the periodic table. The semimetals can exist found along the dividing line betwixt the metals and the nonmetals.
Atoms
Elements are made up of atoms, the smallest particle that has any of the backdrop of the element.John Dalton, in 1803, proposed a mod theory of the atom based on the post-obit assumptions.
| 1. Matter is made up of atoms that are indivisible and indestructible. |
| two. All atoms of an element are identical. |
| 3. Atoms of different elements have different weights and dissimilar chemical backdrop. |
| 4. Atoms of different elements combine in uncomplicated whole numbers to grade compounds. |
| 5. Atoms cannot be created or destroyed. When a compound decomposes, the atoms are recovered unchanged. |
Compounds
Elements combine to form chemical compounds that are oft 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. Past convention, no subscript is written when a molecule contains merely one atom of an element. Thus, h2o is H2O and carbon dioxide is CO2.
Characteristics of Ionic and Covalent Compounds
| Ionic Compounds | Covalent Compounds | |
| Contain positive and negative ions (Na+Cl-) | Exist as neutral molecules (C6H12O2) | |
| Solids suchs every bit 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 forcefulness of attraction between molecules | |
| Separate into charged particles in water to give a solution that conducts electricity | Remain as aforementioned molecule in water and will not deport electricity | |
Determining if a Chemical 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 effigy shown below to assist make up one's mind if the compound is ionic or covalent, or metal.
For each of the following compounds, predict whether you would look it to be ionic or covalent.
(a) chromium(III) oxide, Cr2O3
(b) carbon tetrachloride, CClfour
(c) methanol, CH3OH
(d) strontium fluoride, SrFtwo
Click hither to cheque your answer to Exercise Problem 1
Use the following data to propose a mode of distinguishing betwixt ionic and covalent compounds.
| Compound | Melting Bespeak ( o C) | Boiling Point ( o C) |
| CriiOiii | 2266 | 4000 |
| SrF2 | 1470 | 2489 |
| CCl4 | -22.9 | 76.6 |
| CHthreeOH | -97.8 | 64.7 |
Click here to check your answer to Practice Problem 2
Formulas
A molecule is the smallest particle that has any of the properties of a chemical compound. The formula for a molecule must be neutral. When writing the formula for an ionic compound, the charges on the ions must remainder, the number of postive charges must equal the number of negative charges.
Examples:
| CaCl2 | Counterbalanced formula has 2 positive charges (1 calcium ion with +two charge) and 2 negative charges (2 chloride ions with a -1 charge) |
| Al2(And so4)three | Counterbalanced formula has six positive charges (2 aluminum ions with a +3 charge) and 6 negative charges (three sulfate ions with -2 charge) |
Mixtures Vs. Compounds
The police 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 chemical compound. The constabulary of abiding composition can be used to distinguish between compounds and mixtures of elements: Compounds have a constant composition; mixtures do not. Water is e'er 88.eight% O and eleven.2% H by weight regardless of its source. Brass is an example of a mixture of two elements: copper and zinc. Information technology tin can contain every bit little as 10%, or as much as 45%, zinc.
Another difference between compounds and mixtures of elements is the ease with which the elements tin can exist separated. Mixtures, such as the temper, comprise two or more substances that are relatively easy to separate. The private components of a mixture can exist 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 chemic compounds and mixtures of elements are illustrated by the following case using raisin bran and "Crispix.".
Raisin bran has the following feature properties of a mixture.
- The cereal does not have a abiding composition; the ratio of raisins to bran flakes changes from sample to sample.
- It is piece of cake to physically split up the 2 "elements," to pick out the raisins, for example, and eat them separately.
Crispix has some of the characteristic properties of a compound.
- The ratio of rice flakes to corn flakes is constant; information technology is 1:i in every sample.
- At that place is no mode to separate the "elements" without breaking the bonds that hold them together.
Source: http://chemed.chem.purdue.edu/genchem/topicreview/bp/ch2/
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