INTRODUCTION TO ATOMS CHAPTER 3 section 1

Structure of Atoms Through the years theories about the shape and size of atoms have changed. Scientists used to think that the atom was the smallest particle possible. Now we know that they are made of smaller particles called protons, neutrons, and electrons.

Models through the years

Particles in Atoms The nucleus is the center of the atom. The nucleus is made up of the protons and neutrons. Protons have a positive charge. Neutrons do not have a charge.

Protons=Electrons in number! The number of protons equals the number of electrons. This results in the atom being neutral. The electrons move very rapidly around the nucleus in an area called the electron cloud.

Comparing Particle Masses Electrons are so small that they don’t account for much of the mass of the atom. It would take 2000 of them to make the mass of a proton. Protons and neutrons are about equal in mass. Scientists use a unit called the atomic mass unit (amu) to measure protons and neutrons. Each is equal to 1amu. The mass of the electron is 1/2,000 amu.

Atoms and Elements Elements can be identified by the number of protons in the nucleus of its atoms. This is called the atomic number. Neutrons do not have a charge, so the atom is still neutral. The mass number will be different for the isotopes. Mass number of the atom = protons + neutrons

Isotopes Isotopes of elements have the same number of protons but differ in the number of neutrons. Isotopes are identified by their mass number.