Valence Electrons

Valence Electrons The valence electrons are the electrons in the last shell or energy level of an atom. They do show a repeating or periodic pattern. The valence electrons increase in number as you go across a period. Then when you start the new period, the number drops back down to one and starts increasing again. For example, when you go across the table from carbon to nitrogen to oxygen, the number of valence electrons increases from 4 to 5 to 6. As we go from fluorine to neon to sodium, the number of valence electrons increases from 7 to 8 and then drops down to 1 when we start the new period with sodium A quick way to determine the number of valence electrons for a representative element is to look at which group is it in. Elements in group Ia have 1 valence electron. Elements in group IIa have 2 valence electrons. Can you guess how many valence electrons elements in group VIa have? If you guessed 6 valence electrons, then you are correct! The only group of representative elements that this method doesn't work for is group 0. Those elements certainly have more than 0 valence electrons; in fact, all of them except for helium have 8 valence electrons. Why doesn't helium have 8 valence electrons? Think for a moment about how many electrons helium has - it has a total of only two electrons, so helium only has 2 valence electrons. So generally speaking, the number of valence electrons stays the same as you go up or down a group, but they increase as you go from left to right across the periodic table. The preceding statement works very well for the representative elements, but it comes a bit short of the truth when you start talking about the transition elements. For a chemist, the valence electrons are quite possibly the most important electrons an atom has. "Why the valence electrons?", you might ask. Well, since the valence electrons are the electrons in the highest energy level, they are the most exposed of all the electrons ... and, consequently, they are the electrons that get most involved in chemical reactions. Chemists use a notation called electron dot diagrams, also known as Lewis diagrams, to show how many valence electrons a particular element has. An electron dot diagram consists of the element's symbol surrounded by dots that represent the valence electrons. Typically the dots are drawn as if there is a square surrounding the element symbol with up to two dots per side. (An element will never have more than eight valence electrons.)

Emp ir ical For mu la In chemistry, the empirical formula of a chemical compound is a simple expression of the relative number of each type of atom (called a chemical element) in it. An empirical formula makes no reference to isomerism, structure, or absolute number of atoms. Empirical formulae are the standard for ionic compounds, such as CaCl2, and for macromolecules, such as SiO2. The term empirical refers to the process of elemental analysis, a technique of analytical chemistry used to determine the relative percent composition of a pure chemical substance by element. In contrast, the molecular formula identifies the absolute number of atoms of such element to be found in each discrete molecule of that compound. For example, n-hexane, a chemical compound has the molecular formula CH3CH2CH2CH2CH2CH3, implying that it has a straight chain structure, 6 carbon atoms, and 14 hydrogen atoms. Hexane's molecular formula is C6H14, and its empirical formula would be C3H7 showing a C:H ratio of 3:7.

Per centa ge Composition How to Calculate the Percentage Composition of a Chemical Compound? Example: What is the percentage of the various elements in Sodium Carbonate [Na2CO3] ? Given Atomic Weights: C = 12 ; O = 16 ; Na = 23 The first step is to calculate the Molecular Weight (or the Formula Weight) of the chemical compound by adding the atomic weights of the atoms (elements) that constitute the compound.

Molecular weight of Na2CO3 = (2 x Atomic weight of Na) + Atomic weight of C + (3 x Atomic weight of O) = (2 x 23) + 12 + (3 x 16) = 46 + 12 + 48 = 106 Now, 106 grams of Sodium Carbonate [Na2CO3] contain 46 grams of Sodium [Na], 12 grams of Carbon [C], and 48 grams of Oxygen [O]. So, Percentage of Sodium [Na] in Sodium Carbonate [Na2CO3] = 46/106 x 100 = 43.40%.

Naming Compound Before naming a compound, you have to figure out what kind of compound it is. We will consider three types: Ionic Compounds Without a Transition Metal.

Ionic compounds are formed when a metal gives up its electrons to a non-metal. Basically if the compound contains a metal, it is ionic. But there are different sets of rules for transition metals. A transition metal is an element with an atomic number of 21 to 30, 39 to 48 or 57 to 80.

a. So for a compound with any other metal, apply these rules: The metal ion's name does not change regardless of charge The non-metal's name ends in ide b. In going backwards (from name to formula), we will have more fun. In such a case the total charge of the (+) and (-) ions in the compound has to be ZERO. c.

Polyatomic Ions

When metals are bonded to polyatomic ions, which consist of two or more atoms with one overall charge, the same rules apply, but you have to learn the names and charges of common polyatomic ions. 2. Ionic Compounds With a Transition Metal. The only difference here is that we have to specify the charge of the transition metal ion by using a Roman numeral, and keep in mind that a transition metal is an element with an atomic number of 21 to 30, 39 to 48 or 57 to 80.

3. Covalent Compounds. These are formed from non-metals that share electrons. Because there are many sharing possibilities between two non-metals, the formula cannot be guessed unless we have a naming system that reveals the number of atoms involved.