Unit 15-17 Science Notes Essay

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Units 15-17 Notes
Unit 15
1. Elements
-the components of all the vast number of different materials can be separate into about 115 basic building blocks called elements
-elements are substances that cannot be broken down into other substances
-there are about 90 naturally occurring elements, and another 25 synthetic elements
-can be divided into three classes: metals, non-metals, and metalloids

-most of the elements are metals
-most metals are silver or grey in colour and shiny
-all good conductors of electricity and heat
-all are malleable and ductile
-most metals are solids at room temperature (25oC). Mercury is the only exception. It melts at (-39oC)
-metals react strongly with other substances

-only 17 elements are non-metals
-are grouped together mainly because of their lack of resemblance to metals, rather than their similarities to each other
-contains liquids, solids or gasses

-have properties that are intermediate between metals and non-metals
-some metalloids conduct electricity, but not very well
-Silicon, a metalloid, is used in the manufacture of computer chips

-means that the material can be beaten or rolled into sheets without crumbling

-means that the material can be stretched into long wires

Periodic Table

-each horizontal line or row in the periodic table
-the periods are numbered from 1 to 7

Group/ Family
-each vertical line or column in the periodic table
-the groups/ families are numbered from 1 to 18
-elements of the same group have similar chemical and physical properties

Alkali metals
-group 1, includes lithium, sodium, potassium, rubidium, cesium, francium
-all are soft, shiny, and silver in colour, and very reactive with water
-their compounds tend to be white solids that are soluble in water
-a compound is a chemical combination of two or more elements in a specific ratio

Alkaline-earth metals
-group 2, includes beryllium, magnesium, calcium, strontium, barium, radium
-all are shiny and silver but are not as soft as the alkali metals
-their compounds tend to be white, but they are less soluble than compounds formed by the alkali metals

Noble gases
-group 18, includes helium, neon, argon, krypton, xenon, radon
-are the most unreactive elements

-group 17, includes fluorine, chlorine, bromine, iodine
-these elements are poisonous and react readily with the alkali metals to form salts, such as sodium chloride (table salt)

-compounds produced in neutralization reactions between acid and bases

-the smallest part of an element that still has the properties of the element
-a typical atom is very small—only about 10-10m in diameter

Energy level
-can be thought of as a region of space near a nucleus that may be empty or may contain electrons
-electrons in energy levels nearest the nucleus have the lowest energy
-electrons in energy levels farther away from the nucleus have more energy
-electron in the lowest energy levels are the most tightly held in the atom because they are closest to the positively charged nucleus
-the number of electrons that can exist in the different energy levels varies. The lowest energy level is the one closest to the nucleus. It can hold only 2 electrons. The next energy level is larger and farther from the nucleus. It can hold up to 8 electrons. The third energy level also has room for up to 8 electrons. It is common to discuss the electron arrangement in atoms up to 20 electrons. Beyond that, the pattern becomes more complicated
-an energy level can be empty, partly filled, or completely filled
-partly filled energy levels from two different atoms can overlap, and a pair of electrons can exist in both of them at once. This is the basis for chemical bonding, which you will study later in this section
-electrons and protons are attracted to each other because they have opposite charges.