Classifying Elements: Properties and Structure

1. Science 9: Unit B – Matter and Chemical Change Topic 4: Classifying Elements

2. What is an element? • An element is a pure substance that cannot be broken down into anything simpler. • Everything in the universe is made up of elements. Elements in their simplest form exist as atoms.

3. Elements and Compounds • As stated in the Law of Definite Proportions, compounds are made up of a fixed proportion of elements. Eg. A molecule of water is made up of two hydrogen atoms to one oxygen atom. • Elements can only be destroyed or changed in nuclear reactions. Compounds can be changed in chemical reactions.

4. How are elements organized? • There are over 100 known elements. Elements are organized according to their chemical properties and structure in the periodic table. For example metals are grouped with metals, non-metals with other non-metals and metalloids (have properties of both) are placed between them. • Each element has its own element symbol. Eg. Oxygen = O. Some elements have been known since the beginning of time such as copper. Their symbols reflect their old Latin name (Cu=Cuprum).

5. What kinds of elements are there? • There are three basic groups of elements: • Metals (on the left side of the periodic table) – almost always solid at room temperature, denser than other elements, good conductors of heat and electricity, shiny and malleable. Usually give up/donate electrons in a chemical reaction.

6. Non-Metal Elements • Non-Metals (on the right side of the periodic table) – mostly gases at room temperature, less dense than metals, poor conductors of heat and electricity, tend to be dull. Almost always take in extra electrons in a chemical reaction.

7. Metalloids • Metalloids – (2/3 of the way to the right of the periodic table near the ‘staircase’) – Solid at room temperature. Properties are between those of metals and non-metals. (ie. they are SEMI-conductors of heat and electricity).

8. Families are elements which have similar chemical properties • In addition to these three general groups, there are a number of elements whose properties are so similar that they are grouped together in families. A family is represented as a column in a periodic table. There are four element families you need to know for Science 9. • The reason why they share similar properties is based on their VALENCE ELECTRONS.

9. Orbitals and Valence Electrons • All elements need to have their orbitals filled up with the right number of electrons to be stable. This is why some elements are much more reactive than others, and the Noble Gases who already have their orbitals filled up do not react at all. • Valence electrons are those electrons which exist in the outermost (valence) orbital.

10. The Noble Gases • Column # 18 in the periodic table (far right). • Elements include: Helium (He), Neon (Ne), Krypton (Kr). • Chemical properties: These elements have all of their orbital level filled. They are unreactive and only exist as single atoms in nature (ie. they do not form compounds).

11. Halogen Gases • Column # 17 in the periodic table (right). • Elements include: Fluorine (F), Chlorine (Cl), Bromine (Br), Iodine (I). • Chemical properties: These elements need one more electron to fill all of their orbital levels and are therefore very reactive with metals and hydrogen forming compounds.

12. Alkali Metals • Column # 1 in the periodic table (far left). • Elements include: Lithium (Li), Sodium (Na), Potassium (K), … • Chemical properties: These metals need to get rid of just one electron to have all of their orbital levels filled with the proper # of electrons. They therefore are very reactive with non-metals.

13. Alkali Earth Metals • Column # 2 in the periodic table (left). • Elements include: Beryllium (Be), Magnesium (Mg), Calcium (Ca), … • Chemical properties: These metals need to get rid of two electrons to have their orbital levels full. Because they are further away from their goal, they are less reactive than Alkali Metals or Halogens.