ELEMENTS

ELEMENTS

Remember:• Atoms of one type form an element

– A bunch of gold atoms form gold…etc.• Elements are the simplest form of a

substance • Every element is unique and has its own

chemical and physical properties

Compounds and Molecules

• A molecule is formed when two or more atoms join together chemically.

• A compound is a molecule that contains at least two different elements.

• All compounds are molecules but not all molecules are compounds.

Atoms, Elements, Compounds, Molecules

• O S CH4 FeO2

N2 H H2O H2SO4

Elements are arranged:

Vertically into Groups

Horizontally Into Periods

Each atom has the same number of electrons in it’s outermost shell.

Valence Electrons: Electrons in the outer shell

• The number of outer or “valence” electrons in an atom effects the way an atom joins with other atoms.

• The way atoms join determine many properties of the element.

• This is why elements within a group usually have similar properties.

Each atom has the same number of electron holding shells.

The numbers represent the period as well as the amount of electron holding shells.

Each group has distinct properties

• The periodic Table is divided into several groups based on the properties of different atoms.

Non-reactive

Gases at room temperature

Very Stable

Noble Gases

WHY STABLE?

Noble gases have full valence levels!

• Atoms whose shells are not full tend to interact with other atoms and gain, lose, or share electrons

HYDROGEN (H)Atomic number = 1

CARBON (C)Atomic number = 6

NITROGEN (N)Atomic number = 7

OXYGEN (O)Atomic number = 8

Electron

Outermost electron shell (can hold 8 electrons)

First electron shell (can hold 2 electrons)

• When atoms gain or lose electrons• Charged atoms called ions are created

– Lose electrons---positive charge– Gain electrons--- negative charge– Like charges repel and opposite charges attract

Ionic bonds

NaSodium atom

ClChlorine atom

Na+

Sodium ionCl–

Chloride ion

Sodium chloride (NaCl)

Na Cl Na Cl

+ –

Ionic Bonds• Electrons are transferred• Forms between metals and

nonmetals• Dissolve easily in water, usually

Covalent Bonds

• Sharing of outer electrons• Very strong bond• If “pull” is unequal creates a polar

covalent bond = a polar molecule (ex: water)