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Introduction to Atoms – Chapter 11

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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 1.673 ×10-24 g = 1 amu positive Neutron 1.675 ×10-24 g = 1 amu Neutral (no charge) Electron Outside nucleus in electron clouds 9.109 ×10-28 g < 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.


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