Atoms

Elements and Atoms Terms  Matter is anything that has mass and takes up space.  Elements are the simplest pure substance and they can not be broken into any other substance.  Elements are often called the building blocks of matter because all matter is composed of one element or a combination of two or more elements.  Compound is a pure substance made of two or more elements that are combined chemically in a specific ratio.  Mixture is two or more substances that are in the same space but that are not chemically combined.  Atom is the smallest particle of an element.

Elements and Atoms Examples Discovery activity p6 Phschool.com cgd-2011 Figure 1 - 7

Atomic theory and models  Scientific theory is a well-tested idea that explains a wide range of observations.  Model is a physical, mental visual or other representation of an idea to help people understand what they can not observe directly.  Atomic theory grew as a series of models that developed from experimental evidence. As more evidence was collected, the theory and models were revised.  The models are  Dalton  Thomson  Rutherford  Bohr  Cloud of Electrons  Modern atomic model

Dalton’s Atomic Model

Figure 2 – p8 Elements composed of atoms that can not be divided. All elements of the same atoms are exactly alike and have the same mass. Atoms can not be created nor destroyed. They can not be converted into another atom. Every compound is composed of atoms of different elements combined in a specific ratio.

Thomson’s model He suggested that atoms have negatively charged electrons embedded in a positive space.

Rutherford’s Model An atom is mostly empty space. There are electrons that move around a small positively charged nucleus in the center of the atom.

Bohr’s model The electrons move in a specific orbit around the nucleus.

Cloud Model Electrons move rapidly in every direction around the nucleus. The electrons movement is related to its energy level or the specific amount of energy it has.

Modern Atomic Model The nucleus which contains both protons and neutrons is surrounded by a cloudlike region of electrons.

Valence Electrons Valence electrons are those electrons that have the highest energy level and are held most loosely. The number of valence electrons in a atom of an element determines many properties of that element, including the ways in which the atom can bond with other atoms. Each element has a specific number of electrons ranging from 1-8.

Valence Electrons Electron dot diagrams include the symbol for the element surrounded by dots. Most atoms are more stable when they have 8 valence electrons. Atoms will either give up loosely held electrons or gain electrons in order to become more stable through a chemical bond. A chemical bond is the force of attraction that holds two atoms together as a result of the rearrangement of electrons between them.

Read pages 16+ and consider table

Terms to know Symbol Atomic number Period Group or family Noble gases Halogens Alkali metals

Reactions of the elements Noble gases – these elements have a maximum number of valence electrons so they are stable an nonreactive. (group18) Halogens – these elements have 7 valence electrons and thus readily react with elements that give up or share electrons. (group17) Alkali metals – these atoms have only one valence electron, so they are very reactive. (group 1) Read Review page