WARM-U

PStudying atoms is difficult because they are too small to see or directly observe even with the best scientific tools. Write a similar example of something that can not be studied directly.

CHAPTER 4: ATOMIC STRUCTURE

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Learning Targets-1. I can describe the structure of

atoms.2. I can describe how structure of

an atom affects it’s properties.3. I can create a timeline that

shows the developments that lead to the current model of the atom.

ANCIENT GREEK MODELS OF ATOMS

460-370 BC

Democratis- all matter consisted of extremely small particles that could not be divided he called them atoms from the Greek word Atomos which means “uncut” or “indivisiable”

384-322 BC

Aristotle- no limit to the number of times that matter could be divided

Accepted until the 1800s

THE EXISTENCE OF ATOMS WASN’T SCIENTIFICALLY PROVEN UNTIL THE EARLY 1800S.

John Dalton, 1766-1844, English chemist and teacher

• Studied the behavior of gases in air

• Concluded that gas contains individual particles

• No matter how large or small the sample the ratio of the elements in compounds is always the same

• All elements are composed of atoms

• All atoms of the same element have the same mass and atoms of different elements have different masses

• Compounds contain atoms of more than one element

• In a particular compound, atoms of different elements always combine in the same way

DALTON’S ATOMIC THEORY

DALTONS ATOMIC MODEL

Tiny solid spheres with different masses

Dalton’s theory explained data from many experiments and thus became widely accepted

ALTHOUGH ATOMS ARE INCREDIBLY SMALL THEY CAN NOW BE OBSERVED WITH A SCANNING TUNNELING MICROSCOPE

CHARGED MATERIALS

• Some materials when rubbed gain the ability to attract or repel other materials

• Gain either a positive or negative charge

• Object with like charges repel or push apart, objects with opposite charges attract or pull together

• Charged particles can flow creating an electric current

THOMSON’S MODEL OF AN ATOM

J.J. Thomson, an English physicist 1856-1940

Wires connect the metal disks at opposite ends of a empty glass tube, one disk becomes negatively charged and one becomes positively charged

A glowing beam appears in the space between the plates

The beam is repelled by a negatively charged metal disk or attracted by a positively charged plates brought near it

Thomson hypothesized the ray was a stream of negatively charged particles contained inside atoms, now called electrons, which are part of all atoms and carry a charge of -1.

No matter what metal he used he got the same particles

The mass was always 2000 times smaller than the mass of hydrogen atoms (a proton)

THOMSON’S MODEL

THOMSON’S MODEL

First evidence that atoms are made of even smaller particles

Atom is neutral- negative charges scattered throughout an atom filled with a positively charged mass of matter

ERNEST RUTHERFORD’S GOLD FOIL EXPERIMENT

Ernest Rutherford- 1871-1937

Atom was believed to have its positive charge spread throughout.

Rutherford shot alpha particles (large 2 + atoms) at a very thin sheet of gold foil.

If the current model of the atom was correct the alpha particles should pass though gold the mass and charge being too small to deflect the alpha particles.

RUTHERFORD’S ATOMIC THEORY

Most alpha particles actually passed straight through a small fraction bounced off the gold foil at large angles

Atoms are mostly empty spacePositive charge is concentrated in the nucleus, not evenly distributed, which contains the protons and neutrons and has a positive charge

Positive charge varies among elements

Each nucleus must contain at least one proton, each proton is assigned a charge of +1

All of an atoms positive charge is concentrated in the dense nucleus

Electrons are outside the nucleus

RUTHERFORD’S ATOMIC MODEL

JAMES CHADWICK

1932

James Chadwick, English physicist did experiments that proved the neutrons existed

Concluded that they were neutral because they were not effected by a charged particle

Neutrons are contained in the nucleus and have a mass equal to that of a proton

Compare the mass, location and charge of protons, neutrons, and electrons.

Rutherford's atomic model couldn’t explain chemical properties of elements

required knowledge of electron behavior

Niels Bohr (1885-1962)Focused on ElectronsAgreed with Rutherford that the nucleus of and atom was surrounded by a large volume of space

THE BOHR MODELElectrons are only found in specific circular paths- orbits around the nucleusEach electron orbit has a fixed energy or energy level

An electron cannot exist between energy levels

The energy level closest to the nucleus is the lowest

An electron in an atom can move from one energy level to the next when it gains or loses energy

Energy lost can be in the form of light

THE QUANTUM MECHANICAL MODEL

Rutherford-Bohr described the path of an electron as a large object would behave which was inconsistent with theoretical calculations and experimental results

Electrons move in a much less predictable way then planets in a solar system.

Erwin Schrodinger 1887-1961

Devised and solved a mathematical equation describing the behavior of the electron in hydrogen atom

QUANTUM MECHANICAL (ELECTRON CLOUD) MODEL

Electron Cloud- visual model of the most likely locations for electrons in an atom

Based on the probability of finding an electron with in a certain volume of space surrounding the nucleus is described as a fuzzy cloud where the electron is 90% of the timeMore dense- probability highLess dense- probability low

ATOMIC ORBITALS

• The electron cloud represents all the orbitals in an atom

• An orbital is a region of space around the nucleus where an electron is likely to be found

• Each orbital can hold 2 electrons

• The lowest energy level has one orbital, the second has four, the third 9 and the fourth 16

• Electron configuration- the arrangement of electrons in the orbitals of an atom

• The most stable electron configuration is the one in which the electrons are in orbitals with the lowest possible energies this is called the ground state

DISTINGUISHING AMONG ATOMS

How are atoms of Hydrogen different than atoms of oxygen?

Element of different atoms are contain different numbers of protons

Atomic Number- the number of protons in the nucleus of an atom of a given element

- Identifies an Element

- Atoms are electrically neutral so the number of electrons are also equal to the atomic number

Mass Number- the number of protons and neutrons in the nucleus

• Example- carbon has 6 protons and 6 neutrons so the mass number is 12

• Most the mass of an atom is concentrated in the nucleus

• The number of neutrons in an atom is the difference between the mass number and the atomic number.

• # neutrons = mass # - atomic #

Elements can be represented in the following shorthand notation:

Au

197

79

Or Gold-197

symbolMass #

Atomic #

Isotopes- atoms that have the same number of protons but a different number of neutrons

• Different mass numbers

• Chemically alike- same protons and electrons which are responsible for chemical behavior

Atomic Mass

• The mass of atoms are givin in comparison to carbon-12

• An atomic mass unit (amu) = 1/12 the mass of a carbon-12 atom

• Carbon 12 amu

• Flourine- actual mass 3.155 x 10 –23 g, atomic mass- 18.998 amu

• 1 proton or neutron- 1 amu

• Atomic mass- weighted average mass and relative abundance of isotopes as they occur in nature

• Example- Hydrogen –1, 99 %, 1.0078

Hydrogen- 2, 1% heaver

Hydrogen- 3

Atomic mass- 1.0079