Structure of the Atom

Refer to: Science Probe 10 - Chapter 8

The idea of the atom goes back to Ancient Greece.

We continue by looking at more “recent” ideas of the atom.

Dalton’s Atomic Theory (1808) All matter is made of atoms

(particles to small to see).

Each element has its own kind of atom. Atoms of the same element have the same mass, atoms of different elements have different masses.

Compounds are formed when different atoms link to form “compound atoms” (now called molecules). In a compound all “compound atoms” are alike.

Atoms cannot be created or destroyed.

Modern Atomic Theory Even though Dalton’s work was

200 years ago, the basis of it is still valid with only a few modifications.

Dalton thought atoms were solid spheres like billiard balls, but we know atoms are made up of smaller parts

Sub Atomic Particles (smaller than atoms)

Atoms are made up of three main type of particles:

Protons Found in nucleus (center part) of

atom Positive charge (+1) Mass 1 atomic mass unit (a.m.u.)

Neutrons Found in nucleus of atom Neutral ( 0 charge) Mass 1 a.m.u.

Electrons Found moving outside the nucleus Negative charge (-1) Mass approx 1/1800 a.m.u.

Atomic Number Number of protons in the atom Each atomic number is unique and

defines each atom E.g. If there are 8 protons, the

element is… oxygen! If the element is oxygen, there must

be 8 protons

The atomic number is found at the top of each square on the periodic table.

Atomic number

See Periodic Table Here (or text p. 516)

If you have a periodic table you can instantly find the number of protons.

Number of Electrons In any neutral (uncharged) atom

the number of protons is always equal to the number of electrons.

The number of electrons, therefore, is also equal to the atomic number.

If there are 8 protons, there will be 8 electrons.

Mass Number Total number of all protons and

neutrons in an element always a whole number Note - electrons are too small

to effect total mass An element with 3 protons and 4

neutrons will have a mass number of 7

Mass Number To find mass number from a periodic

table, look under the symbol (or name) Round off to a whole number (decimals

will be explained soon!) See periodic table

Mass number = 12 (rounded off)

Number of Neutrons Can be determined from the periodic

table. If mass number = protons +

neutrons, then: Neutrons = mass number - protons or Neutrons = mass number - atomic

number

Calculating Protons, Neutrons and Electrons Find number of protons, neutrons

and electrons for each of the following.

Carbon Na Element #5

Assignment Do PNE Worksheet

Isotopes Isotopes are atoms of the same

element that have different masses.

Why are these masses different?? Since number of protons doesn’t

change, the change in mass must be due to different number of neutrons.16 amu 17 amu

8 protons 8 neutrons 8 protons 9 neutrons

Example Hydrogen exists in 3 different

isotopes Most hydrogen atoms have one

proton, (one electron) and no neutrons

Mass is 1 a.m.u. This is also called Hydrogen-1 or

Protium

Some Hydrogen atoms have one neutron as well as the one proton and electron.

This is a different isotope - it is still hydrogen, but it has a mass of 2 a.m.u.

This isotope is called Hydrogen-2 Also called Deuterium

A third form of hydrogen has 2 neutrons with 1 proton and 1 electron.

This gives a mass of 3 a.m.u. This is called Hydrogen-3 Also called Tritium

Atomic Mass(Decimal Number) The mass reported on the periodic

table is actually a weighted average of the masses of all the isotopes of an element.

This is why the periodic table show decimal number.

For most atoms, one isotope is much more common than the other(s).

Assignment Do Isotope Worksheet