Presentation on theme: "Looking for Patterns in Chemical Reactivity. Elements and Compounds An element is a pure substance that cannon be broken down into simpler substances."— Presentation transcript:
Elements and Compounds An element is a pure substance that cannon be broken down into simpler substances by ordinary means such as a chemical reaction. (Iron, Oxygen, and Carbon are all elements.) A compound is a pure substance made up of two or more elements that are chemically combined (Water, or H 2 O, is a compound.)
Atoms and Subatomic Particles All atoms are composed of protons, neutrons, and electrons. Protons are positively charged particles found in the nucleus of an atom. Neutrons are uncharged particles that are also found in the nucleus. Electrons are negatively charged particles that orbit the nucleus of an atom.
Atoms and Subatomic Particles Protons and Neutrons are about the same size and mass. Electrons are much smaller than Protons and Neutrons. Protons and Electrons have charges that are equal in size but opposite in sign.
Atomic Number and Atomic Mass Atomic number is the number of protons and electrons in each atom of that element The atomic mass is the number of protons and neutrons in an atom (these make up the nucleus of an atom) By subtracting the atomic number from the atomic mass you can find the number of neutrons
Isotopes An isotope of an element has the same atomic number but different mass numbers because the number of neutrons has changed. For example: we can have Carbon-12 with 6 protons and 6 neutrons but we can also have Carbon-14 with 6 protons and 8 neutrons
Properties of Metals, Non-metals, and Metalloids Malleable – able to be pounded or formed into a shape Ductile – able to be drawn into a wire
Organization of the Periodic Table The periodic table arranged elements into periods (horizontal rows) and groups (vertical columns) according to their atomic numbers. Elements in the same group have similar chemical properties.
Electron Shell Patterns Period 1 elements have a single electron shell, while Period 2 elements have two electron shells and Period 3 elements have 3 electron shells. The first electron shell can hold two electrons while the second and third shells can each hold eight electrons. Elements in the same group have the same number of electrons in their outermost shell.
Predicting Chemical Reactivity Electron shell diagrams are useful because they show the number of electrons in the outer shells. Knowing this is the key to understanding the organization of the periodic table and the chemical properties of elements. Elements in the same group have similar chemical properties because they have the same number of electrons in their outer shell.
Valence Electrons Knowing the number of outer shell electrons will help us predict the formation of compounds, name the compounds, and write their chemical formulas. The outer shell is called the valence shell and the electrons that occupy it are called valence electrons. A chemical bond forms between two atoms when their valence electrons form a stable arrangement together.
Valence Electrons The chemical properties of elements are related to the energy changes that take place when their atoms lose, gain, or share electrons to obtain a filled valence shell. Metals are elements that tend to lose their valence electrons relatively easily and this accounts for many of their physical and chemical properties. One important property of valence shells is that they generally like to be full.
Ions Any atom or group of atoms that carries an electrical charge is called an ion. When a neutral atom gives up one or more electrons, the positively charged ion that results is called a cation. HINT: cation – think t or + for positive When a neutral atom gains one or more electrons, the negatively charged ion that results is called an anion. HINT: anion – think n for negative
Cations Adding energy to a neutral sodium atom removes its one valence electron and leaves a positively charged sodium ion (cation) and a free electron.
Anions Adding an electron to a neutral chlorine atom results in a negatively charged chlorine ion (anion) and the release of energy.
Electron Dot Diagrams Electron dot diagrams depict valence electrons as dots. The inner electrons and the atomic nuclei are represented by the symbol for the element and dots are placed one at a time moving around the symbol.