Modern Atom

Basic Structure

Consists of tiny nucleus and the electrons that move about it

Nucleus diameter about 10-13 cm Electron is average distance of 10-8 cm

from the nucleus

Nucleus

Chemistry is mostly about the electrons Nucleus has a very high density (a lot of

mass and small area)

Nuclear Contents

Protons – positively charged with charge about equal to but opposite of the electrons

Neutrons – no charge and about same mass as protons

What Causes Different Properties

Electrons are the parts that intermingle with other atoms

Different number of electrons (thus protons) causes different interactions and thus different behavior

Atoms Are Very Small In Mass

Hard to express in grams Unit of atomic mass – amu

– 1 amu = 1.66 x 10-24 g

Masses of Subatomic Particles

Proton 1.0073 amu Neutron 1.0087 amu Electron 5.486 x 10-4 amu It takes 1836 electrons to equal the

mass of 1 proton

In General

In a neutral atom the number of protons is equal to the number of electrons

The number of protons determines the identity of the atom

All atoms of the same element have the same number of protons

Isotopes – are atoms of the same element with different numbers of neutrons (thus different masses)

Isotopes are also called Nuclides Atomic mass on the periodic table is the

weighted average of the masses of all naturally occurring isotopes

Ions – atoms with different numbers of protons and electrons

Notation

Mass number shows the total number of protons and neutrons

Atomic number shows the number of protons