ChemistryWilbraham

StaleyMatta

Waterman

Chapter 4: Atomic Structure

Copyright © 2005 Pearson Education & Prentice-Hall, Inc.

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Element Rap

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Discoveries of the Atom

In 1774, Antoine Lavoiser (Aun-twain la-vwa-zyey) preformed experiments and measurements that led to the law of conservation of matter.

Matter can be neither created nor destroyed.

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In 1799, Joseph Proust (Proost) made measurements on chemical reactions and compounds and developed the law of constant composition.

a pure chemical compound is always has the same percentage composition of each element by mass, regardless of how the substance was made or where the substance is found.

So, whenever we talk about water, we know that there are 2 atoms of hydrogen and 1 atom of oxygen in a molecule of water.

As soon as the composition of a molecule changes, then you have a different substance with different properties.

Substance

Formula Composition Properties

Water H2O 2 atoms of hydrogen, 1 atom of oxygen

Clear, colorless, homogeneous liquid that is good for human consumption

hydrogen peroxide

H2O2 2 atoms of hydrogen, 2 atoms of oxygen

Clear, colorless, homogeneous liquid that is harmful if consumed by human (poisonous)

Which states:

1. All matter is composed of tiny, identical particles, called atoms, that cannot be destroyed or created.

2. Each element has atoms that are identical to each other in all of their properties, and these properties are different from the properties of all other atoms.

3. Chemical reactions are simple rearrangements of atoms from one combination to another in small whole-number ratios.

John Dalton’s atomic theory

Scientific theory

provides new predictions that are tested by experiments to support or disprove the theory.

scientists can never prove a theory to be true.

experiments are used to support a theory.

..?.atomic theory led to...

John Dalton proposed the law of multiple proportions

Which states:

When two elements can be combined to make two different compounds, and if samples of these two compounds are taken so that the masses of one of the elements in the two compounds are the same in both sample, then the ratio of the masses of the other element in these compounds will be a ratio of small whole numbers.

In 1834, Michael Faraday showed that electrical current could cause chemical reaction to occur, demonstrating the electric nature of elements.

In the 1870s, Sir William Crookes developed what is know today as cathode ray tube.

He mistakenly thought that the cathode rays were negatively charged molecules instead of electrons.

How can you describe the structure of the

nuclear atom?

J.J. Thomson determined that cathode rays were a fundamental part of matter he called electrons.

He determined their charge to mass ratio (e/m= -1.76 x 108 coulomb’s gram-1)

Led to the “plum pudding” model of the atom, which had electrons bathed in a sea of position charges similar to raisins in English pudding.

Cathode-ray tube

Cathode-ray tubeA cathode ray is deflected

by electrically charged metal plates, as shown

beside. A positively charged plate attracts the cathode

ray, while a negatively charged plate repels it.

Thomson knew that opposite charges attract

and like repel, so he determined the cathode ray to be stream of negatively

charged particles moving at high speed. Thomson called these particles corpuscles;

later named electrons.

Checkpoint

How do negatively charged plates affect the path of cathode rays?

Checkpoint

How do negatively charged plates affect the path of cathode rays?

A negatively charged plate repels a cathode ray.

Subatomic Particles

What are the three kinds of subatomic particles?

Subatomic Particles

The three kinds of subatomic particles are electrons, protons, and neutrons.

Electrons are negatively charged particles shown by Thomson’s experiments

Ernest Rutherford was interested in radioactive materials and identified alpha and beta particles in his research.

A alpha particle contains two protons and two neutrons, the nucleus of a helium atom.

A beta particle is a naked negatively-charged electron

Through the gold foil experiment Rutherford deduced the nuclear model of the atom.

Rutherford’s gold foil experiment

Deduces the nuclear model of the atom.

In 1911, Rutherford and co-worker at University of Manchester, England, wanted to test the existing plum-pudding model of atomic structure.

Their test used alpha particles, which are helium atoms that have lost their two electrons and have a double positive charge- two remaining protons.

A narrow beam of alpha particles were directed at a very

thin sheet of gold foil.

Rutherford’s gold foil experiment

Rutherford’s results:most alpha particles

went straight through the gold foil, or were

slightly deflected. However, what was surprising is that a small fraction of the

alpha particles bounced off the gold

foil at very large angles.

Rutherford’s gold foil experiment

Rutherford concluded that most of the alpha particles pass through the gold foil because the atom is mostly empty space. The mass and positive charge are concentrated in a small region of the atom. Rutherford called this region the nucleus. Particles that approach the nucleus closely are greatly deflected.

In 1919, Rutherford discovered that the basic unit of positive charge in the atom and named it proton.

The proton has a positive charge which is exactly equal in magnitude to the electron charge.

mass of 1.67 x 10-24 which is 1836 times heavier than the electron.

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In 1932, James Chadwick discovered a very penetrating form of radiation.

The third major particle that makes up the atom, and it was named the neutron since it is neutral.

The neutron has a mass almost equal to that of the proton.

What are their relative charges? and symbol?

Electrons?

Protons?

Neutrons?

Name Symbol Relative Charge

Electrons -1 e or e-

Protons +1 p

Neutrons 0 n

Structure of the Atom

Principal energy levels (shells)

Sub-levels (sub-shells)

Orbitals

Electronic Structure of the Atom

Electronic Configuration

abbreviated electronic configurations

Valance electrons

How to count valance electrons

Distinguishing Among Atoms

What makes one element different from another?

