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!

6 (Same as atomic number)

6 (same as protons)

6 (Atomic mass – atomic number: 12 – 6 =6)

• Number of Protons?

• Number of Electrons?

• Number of Neutrons?

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?

Isotopic SymbolX = element symbolA = mass number (# protons + # neutrons)Z = atomic number (# proteins)N = # neutronsA – Z = N

A typical isotopic symbol takes this form:

Ex: The isotopic symbol for carbon would be:

AZ

126

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:1. The energy of an electron must match one of the

energy levels in the atom.2. Each energy level can hold only a certain number of

electrons, and no more.3. 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.

Solving Problems1. Looking for: • …formula for a compound

2. Given• … Fe3+ and O2–

3. Relationships:• Write the subscripts so that the sum of the oxidation numbers equals zero.

4. 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 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!