CH.3 ELEMENTS, ATOMS AND ISOTOPES

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

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

III. Atoms of different elements combine in simple whole-number ratios to produce compounds (stable groups of atoms.)*

IV. Atoms cannot be created or destroyed.H2 H2O NH3 CH4

Abundance of Elements in Earth’s Crust

Sub Atomic Particles (smaller than atoms)1. Protons

– Found in nucleus (center part) of atom– Positive charge (+1)– Mass 1 atomic mass unit (a.m.u.)

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

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

Atomic Structure

Atoms are electrically neutral, as the number of protons equals the number of electrons.

There have been many models of the atom proposed by scienctist over the years. The most recent two are:Bohr Model

• Protons and neutrons crammed into the center called the nucleus

• Electrons move in definite orbits around the nucleus

Models of the atom:

The electron cloud model:- the most accepted model today

• Almost the same as the Bohr model.

• Around the nucleus is an electron cloud.

• Within the cloud there are energy levels that electrons can jump from.

Atomic Model

The nucleus and electron cloud are separated by vast space, for example:

• An atom the size of Dodgers Stadium and parking lot would contain a pea sized nucleus containing 95.95% of the atom’s mass!!!!!!

XAZ

Atomic

Number

mass number (A) - sum of the number of protons and neutrons

Mass Number

Symbol of the atom

atomic number (Z) - the number of protons in the atom

• Isotopes - atoms of the same element having different masses

– contain same number of protons– contain different numbers of neutrons

Isotopes of Hydrogen

H H H 31

21

11

Hydrogen(Hydrogen - 1)

Deuterium(Hydrogen - 2)

Tritium(Hydrogen - 3)

• Isotopes of the same element have identical chemical properties

• Some isotopes are radioactive

• Atomic Mass and mass number are close, but not exactly the same….atomic mass is the average of all known isotopes and their relative abundance in nature

• Mass number and atomic mass can help determine how many neutrons there are in an atom (subtract mass number—atomic number)