Atomic Structure

Defining the Atom

Defining the AtomEssential Question

How did the concept of the atom move from philosophy to science?

Defining the Atom

• An atom is the smallest particle of an element that retains its identity in a chemical reaction.

Philosophy or Science?

• Democritus believed that matter consisted of tiny, indivisible, unchangeable particles called atoms.

• Until recent centuries the existence of the atom was a philosophical question rather than a scientific one.

Philosophy to Science

• Greater accuracy in measurement allowed for experiments which indicated the existence of atoms.

• Law of Conservation of Mass, Law of Definite Proportions, and Law of Multiple Proportions were clues to the existence of atoms.

Dalton’s Atomic Theory

• 1) All elements are composed of tiny indivisible particles called atoms.

• 2) Atoms of one element are identical. The atoms of one element are different from those of any other element.

Dalton’s Theory (cont.)

• 3) Atoms of different elements can physically mix together, or can chemically combine in simple whole-number ratios to form compounds.

• 4) Chemical reactions occur when atoms are separated, joined, or rearranged. Atoms of one element, however, are never changed into atoms of another element as a result of a chemical reaction.

Sizing Up the Atom

• A copper penny contains about 2.4 x 1022 atoms.

• Earth’s population measures about 6 x 109 people.

• There are about 4 x 1012 times as many atoms in a penny as people on the earth.

Sizing Up the Atom

• A line of 100,000,000 copper atoms side by side would be only 1 cm long.

Sizing Up the Atom

• Sizes of atoms are expressed in picometers (trillionths of a meter).

• 1 cm is the same fractional part of 600 miles as 100 picometers is of 1 cm!

Sizing Up the Atom

• Dry air contains 0.002% Neon.

• Yet there are about 5 x 1017 atoms of Neon in every breath you inhale!

• That’s 500,000,000,000,000,000 atoms!

Sizing Up the Atom

• In Summary:

• Atoms are itty-bitty.

Atomic Structure

Structure of the Nuclear Atom

Structure of the Nuclear Atom

Essential Question

What are the three principle sub-atomic particles and what part do they play in the structure of the atom?

The Structure of the Nuclear Atom

• Atoms are known to be divisible.

• Sub-atomic particles include neutrinos, quarks, baryons, hadrons, fermions, bosons, mesons, leptons, photons, gravitons, gluons, electrons, protons and neutrons.

• Not to mention futons, croutons, Nissans and morons.

The Structure of the Nuclear Atom

We will be most concerned with protons, neutrons and electrons.

Subatomic ParticlesThe Electron

• Discovered by Thomson

• Negatively charged particles

• Relative charge = 1–

• Symbol = e–

• 1 / 1840 the mass of a proton

• Reside outside of the nucleus

Subatomic ParticlesThe Proton

• Discovered by Goldstein

• Positively charged particles

• Relative charge = 1+

• Symbol = p+

• Mass of 1 atomic mass unit (amu)

Subatomic ParticlesThe Neutron

• Discovered by Chadwick

• Neutrally charged particles

• Relative charge = 0

• Symbol = n0

• Mass of 1 atomic mass unit (amu)

The Atomic Nucleus

• Discovered by Rutherford (Gold Foil Experiment)

• The tiny central core of an atom

• Composed of protons and neutrons

The Atomic Nucleus

• The nucleus is highly compacted and extremely dense.

• Most of the mass of atoms is in the nucleus.

• Nuclei have a density of about 2 x 108 metric tons / cm3 !

Atomic Structure

Distinguishing Among Atoms

Distinguishing Among AtomsEssential Question

What part do protons, neutrons and electrons play in the identity and characteristics of atoms?

Atomic Number

• The number of protons determines the identity of the element.

• The number of protons is indicated by the atomic number.

Mass Number

• The mass number indicates the number of protons AND neutrons in the nucleus.

• How do you find the number of neutrons from the mass number of a given element?

Isotopes

• Isotopes are atoms of that have the same number of protons (the same element) but have different numbers of neutrons.

• They, therefore, have different mass numbers and different masses.

Atomic Mass

• Masses of atoms are incredibly small and impractical to express in grams

• It is more practical to compare the masses of atoms to a different standard.

Atomic Mass Unit

• An AMU (atomic mass unit) is defined as 1/12 the mass of a Carbon-12 atom.

• Since the Carbon-12 atom is made up of 6 protons and 6 neutrons, an AMU is approximately equal to the mass of one proton or one neutron.

Atomic Mass

• An element’s atomic mass is a weighted average mass of all the atoms in a naturally occurring sample of the element.

• To calculate atomic mass, multiply the mass of each isotope by its percent abundance (expressed as a decimal) and add the products.