Chapter 14- The AtomThe Big Idea

Atoms are the incredibly small building blocks of matter

14.1 Discovering the Invisible Atom

How small are they?So small that the number of them in a baseball is roughly equal to the number of ping pong balls that would fit in a hollow sphere the size of the earth.We can’t see them with the best light microscopes made because they are smaller than wavelengths of visible light.

Where did they come from?

The origin of atoms goes back to the the birth of the universe. Hydrogen (H) was the first.The lightest atom – hydrogen (H) makes up over 90% of the known universe.Heavier atoms are produced in stars. Extremely high heat and pressure caused hydrogen atoms to fuse together making the heavier atoms

A Breath of Air

1022 atoms of air make up a liter of air

There are about 1022 liters of air in the atmosphere

You have roughly a liter of air in your lungs- every time you exhale, you exhale roughly the same number of atoms as there are liters of air in the universe

14.2 Evidence for Atoms

First direct evidence1827- Robert Brown (Scottish botanist)Brown was examining pollen grains in a drop of water under a microscopeHe noticed that they were moving

Brownian MotionAt first he thought they were a life form but later found that dust and soot moved the same way

Movement was called Brownian motionEinstein was later able to explain that Brownian motion results from collisions between invisible atoms and visible particlesThe grains were moving because they were being jostled by atoms in the water

Can we see atoms?

We still can’t see atoms directly but can generate images of them indirectly using a scanning probe microscope.

Opened up the field of Nanotechnology

Atomic StructureEarly 1900’s scientists cane to realize that atoms are made of smaller particles called subatomic particles

Protons, neutrons and electrons.

Protons and neutrons are bound together at the atom’s center to form the atomic nucleus

The nucleus makes up most of the atom’s mass

Orbiting the nucleus are tiny electrons.

Atoms are mostly empty space

Atoms are mostly empty space- a core nucleus with tiny electrons buzzing around it.

This is like our solar system.

Read page 263 to find out why we don’t just pass through each

other if we are mostly empty

space.

ElementsEverything in the universe is made up of slightly more than 100 types of distinct atoms called elements.

The variety in matter is caused by combinations of these atoms.

Of the 100 or so elements, only 90 are naturally found in nature. The rest are man-made.

The periodic tableElement- def.- a material made up of only one type of atom

All of the elements are listed in a chart called the periodic table.

Each element is designated by a one, two or three letter symbol that comes from letters of the element’s name. Some of the names came from Latin

Ex- gold’s symbol Au comes from its Latin name- aurum; lead’s symbol Pb comes from plumbum

Elements & TableOnly the first letter in an element’s name is capitalized.

Cobalt is Co; CO means a compound made with carbon (C) and oxygen (O).

A vertical column is called a GROUP. Atoms in the same group have similar properties.

A horizontal row is called a PERIOD. As you go across a row, the properties of matter gradually change. This change is called a Periodic Trend.

Each period corresponds to one full cycle of a trend. For example, one trend is that the size of atoms gradually decreases in moving from left to right across any period. Atomic size is a periodic (repeating) property.

14.3 Protons and Neutrons

Proton- a particle in the atomic nucleus that carries a positive electric charge.

Heavy- about 2000 times heavier than an electron.

In a neutral atom, the number of positively charged protons equals the number of negatively charged electrons orbiting around the atom.

The quantities are the same but the charges are opposite

Example- oxygen has 8 protons and 8 electrons

Atomic Number

Each element is identified by its atomic number

This is the number of protons in its nucleus.

The number of protons makes the atom what it is. If somehow the proton number changes, so does the atom ( this is difficult to do- nuclear fission or fusion)

NeutronsNeutrons are also located in the nucleus

Neutrons are particles that have about the same mass as protons but have no electric charge. They are electrically neutral. (that’s how it got its name)

ElectronsExtremely small nuclear particles that have about 1/2000 the mass of a proton or neutron

They have an electrically negative charge (opposite charge of proton)

They are the only particle that participate in chemical reactions

They are arranged in electron shells

NanotechnologyPlease read the top of page 268 about nanotechnology.

There will be worksheet questions and test questions on this material

14.4 Isotopes and Atomic Mass

An atom has a definite number of protons but its number of neutrons can vary.

For example, there are three forms of hydrogen-

One with 1 proton and 0 neutrons

One with 1 proton and 1 neutron

One with 1 proton and 2 neutrons

They all have the properties of hydrogen because they have the same number of protons

These different forms where the neutron number varies are called isotopes

IsotopesIsotopes are identified by their mass number

Mass Number- the total number of protons and neutrons they contain. So mass number is the total number of nucleons (particles in the nucleus)

The average mass number for each atom is listed on the periodic table. It is a fractional number because it is a weighted average of all the isotopes and their abundance in nature.

To use this number, round it to the nearest whole number.

To write symbols and isotopes

Using the atomic and mass numbers

Atomic number = # of protons; # of electrons in neutral atom

Mass number (rounded to whole number) = # proton + # neutrons

Mass number – atomic number (protons) = # neutrons

Example: uranium

Atomic number 92 (92 protons, 92 electrons)

Mass number 238 238 – 92 = 146 neutrons

14.5 Electron shellsElectrons orbit the nucleus in distinct shells

Each shell can only hold a certain number of electrons

There are at least seven shells and an atoms location on the periodic table reflects how many shells it has.

Shell 1 – can hold only 2 electrons

Shell 2- 8 shell 5 – 18

Shell 3 – 8 shell 6 - 32

Shell 4 -18 shell 7 - 32

Valence electronsThe electrons in the outermost shell of an atom are called the valence electrons

These are the electrons that are exposed to the environment and are the first to interact other atoms.

They are the ones that participate in chemical bonding.

ATOMS COMBINE TO FORM MOLECULES, WHICH ARE TIGHTLY HELD GROUPS OF ATOMS, BY WAY OF VALENCE ELECTRONS. VALENCE ELECTRONS ARE THE ONLY ONES THAT PARTICIPATE IN A REACTION.