1. Introduction to Atoms – Chapter 11
Section 2 – The Atom pp

2. DEFINITIONS
amu – the SI Unit used to express the masses of the particles of an atom
Atomic number – the number of protons in the nucleus of an atom

3. DEFINITIONS
Mass number – the sum of the particles in the nucleus (protons + neutrons)
Atomic mass – the weighted average of the masses of all the naturally occurring isotopes of an element

4. PARTS OF AN ATOM Parts of the atom Where Found? Size Charge (if any)
Proton
In nucleus of atom
1.7 ×10-24 g = 1 amu
positive
Neutron
Same size as proton = 1 amu
Neutral (no charge)
Electron
Outside nucleus in electron clouds
‹ 1 amu
is almost zero
negative

5. HELIUM ATOM
shell
proton N + - + N -
neutron
electron

6. ISOTOPES
Isotopes of an element share the same physical and chemical properties.
How can you tell one isotope from another? You can identify each isotope by its mass number.

7. Isotopes
Isotopes are atoms that have the same number of protons but have different numbers of neutrons.
The proton # never changes for an element, only the number of neutrons or electrons can change.

8. Isotope Notation Chlorine-35 Cl-35 35 is the mass number Na-23
Sodium-23
23 is the mass number

9. Ions Ions are charged particles.
An atom that loses one or more electrons becomes a positively charged ion.
An atom that gains one or more electrons becomes a negatively charged ion.

10. FORCES AT WORK IN ATOMS
Gravity – because the masses of particles in atoms is small, the force of gravity within atoms is very small.
Electromagnetic force – Particles with the same electric charge repel; particles with opposite charges attract. Protons and electrons are attracted to each other because they have opposite charges.

11. FORCES AT WORK IN ATOMS
3. Strong Force – the force that hold protons in the nucleus together. The strong force only acts on small distances
4. Weak Force – is an important force in radioactive atoms. In certain unstable atoms a neutron can change into a proton and an electron. This force plays a key role in this change.