Distinguishing Among Atoms

Elements are different because they contain different numbers of protons.

Distinguishing Among Atoms

Atoms are composed of protons, neutrons, and electrons.

Protons and neutrons make up the nucleus.

Electrons surround the nucleus.

Atomic number

The atomic number of an element is the number of protons in the nucleus of an atom of that element.

ex: all hydrogen atoms have one proton, the atomic number is 1.

All oxygen atoms have eight protons, the atomic number is 8.

The atomic number identifies an element.

In table 4.2:

The number is protons equals the number of electrons.

Atoms are electrically neutral. Thus, the number of electrons (negative charged particles) must equal the number of protons (positively charged particles).

Understanding Atomic Number

The element nitrogen (N), shown here is liquid form, has an atomic number of 7. How many protons and electrons are in a neutral nitrogen atom?

The atomic number gives the number of protons, which in a neutral atom equals the number of electrons.

The atomic number of nitrogen is 7, which means that a neutral nitrogen atom has 7 protons and 7 electrons.

Practice Problems:Complete this table:

ElementAtomic number Protons Electrons

K 19 (a) 19

(b) (c) (d) 5

S 16 (e) (f)

V (g) 23 (h)

Mass Number

The total number of protons & neutrons in an atom is called the mass number.

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

Number of neutrons = mass number - atomic number.

The chemical symbol Li appears with two numbers written to the left.

What does it mean?

What do the numbers stand for?

The chemical symbol for Lithium, Li appears with two numbers written to the left.

The mass number is the superscript.

The atomic number is the subscript.

You can also refer to atoms by using the mass number and the name of the element.

Ex: 197 79Au may be written as gold-197.

How many electrons does gold have?

Gold has 79 electrons.

How do you calculate mass number?

How do you calculate mass number?

Mass number = number of neutrons + atomic number.

Isotopes

Isotopes: are atoms that have the same number of protons but different numbers of neutrons.

Because isotopes of an element have different numbers of neutrons, they also have different mass numbers.

Isotopes are chemically alike because they have identical numbers of protons and electrons, which are the subatomic particles responsible for chemical behavior.

Chemical symbols of Isotopes

Diamonds are a naturally occurring form of elemental carbon. Two stable isotopes of carbon are carbon-12 and carbon-13. Write the symbols for each isotope using superscripts and subscripts to represent the mass number and atomic number.

Diamonds are a naturally occurring form of elemental carbon. Two stable isotopes of carbon are carbon-12 and carbon-13. Write the symbols for each isotope using superscripts and subscripts to represent the mass number and atomic number.

Isotopes are atoms that have the same number of protons but different numbers of neutrons. The composition of a atom can be expressed by writing the chemical symbol, with the atomic number as a subscript and the mass number as a superscript.

Based on Table 4.2, the symbol for carbon is C and the atomic number is 6. The mass number for each isotope is given by its name. For carbon-12, the symbol is 12

6C. For carbon-13, the symbol is 136C.

Atomic Mass

Mass of a proton or neutron is very small (1.67 x 10-24 g).

Mass of an electron is 9.11 x 10-28 g).

The mass of even the largest atom is incredibly small.

An atomic mass (amu) is defined as one twelfth of the mass of a carbon-12 atom.

helium-4 atom, with a mass of 4.0026 amu, has about 1/3 the mass of a carbon-12 atom.

Carbon-12 has six protons and six neutrons in its nucleus, and its mass is set as 12 amu.

What is the atomic mass of an element?

Atomic mass is a weighted average mass of the atoms in a naturally occurring sample of element.

The atomic mass of copper is 63.546 amu. Which of copper’s two isotopes is more abundant: copper-63 or copper-65?

Analyze

The atomic mass of an element is the weighted average mass of the atoms in a naturally occurring sample of the element. A weighted average mass reflects both the mass and the relative abundance of the isotope as they occur in nature.

Solve

The atomic mass of 63.546 amu is closer to 63 than to 65. Because the atomic mass is a weighted average of the isotopes, copper-63 must be more abundant than copper-65.

Practice Problems:

Boron has two isotopes: boron-10 and boron-11. Which is more abundant, given that the atomic mass of boron is 10.81?

Practice Problems:

There are three isotopes of silicone; they have mass numbers of 28, 29, and 30. The atomic mass of silicon is 28.086 amu. Comment on the relative abundance of these three isotopes.

To calculate the atomic mass of an element, multiply the mass of each isotope by the natural abundance, expressed as a decimal, and then add the products.

Atomic mass of carbon = (12.00 amu x 0.9889) + (13.003 amu x 0.0111) = 12.011 amu

Facts and Figures

Carbon-14 Dating

All living organisms contain carbon-12 and carbon-14 in a fixed ratio. After an organism dies, this ratio changes as the carbon-14 decays. Paleontologists and archaeologists use this fact to establish the age of fossils and ancient artifacts.

Review

The periodic table

How many elements are in period 2?

In group 2A?

Review

The periodic table

How many elements are in period 2?

There are eight elements in period 2.

In group 2A?

There are 6 elements in group 2A.

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