The Atom

The Atom

Matter

– Anything that takes up space and has weight

Physical Forms of Matter

– Solids

– Liquids

– Gases

Chemical Forms of Matter

– Elements

– Compounds

– Mixtures

The Atom

Elements

– 63 elements arranged in Periodic Table by

Mendeleev in 1869

– 92 elements now recognized as natural

– 117 total elements recognized if we include

man-made

The Atom

Compounds

– Substances made from two or more elements

combined together

– The smallest particle of a compound, which retains the

properties of the compound, is called a “molecule”

Mixtures

– Substances which contain a mixture of different

elements and compounds

The Atom

Atom

– Democritus (530 BC) “All matter is formed from

atoms.”

– John Dalton (1808) “Some substances cannot be

broken down into other substances. These pure

substances are elements. The atom is the smallest

particle of an element.”

The Atom

Are atoms real?

– Indirect evidence

• Behavior of gases

• Chemical combinations

• Brownian motion

– Direct evidence

• X-ray crystallography (1912)

• Atomic scale microscopy (1980)

The Atom

Periodic Table

– Arranged in columns and rows which show the

relationship between the different elements

The Atom

Elements on left side of periodic table tend to be solids

– Sodium, potassium, magnesium, calcium

Elements on right side of periodic table tend to be gases

– Oxygen, helium, neon, chlorine

Only one element is a liquid at normal atmospheric

temperatures and pressures

– mercury

The Atom

Subatomic Particles

Size Mass Charge

Protons Same as neutron 1 +

Electrons1/1000 of the proton or neutron 0 -

Neutrons Same as proton 1 no charge

The Atom

Atomic Structure

– Nucleus

• Dense central region

• Contains protons and neutrons

– Shells

• Energy levels

• Contain electrons

Rutherford model vs. Bohr model

The Atom

Rutherford Model

– Similar to planets in our solar system, fixed positions

– Not mathematically possible

Bohr Model

– Electrons can move between higher and lower energy levels

– Drop in level releases photon of energy in form of light

– Light emitted or absorbed determines spectrum (an atom’s

“fingerprint”)

– Mathematically possible

The Atom

Construction of atoms

– Atomic number

• Number of protons in atomic nucleus

• Determines which element is present

• Equals the number of electrons in a neutral atom

– Atomic mass

• Reflects the mass of protons and neutrons in the atomic

nucleus

• Is not influenced by the number of electrons

He4

2symbol

atomic number

atomic mass

The Atom

Energy shells

– Number of electrons equals the number

of protons

– Innermost (1st) can hold 2 electrons

– 2nd shell can hold 8 electrons

– 3rd shell can hold 18 electrons

– Capacity increases in additional shells

– Outer shell can only hold 8 no matter

what capacity

The Atom

helium carbon sodium

He2

4C

6

12Na

11

23

The Atom

Atoms in combination

– Covalent compounds

• Composed of 2 or more elements joined together by

covalent bonds

• Sharing of electrons

– Ionic compounds

• Composed of 2 or more elements joined together by ionic

bonds

• Transfer of electrons from one atom to another

The Atom

Elements in vertical columns have same number of electrons in their outer shells

Outer shell electrons called “valence” electrons

Valence electrons determine which elements react with each other

Chemical bonds link atoms together

– Covalent

– Ionic

The Atom

Columns IA and VIIA

– Gives up and takes on one electron (ionic bonds)

Columns IIA and VIA

– Gives up and takes on two electrons (ionic bonds)

Column IVA

– Shares four electrons (covalent bonds)

Columns VIIIA (0)

– Does not react because outer shell is full (no bonding)

The Atom

Ions

– Charged derivative of an atom

– Positive if the atom loses electrons

– Negative if the atom gains electrons

Fourth state of matter = Plasma

– Ionized gases in which electrons have been stripped

from atoms

– Formed at high temperatures

The Atom

Isotopes

– Multiple forms of the same atom

– Same atomic number

– Different mass number

• Differs in neutron content

• Many are radioactive (e.g.. Carbon-14)

The Atom

Radioactivity

– Spontaneous release of energetic particles

• Becquerel (late 1800’s)

• Marie Curie (early 1900’s)

– Types of radioactive decay

• Alpha

• Beta

• Gamma

– Half-life

• Time it takes for ½ of radioactive isotopes to decay

The Atom

Atomic energy

– Nuclear fission

• Splitting of a large atom’s nucleus

– Atomic bomb

– Nuclear reactors

– Nuclear fusion

• Joining two small atomic nuclei together to form a larger

atom

– Hydrogen bomb

– Energy for the future?