Atoms, Elements, and Compounds Physical Science, Chapter 14, 15, and 16

All About Atoms

What is an atom? Basic unit of matter Definition comes from ancient Greek: means indivisible or uncuttable Can’t be broken down into smaller pieces

What are atoms made of? Protons – positive charge Neutrons – neutral, no charge Electrons – negative charge

Structure of the Atom In order to understand atoms, we need to understand the idea of electric charge. We know of two different kinds of electric charge and we call them positive and negative. Positive and negative: Attract Positive and positive: Repel Negative and negative: Repel

Electric charge in matter We say an object is electrically neutral when its total electric charge is zero. The charge on a complete atom is ZERO Atoms are neutral

Early model of the atom In 1897 English physicist J. J. Thomson discovered particles that were too small to be atoms. These negative particles were eventually called “electrons.” Thomas thought the electrons were evenly distributed inside the atom, like raisins in bread. (Bread +, raisins – )

Testing the model In 1911, Ernest Rutherford, Hans Geiger, and Ernest Marsden did an experiment to test Thomson’s model. They discovered that atoms are mostly empty space!

Atomic structure video http://www.teachertube.com/viewVideo.php?video_id=247101

Inside an atom Protons and neutrons are much larger and more massive than electrons. The mass of the nucleus determines the mass of an atom. The electrons surround the nucleus and form the electron cloud.

Electron Cloud

Forces – Electromagnetic Force Electrons (-) are attracted to the protons (+) in the nucleus because of the difference in electric charge. Momentum (kinetic energy!) keeps electrons from falling into the nucleus Electrons “orbit” the nucleus just like Earth orbits the Sun

Forces – Strong Nuclear Force Normally, positively charged protons repel each other Strong nuclear force holds the protons and neutrons together in the nucleus Stronger than electromagnetic force.

How are elements different? Atoms of different elements contain different numbers of protons in the nucleus. Because the number of protons is so important, it is called the atomic number. Each element has a unique atomic number

How to “read” an element Atomic Number Element Symbol Element Name Atomic Mass

How to build an element Atomic number = number of protons Number of protons = number of electrons Atomic mass = number of protons + neutrons

Let’s practice! Number of Protons? Number of Electrons? Number of Neutrons? 6 (Same as atomic number) 6 (same as protons) 6 (Atomic mass – atomic number: 12 – 6 =6)

An exception: Isotopes Isotopes are atoms of the same element that have different numbers of neutrons. Remember: the mass number tells you the number of protons plus the number of neutrons. How are these carbon isotopes different?

X C Isotopic Symbol A typical isotopic symbol takes this form: X = element symbol A = mass number (# protons + # neutrons) Z = atomic number (# proteins) N = # neutrons A – Z = N A typical isotopic symbol takes this form: Ex: The isotopic symbol for carbon would be: A Z 12 6 X C

Radioactivity Almost all elements have one or more isotopes that are stable. “Stable” means the nucleus stays together. Carbon-14 is radioactive because it has an unstable nucleus.

Carbon dating Unstable Carbon-14 is used in carbon dating Carbon-14 breaks down at a predictable rate Scientists can use this to estimate the age of organic matter (matter which was once alive)

More about electrons The colors of clothes, paint, and everything else around you comes from electrons! Each element has its own characteristic pattern of colors called a spectrum.

Electrons in atoms Each individual color in a spectrum is called a spectral line because each color appears as a line in a spectroscope. A spectroscope is a device that spreads light into its different colors.

Bohr model of the atom Danish physicist Neils Bohr proposed the concept of energy levels to explain the spectrum of hydrogen. When an electron moves from a higher energy level to a lower one, the atom releases the energy difference between the two levels. The energy comes out as different colors of light.

Energy levels Electrons can absorb or emit energy only at specific wavelengths Like going up or down steps. http://www.youtube.com/watch ?v=kJBcXFsFa7Y (to about 5 min) Fireworks are different colors depending on what element they are made with 14.2 Clicker

Rules for energy levels Inside an atom, electrons always obey these rules: The energy of an electron must match one of the energy levels in the atom. Each energy level can hold only a certain number of electrons, and no more. As electrons are added to an atom, they settle into the lowest unfilled energy level.

Bioluminescence

The Periodic Table

Remember Physical Properties? Physical properties are seen through direct observation Physical properties include color, texture, density, and state (solid, liquid, or gas). Melting point and boiling point are also physical properties. A physical change does not result in a new substance being formed.

Chemical Properties Properties that can only be observed when one substance changes into a different substance are called chemical properties. Any change that transforms one substance into a different substance is called a chemical change.

Periodic Table The periodic table organizes the elements according to how they combine with other elements These are chemical properties. The periodic table is organized in order of increasing atomic number.

Atomic Number Remember, the atomic number is the number of protons in the nucleus of that element. If the atom is neutral, it will have the same number of electrons as protons.

Periodic Table The periodic table is divided into periods and groups. Each horizontal row is called a period. Each vertical column is called a group.

Groups of the Periodic Table The first group is known as the alkali metals. The alkali metals are highly reactive. This group includes the elements lithium (Li), sodium (Na), and potassium (K). The group two metals include beryllium (Be), magnesium (Mg), and calcium (Ca). They also bond easily with oxygen.

