Chapter 1: Atomic Structure and the

Periodic Table

1.1 Atoms are the smallest form of elements.

• All matter is made of atoms. About 100

different atoms, or elements, make up

everything on Earth.

Hydrogen is the most abundant element

in the universe, it makes up 90% of all

elements in the universe.

Oxygen is the most abundant element in

the human body and in Earth's crust.

• The structure of an atom

• Nucleus: is at the center of the atom, and is a

combination of protons and neutrons.

Protons: are positively charged particles.

Neutrons: are particles that have NO

electrical charge.

Protons and neutrons have approximately

the same mass.

Electrons: are tiny particles with a

negative charge.

o They form a cloud, because at the same time

that they repel each other, they are attracted to

the positively charged protons in the nucleus.

(2000 times smaller than protons and

neutrons)

What part of the atom has a positive

charge, negative charge and no charge?

What is the smallest sub-atomic particle?

• Atomic Number: is the number of protons in

an atom’s nucleus.

• Atomic mass number: is the combined

number of protons and neutrons in a nucleus.

• Isotopes: are atoms of an element that have a

different number of neutrons.

For example, most carbon atoms contain 6

protons and 6 neutrons. Some carbon

atoms, however, have 6 protons and 8

neutrons.

Chemists will often put the atomic mass

number after the element's name, such as

carbon-12 or carbon-14, to signify

different isotopes.

How would we write these isotopes? What is

the difference between each isotope?

Atomic Mass: of an element is the average

mass of all the element’s isotopes.

• Ions are atoms that have lost or gained one or

more electrons.

They no longer contain equal numbers of

protons and electrons, so they have an

electric charge.

Atoms that have lost electrons are

positive ions; atoms that have gained

electrons are negative ions.

1.2 Elements make up the periodic table.

• Mendeleev’s Periodic Table

A Russian chemist, Dmitri Mendeleev,

was the first to produce a periodic table

of the elements.

His periodic table was organized by

increasing atomic mass and similarities in

chemical properties.

The Na atom is now Na+ because it has one more positive proton than negative electrons.

Sodium ion (Na+) 11 protons & 10 electrons

Sodium atom (Na) 11 protons & 11 electrons

An electron is lost

An electron is gained

Chlorine atom (Cl) 17 protons & 17 electrons

Chlorine atom (Cl-) 17 protons & 18 electrons

The Cl atom is now Cl-

because it has one more negative electron than positive proton.

• The periodic table has a system of

organization.

The number at the top is the atomic

number.

The big bold letters are the chemical

symbol

The name of the element is below the

chemical symbol.

Under the name is the atomic mass

number.

• The periodic table is organized into groups

and periods.

In a group (vertical column) of the

periodic table, elements show similarities

in their physical and chemical properties.

In a period (horizontal row) of the

periodic table, the physical and chemical

properties change in a predictable way

from one to the next, and the atomic

number increases by one across a period.

How is the modern periodic table different

then Mendeleev’s?

• An element's position in the periodic table

dictates what type of ion it will form.

Atoms at the left of the table tend to lose

an electron to form positive ions

Atoms at the right of the table tend to

gain electrons to form negative ions.

Atoms in group 18 rarely form ions at all.

Atoms in groups 3-12 all form positive

ions.

• Atomic size and density show patterns in

the periodic table.

Density generally increases from the top

of a group to the bottom of a group.

Elements to the left and right sides of the

periodic table are less dense than the

elements in the middle of the table.

Atomic size increases, or gets larger, from

top to bottom.

Atomic size decreases, or gets smaller,

from left to right.

1.3 The periodic table is a map of elements.

• The periodic table has distinct regions

Metals (Yellow)

Nonmetals (Green)

Metalloids (Purple)

• The element's position on the periodic table

determines how reactive it is

Reactivity is a measure of how likely an

element is to undergo a chemical

change.

• Most elements are metals.

Metals are elements that conduct

electricity and heat well and have a

shiny appearance.

• The most reactive metals in the periodic

table.

Alkali metals in Group 1 (most reactive)

Alkaline earth metals in Group 2

(more reactive than most other metal

elements, but not as reactive as those in

group 1)

• Transition metals in the middle of the

periodic table, like iron and copper, are less

reactive and were among the first known

elements.

• Rare earth elements are the elements in

period 6, the portion taken out of the body of

the periodic table.

• Nonmetals

Most nonmetals are gases at room

temperature.

In the solid state, nonmetals have dull

surfaces and can be hammered into

shape or drawn into a wire.

Halogens in Group 17 are very reactive

nonmetals.

The noble gases in Group 18 almost

never react with other elements.

• Metalloids

Have properties of both metals and

nonmetals.

Metalloids make up the semiconductors

in electronic devices.

The metalloid silicon is the second most

abundant element in the Earth's crust.