Elements & Atoms

Elements & Atoms

What is chemistry?

• Chemistry is the study of matter and its interactions.

• Matter is anything that has volume (takes up space) and has mass (weight).

• All matter is made of atoms

• Atoms are the building blocks of matter, sort of how bricks are the building blocks of houses

• Atoms are the smallest individual particles of matter which still have their own properties.

An atom refresher

• An atom has three fundamental particles:

• Proton = positive charge

• Neutron = no charge

• Electron = negative charge

• The proton & neutron are found in the center of the atom, a place called the nucleus.

• The electrons orbit the nucleus in the electron cloud.

Mass of the Subatomic Particles

• Protons = 1 AMU

• Neutrons = 1 AMU

• Electrons = 0 mass

AMU = Atomic Mass Unit; an incredibly small unit of mass created only to describe the mass of atoms and their particles

Structure of the atom

• Structure of the atom

• Nucleus: positively-charged center core which contains the protons (+) and the neutrons (no charge). The nucleus contains all the mass of the atom.

• Protons (+) and electrons (-) have equal but opposite charges

• Electrons move in the space outside the nucleus called the electron cloud

• The nucleus is incredibly small and contains all the mass of the atom. Atoms are mostly empty space.

Diagram of an Atom

What are elements?

• The elements are different types of atoms.

• The periodic table lists all the elements that are known to man.

• The periodic table organizes all the elements in rows and columns based on their properties.

Information & picture from Chem4kids at http://www.chem4kids.com/files/atom_structure.html

Atoms of Elements

• Atoms are arranged on the periodic table by increasing atomic number. (1 to 117)

• Atomic Number- the number of protons an atom has in its nucleus

• Mass number: the total number of protons and nuetrons an atom has in its nucleus

• Mass number – atomic number = # of neutrons

• *The number of electrons equals the number of protons in a neutral atom.

Example

• Chlorine

Atomic Number 17

Mass Number 35

Number of protons:

Number of nuetrons:

Number of electrons

Flip Card – 2 cuts, 3 flaps

Outside

Number of Protons

Number of Neutrons

Number of Electrons

Inside

Atomic Number

Mass Number – Atomic Number

Same as the number of protons

Element

Atomic #

Mass # # of + # of - # of neutrons

Iron- Fe 56Silicon- Si

28

Calcium- Ca

41

Bromine- Br

80

Sulfur- S 33

Periodic Table

8

OOxygen

16

Atomic Number:

The symbol of the element

Elements Name

Atomic Mass/Weight:

Number of protons + neutrons.

Graphic from http://education.jlab.org/atomtour/fact2.html

More about Elements..

• Elements are the building blocks of all matter.

• The periodic table is a list of all of the elements that can build matter. It’s a little like the alphabet of chemistry.

• The periodic table tells us several things…

Properties of Metals

Metals Located in the

center and the left side of the periodic table.

Properties of Metals

• Metals are usually solids at room temperature

• Good conductors of heat and electricity

• High melting points and boiling points (except Hg)

• High densities (except some group 1 metals)

• Metals are shiny and reflect light

Properties of Nonmetals

• Nonmetals are located to the right of the stair step line.

Properties of Nonmetals• The nonmetals exist in all three of the states of matter at

room temperature: gases (such as oxygen), liquids (such as bromine), and solids (such as carbon).

• Nonmetals do not conduct electricity or heat very well.

• Nonmetals have much lower melting and boiling points than metals. (Consider all of the ‘red’ ones.)

• Nonmetals are not nearly as dense as metals.

• The nonmetals have no metallic luster, and do not reflect light.

• Nonmetallic elements are very brittle, and cannot be rolled into wires or pounded into sheets.

Properties of Semimetals (Metalloids)

Metalloids are located on the staircase line, dividing metals from non metals.Have a mixture of metallic and nonmetallic properties.Semi-conductors: Can conduct heat and electricity to some extent.

..

Groups Valence Electrons

• Columns of elements are called groups or families.

• Elements in each group have similar but not identical properties.

• For example, lithium (Li), sodium (Na), potassium (K), and other members of group 1 are all soft, white, shiny metals. (not H)

• All elements in a group have the same number of outermost electrons.

• Valence electrons are critically important since they determine the reactivity of an atom.

• How can you determine how many valence electrons an atom has?

• For groups 1,2, and 13-18- the group number is the number of valence electrons (drop the 1 in 13-18)

How many valence electrons do the following atoms have?* Ca

• Cl

• P

• Li

• C

• Ar

• N

• H

• He

• Se

• Ba

• I

• Al

Another Way to Represent Atoms- Lewis Diagrams

• Lewis Diagrams- a model of the atom in which the element’s symbol represents the nucleus and the inner electrons and dots around the symbol represent valance electrons.