Groups of the Periodic Table – Halogens The halogens tend to be toxic gases or liquids in their pure form. Fluorine (F), chlorine (Cl), and bromine (Br) form salts when they bond with alkali metals.

Groups of the Periodic Table Noble Gases The noble gases group includes the elements helium (He), neon (Ne), and argon (Ar). These elements naturally do not form chemical bonds with other atoms and are almost always found in their pure state.

Phases of Elements Most of the pure elements are solid at room temperature. Only 11 naturally occurring elements are a gas. Only 2 elements (Br and Hg) are liquid at room temperature.

Electrical conductivity Electricity is the movement of electric charge, usually electrons. Some materials allow electrons to flow easily through them. We call these materials electrical conductors.

Thermal conductivity Like copper, most metals are also good thermal conductors. That is one reason pots and pans are made of metal.

Insulators Elements on the far right of the table are called non-metals. Nonmetals make good insulators. An insulator is a material which slows down or stops the flow of either heat or electricity.

Metals and metal alloys An alloy is a solid mixture of one or more elements. Most metals are used as alloys and not in their pure elemental form. Example: Titanium combines the strength of steel with the light weight of aluminum. It is used in racing bicycles and airplanes.

Nitrogen, oxygen and phosphorus Nitrogen is the most abundant element in the atmosphere! Oxygen and nitrogen are crucial to living animals and plants. Phosphorus is a key part of DNA, the molecule responsible for carrying the genetic code.

Carbon and carbon-like elements Almost all the molecules that make up plants and animals are constructed around carbon. Carbon is essential to life! The chemistry of carbon is so important it has its own name, organic chemistry.

Carbon and carbon-like elements Pure carbon is found in nature as either graphite or diamond. Silicon is the second most abundant element in the Earth’s crust, second only to oxygen. Why are carbon and silicon important?

What’s so great about silicon? It makes up sand, glass, gemstones like amethyst and opal, and most of the rocks on the surface of the Earth. Pure silicon is used to make microchips, also known as computer chips. These microchips can be found in many devices, including computers, cell phones, and microwaves. Ever heard of Silicon Valley?

Creating Compounds

What is a compound? Two or more elements combined to make something new For example, when hydrogen and oxygen combine to make water This is a chemical change A chemical bond forms when atoms transfer or share electrons.

Chemical Formulas A molecule’s chemical formula tells you the ratio of atoms of each element in the compound.

Types of Bonds A covalent bond is formed when atoms share electrons. In water, the atoms share their electrons Ionic bonds are bonds in which electrons are transferred from one atom to another. In salt (NaCl) the positively charged sodium is attracted to the negatively charged chlorine

Reactivity In chemistry, reactive means an element easily forms chemical bonds, often releasing energy. Some elements are more reactive than others. The closer an element is to having the same number of electrons as a noble gas, the more reactive the element is.

Valence electrons Each energy level can only hold a certain number of electrons. The electrons in the highest energy level are called valence electrons Valence electrons determine how elements bond

Lewis Dot Diagrams A clever way to keep track of valence electrons is to draw Lewis dot diagrams. A dot diagram shows the element symbol surrounded by one to eight dots representing the valence electrons. What is the dot structure for nitrogen?

Chemical Formulas and Oxidation Numbers All compounds have an electrical charge of zero This means they are neutral. An oxidation number indicates the charge on the atom (or ion) when electrons are lost, gained, or shared in chemical bonds.

Ionic Bonds On the periodic table, strong electron donors are the left side (alkali metals). Strong electron acceptors are on the right side (halogens). The further apart two elements are on the periodic table, the more likely they are to form an ionic compound.

Covalent Bonds Covalent compounds form when elements have roughly equal tendency to accept electrons. Elements that are nonmetals and close together on the periodic table tend to form covalent compounds.

Oxidation and Chemical Formulas Remember, the oxidation numbers for all the atoms in a compound must add up to zero.

Solving Problems Iron and oxygen combine to form a compound. Iron (Fe) has an oxidation number of 3+. Oxygen (O) has an oxidation number of 2–. Predict the chemical formula of this compound.

Chemical Formula is Fe2O3 Solving Problems Looking for: …formula for a compound Given … Fe3+ and O2– Relationships: Write the subscripts so that the sum of the oxidation numbers equals zero. Solution Two iron atoms = 2 × (3+) = 6+ Three oxygen atoms = 3 × (2–) = 6– Chemical Formula is Fe2O3

Chemical Formula Practice Compounds can contain more than 2 elements How many atoms of each element are in Al2(SO4)3? Aluminum (Al): Sulfur (S): Oxygen (O):

Carbon again! Carbon has four valence electrons. Two oxygen atoms can bond with a single carbon atom, each oxygen sharing two of carbon’s four valence electrons. The bonds in carbon dioxide (CO2) are double bonds because each bond involves 2 electrons.

Organic Compounds Organic chemistry is the branch of chemistry that specializes in carbon compounds, also known as organic molecules. Plastic, rubber, and gasoline are important carbon compounds. Scientists classify organic molecules into four basic groups: carbohydrates, proteins, fats, and nucleic acids.

Carbon is essential to life! Carbon Chemistry Carbon molecules come in three basic forms: straight chains, branching chains, and rings. All three forms are found in important biological molecules. Carbon is essential to life!