Chapter 12: Atoms and the Periodic Table

• Use of materials around us• Wet clay harden into ceramic when heated by fire• 5000 B.C. pottery fire pits gave way to furnaces hot

enough to refine copper ores to produce metallic copper.

• 1200 B.C. Hotter furnaces were converting iron ore into iron. This allowed the production of metal tools and weapons, and many achievements of the ancient Chinese, Egyptians and Greek civilizations.

• 20th Century: Materials are made of atoms. Control of atoms to make new materials.

• This chapter: What are atoms?

12.9 The Shell model

• The model: electrons behave as if they are arranged in a series of concentric shells. The shell is a region of space about the nucleus where the electrons reside.

• There are at least 7 shells, each one can hold a certain number of electrons

• The number of shells is equal to the number of periods in the periodic table

• Valence electrons are the electrons at the outer most shells. They the first to interact, so they participate in the chemical bonding.

• What are the valence electrons in each group?

12.1 Atoms are Ancient and Empty

• Atoms originated at the birth of the universe.• Hydrogen (H) is the most abundant (more than 90%).• Helium (He) is the next most abundant.• Heavier atoms were produce in stars where great pressures caused

H atoms to become heavier atoms (fusion).• All the atoms that occur naturally on earth (except H) are produced

in stars.• Atoms are ancient: recycling in the universe, moving from one

object to another (like our bodies).• If you put ink in the ocean, the atoms will move all over the world’s

oceans.

12.1 Atoms are Ancient and Empty

• Atoms are small: there are more than 10 billion trillion of them in each breath you exhale.

• They can’t be seen by eye. Brownian Motion(1827).• Today: atoms are made up of: electrons, protons, neutrons.• Atoms are different because the number of e,p,n is different.• Nucleus is much smaller than the atoms, but has most of the mass.• Electrons move in the space around the nucleus.• They define the volume of the atom. But because they are small, far

apart from each other and from the nucleus, the atoms are indeed mostly empty space.

• So what keep the atoms from merging into each other?• Space is filled with electric field. Outer electrons repel each other.

14.2 The Elements

• Atoms make up matter around us: Stars, steel, ice cream, our bodies.

• What is the number of different type of atoms??? SMALL• Only 90 atoms are found in nature. About 12 atoms are man made.• Different substances are made of combination of different atoms.• A material that is made up of one type of atoms is called an

element.• Examples: gold, nitrogen, graphite, diamond.• Periodic table: atomic symbol (C:Carbon, Cl:Cloor, Au:Aurum,

Co/CO).

12.3 Protons and Neutrons

• Protons– Positive charge– Same quantity of charge as electron, but opposite – Number of protons in the atom is equal to the number of

electrons, so the atoms is electrically neutral. Example: Oxygen has 8 electrons and 8 protons.

– 2000 times as massive as an electron– Atomic number = is number of protons (to identify elements)– Periodic table list atoms in order of increasing atomic number

The periodic Table

• Neutrons– Mass of atomic nucleus must be made up of more than protons– Neutrons have the same mass as protons – Have no electric charge (neutral)– Both neutrons and protons are called nucleons.– See table 14-1

Isotopes and Atomic Mass

• How does the number of neutrons affect the element• Most hydrogen atoms have no neutrons, some have one neutron,

others have 2 neutrons• Most iron atoms have 30 neutrons, some have 29 neutrons.• Atoms of the same element that contains different number of

neutrons are called isotopes of the element• Isotopes are identified by the mass number

Atomic mass

• Atoms interact with each other electrically (mostly electrons). So the number of neutrons does not effect the interaction.

• Therefore isotopes share many common properties• The number of neutrons affect the mass of the atom• Atomic mass=masses of electrons+neutrons+protons• Atomic mass unit (amu)• Atomic masses in the periodic table are in amu• It is the average atomic masses of all isotopes

12.4 The Periodic Table:metals, non-metals and metalloids

Periods and Groups

Atomic size trend

12.5 Physical and Conceptual Models

• Atoms are so small so that the number of them in a ping pong id equal to the number of ping-pongs in a sphere the size of earth

• The reflection of light allows our eyes to see things. Atoms are so smaller than the wavelength of visible light.

• We can not see atoms but can generate images indirectly (STM)• Physical model: that can replicates the object at a more convenient

scale. Not suitable for the atom• Conceptual model:

12.6 Identifying Atoms with the Spectroscope

• Each color of the visible light has a frequency (white light?)• A spectroscope is an instrument used to observe the color

components of any light source• It is used to analyze the light emitted by elements• Light is emitted by atoms subjected to any form of energy. The

atoms of a given element emit only certain frequencies of light.• Sodium atoms emit yellow light. Neon atoms emit red-orange light• When a spectroscope is used, light is composed of discrete

(separate from each other) frequencies or colors. These frequencies are called Atomic spectrum. It is the element fingerprint.

12.7 The Quantum Hypothesis

• Max Planck (1900): The light energy is quantized the same way matter is (the mass of a gold brick is equal to the number of atoms in the brick multiplied by the mass of one atom)

• Also the electric charge on an object is a whole-number multiplied by the charge of electron (or a proton)

• Mass and charge are quantized.• Planck: a beam of light energy is not continuous steam of energy

but consists of small discrete packets of energy (quantum) • Einstein: These quanta of light behave like tiny particles (called

photons)

Bohr: Atomic spectra

• Bohr explained the formation of the atomic spectra:• An electron has more potential energy when it is further from the

nucleus• Absorption: When an atoms absorbs a photon of light it is absorbing

energy• This energy is acquired by one electron• The electron (once gain energy) will move away from the nucleus.• Emission: when a high energy electron losses some of its energy it

moves closer to the nucleus• The energy lost is emitted as a photon.

Bohr: Principal Quantum number n

• Because light energy is quantized, the energy of an electron in the atom must also be quantized (can not take any value)

• Within the atom, there must be discrete energy levels (like stairs), and therefore limited number (n) of energy levels (orbits). The lowest energy level has a principal quantum number n=1.

• Bohr developed a conceptual model (planetary model) for the atom where the electron is restricted to certain distances from the nucleus determined by its energy.

• Photons are emitted by the atoms as the electrons move from a high energy level (orbit) to a low energy level (orbit). The energy emitted is equal to the difference in energy between the two levels. Because electrons are restricted to discrete orbits, only certain light frequencies are emitted.

12.9 The Shell Model

• Electrons behave as if they are arranged in series of concentric shells

• A shell is a region in space about the nucleus where the electrons reside

• There are at least 7 shells each can hold a certain number of electrons

• The 7 shells accounts for the 7 periods in the periodic table• Valence electrons occupy the outermost shells. They are the first to

interact with other atoms. They are the ones that participate in the chemical bonding

• See valence electrons in figure 12.30