• Example:

• What are INCORRECT ways that dots are placed around the symbol?

Write out the Lewis Dot Diagrams for the following elements.

Ca

Cl

P

Li

C

Ar

N

H

He

Se

Ba

I

Al

Period- Rows of Elements

• Each horizontal row of elements is called a period.

• The elements in a period are not alike in properties.

• In fact, the properties change greatly across every given row.

• The first element in a period is always an extremely reactive solid. The last element in a period, is always an inactive gas.

• Elements in a period have the same number of energy levels.

Identify the following elements:

• Group 2, period 4

• Ca

• Group 16 , period 3

• S

• Group 8, period 4

• Fe

• Period 3, noble gas

• Ar

Atom Models

• There are two models of the atoms we will be using in class.

• Bohr Model

• Lewis Dot Structure

Bohr Model

• The Bohr Model shows all of the particles in the atom.

• In the center is circles representing the nucleus. Each circle represents a single neutron or proton. Protons should have a plus (+). Neutrons should have an N.

• In a circle around the nucleus are the electrons. Electrons should have a minus sign.

++++

N

N

--

--

NucleusNucleus

11stst Energy Level Energy Level

22ndnd Energy Level Energy Level

33rdrd Energy Level Energy Level

So let’s try it….• How to draw a Lithium atom

• First, look at the Periodic Table

• Second, determine the number of protons (Look @ the atomic number)

• Then determine the number of neutrons (mass # – atomic #)

• Then determine the number of electrons (Look @ the # of protons)

3

LiLithiumMass # = 7

Electrons have special rules….

• Electrons live in something called shells or energy levels.

• You can’t just shove all of the electrons into the first energy level of an electron.

• Only so many electrons can be in any certain energy level.

• 1st energy level = 2 electrons

• 2nd energy level = 8 electrons

• 3rd energy level = 18 electrons

• The electrons in the outer most energy of any element are called valance electrons.

Lewis Dot Structure

• The Lewis Dot Structure is a bit different from the Bohr model.

• It only shows the element symbol and it’s outer most electron shell.

• The symbol represents the nucleus and the inner electrons.

• Dots around the nucleus represent the valence electrons.

O• ••• •

•

++ ++

++ ++

++

++++

++

--

--

-- --

----

-- --

Determining the number of Valence Electrons

• It is very easy to determine the number of valance electrons (outermost electrons) for many element from the periodic table.

• For groups 1 and 2 and 13-18, the number of valence electrons is equal to the element’s group number.

• Group 1 and 2 elements have 1 or 2 valence electrons.

• Groups 13-18 have 3-8 valance electrons. (Simply drop the 1 from the group number.)

Which group and how many valence electrons (outermost electrons) do the following elements have? Draw the Lewis Structure.

• Ca

• Cl

• P

• Li

• C

• Ar

• N

• H

• He

• Se

• Ba

• I

• Al

Importance of Valence Electrons

• Valence electrons are critical to an element’s behavior.

• Valence electrons are the only particles that interact when atoms come into contact with one another.

• Elements in a group have the same number of valence electron causing them to have similar properties. In other words, elements in a group behave generally the same since they have the same number of valence electrons.

Elements in Periods

• Elements in a period have the same number of energy levels but they have differing number of valence electrons.

• Because elements in a period have differing numbers of valence electrons, they have very different properties. For example:

• Sodium is in period 3 and is a very reactive, solid metal.

• Argon is in period 3 and it is a very stable, nonreactive gas which is a nonmetal.

Isotopes

• All atoms of a given element have the same number of protons but could have different numbers of neutrons.

• Atoms with the same number of protons but different numbers of neutrons are called isotopes.

• The major difference between isotopes of an atom is the amount of neutrons not its properties.

• The mass number states the number of protons and neutrons within the nucleus of an atom

• Isotopes have different mass numbers. The atomic number for the element is the same.

• For example: (note the different way they are represented)

How isotopes are written:

• C12 and C13 or Chlorine-37 or Chlorine-35

• Only the mass number is given. The atomic number is not given since it never changes for an element.

Another way to represent the isotope is by

•zX : the X = symbol

• A= mass number

• Z = atomic number

• For example

Identify the following

1)

Atomic Number Mass Number Number of Protons

Number of Neutrons Number of Electrons

2)

Atomic Number Mass Number Number of Protons Number of Neutrons Number of Electrons