Chapter 5 Atomic Structure & the Periodic Table

5.1 Atoms are the smallest form of elements.

• 2400 years ago, Greek philosophers believed everything was made up of air, fire, water, and earth. Everything else was a mixture of these 4 substances.

• Today, we know there are about 100 basic substances, known as the elements.

• Sometimes, elements can be found by themselves. Most often they are found combined with other elements to make new substances.

Atoms in the Earth & Living Things

• Hydrogen atoms make up about 90% of the mass of the universe.

Earth's Crust

47%

5%8%

28%

12%

Oxygen

Iron

Aluminum

Silicon

Other

Humans

61%

3%

3%

10%

23%Oxygen

Nitrogen

Other

Hydrogen

Carbon

Element Names

• Magnesium = region in Greece known as Magnesia

• Lithium = Greek word lithos, meaning stone

• Neptunium = after the planet Neptune• Hydrogen (H), Sulfur (S), Carbon (C)• Gold (Au), Lead (Pb), Iron (Fe), Copper

(Cu) = symbols come from latin names.

Dalton’s Atomic Theory

• British scientist, John Dalton, proposed that each element is made of tiny particles called atoms.

• All of the atoms in a particular element are identical but are different from atoms of all other elements.

• Atoms cannot be divided into anything simpler. (This was found to be untrue when protons, neutrons & electrons were discovered).

Structure of Atoms

• Atoms can either be positively or negatively charged.

• Particles with the same type of charge repel each other.

• Particles with different charges attract each other.

• Atoms are composed of protons, neutrons & electrons.

The Atomic Model

Same Mass

Uncharged; inside nucleus

Positive Charge; inside nucleus

Negative charge; move around outside nucleus (electron cloud)

Contains protons & neutrons; positively charged

.

Millions of atoms could fit in a space the size of this dot.

It would take you 500 years to count all the atoms in a grain of salt!!

Relative Sizes• The electron cloud is about 10,000 times the

size of the nucleus.• Electrons are MUCH smaller than protons or

neutrons• Electrons remain evenly spread out in the

electron cloud because they repel each other. However, they are attracted to the nucleus because it is positively charged.

• Neutral atoms have no overall electrical charge because they have an equal number of protons and electrons.

Atomic Number

• The identity of an atom is determined by the number of PROTONS in its nucleus. This is called the ATOMIC NUMBER!

• Hydrogen is atomic number 1. Therefore, it has one proton in its nucleus.

• How many protons does Lithium have in its nucleus?– Nitrogen?– Gold?

Atomic Mass

• Atomic Mass = the total number of protons and neutrons in an atom’s nucleus.

• Atoms of a certain element always have the same number of protons. However, they don’t always have the same number of neutrons. Therefore, not all atoms of a certain element have the same atomic mass.

Isotopes

• Isotopes are atoms of the same element that have a different number of neutrons.

• Ex: Chlorine– All chlorine atoms have 17 protons. – Some chlorine atoms have 18 neutrons and

some have 20 neutrons. These are chlorine isotopes.

• Isotopes have different numbers of neutrons!

How to find the number of neutrons in a particular isotope.

Atomic Mass – Atomic Number = # of neutrons# of protons &

neutrons# of protons

Ions

• An ion is formed when an atom loses or gains one or more electrons.

• Since the number of electrons in an ion is different from the number of protons, an ion does have an overall electric charge.

• See pgs. 142 & 143