Chapter 17

Properties of Atoms & the Periodic Table

http://www.youtube.com/watch?v=d0zION8xjbM&feature=fvw

Sec. 1: Structure of the Atom

Each element has a chemical symbol.Chemical Symbol—consists of 1 capital

letter or 1 capital letter plus 1 or 2 lower case letters.

This system is used world-wide, so everyone knows what the symbols mean.

Atomic ComponentsAtom—the smallest piece of matter that

retains the properties of the element.Atoms are composed of protons, neutrons,

and electrons.Protons & neutrons are found in the

nucleus.Protons—particles with a charge of +1Neutrons—neutral particles (no charge)Electrons—particles with a charge of -1

Quarks—Even Smaller Particles

Electrons are not made of smaller particles, but protons & neutrons are.

Protons & neutrons are made of smaller particles called quarks.

Scientists have found 6 different quarks.Protons are made of 3 quarks tightly held

together—neutrons are made of a different arrangement of quarks.

Models of the AtomAtoms are difficult to visualize, so we use

models to represent them.Democritus proposed the idea of atoms.In the 1800s, John Dalton proved atoms

existed. He believed atoms were solid spheres.

The model of the atom has been changing ever since.

The Electron CloudWe now believe that electrons are found in

an electron cloud.Electron Cloud—the area around the

nucleus where electrons are likely to be found.

The electron cloud is much larger than the nucleus.

It is impossible to tell exactly where an electron will be inside the cloud.

Sec. 2: Masses of Atoms

Most of the mass of an atom is in the nucleus.

Protons & neutrons are about the same size.Electrons are much smaller.Atomic number—the number of protons in

an atom. (always a whole number) Atoms of different elements have different #’s of

protons & different atomic #’s. The atomic # of an element is on the periodic table

Mass Number & IsotopesMass Number—the sum of the # of protons

& the # of neutrons.Mass # = # of neutrons + atomic #

If you know mass # & atomic # you can find the # of neutrons.# of neutrons = Mass # - Atomic #

Not all atoms of the same element have the same # of neutrons.

Isotopes—atoms of the same element with different numbers of neutrons

Isotopes & Atomic MassDifferent isotopes of elements have different

properties.You can distinguish isotopes by writing the

name of the element followed by the mass #.Ex. Carbon-12 and Carbon-14

Each element has an average atomic mass.Average atomic mass (or atomic mass)—

the weighted average mass of the mixture of isotopes. (found on the periodic table)

Atomic mass is always closest to the most abundant isotope.

Sec. 3: The Periodic TablePeriodic Table—an organized

list of all known elements that are arranged according to their properties.

The 1st person to organize elements was Dmitri Mendeleev in the 1800s.

Mendeleev organized the elements by their atomic mass and left spaces for unknown elements.

The Modern Periodic TableMendeleev’s table was mostly correct, but it is

more accurate to organize elements in order of increasing atomic number.

GroupsGroups (or families)—vertical

columns on the periodic table.They are numbered 1-18.Elements in each group have similar

properties.

PeriodsPeriods—horizontal

rows of elements on the periodic table. Periods are

numbered 1-7Elements increase

by one proton as you move from left to right across a period.

Electrons & The Periodic Table

The periodic table organizes elements based on where their electrons are located.

Electrons (e-)—are located in different energy levels around the nucleus.

Elements in the same group have electrons arranged similarly which gives them similar properties.

Electron ShellsThe number of energy levels or electron

shells is determined by the period number.Period 1: 1 electron shell and can hold 2 e-

Period 2: 2 electron shells & can hold 8 e-

Period 3: 3 electron shells & can hold 8 e-

As you move down the periodic table, 1 electron shell is added each time.

The outer electron shell must be full (usually with 8 e-) to be stable.

Electron Dot Diagrams

You can show the number of electrons in the outermost electron shell by using an electron dot diagram.

Dot diagrams use valence electrons which are the electrons in the outer electron shell.

Electron Dot Diagram—uses the symbol of the element and dots to represent the valence electrons.