The atomCHAPTER 1: ATOMS & ELEMENTS (NOTES) (1) WHAT IS AN ATOM?

-Aristotle believed matter was infinitely divisible.

-Democritus believed that matter was made up of minuscule particles (which he called atomos) that couldn’t be divided and he was right.

- Both were philosophical theories that weren’t backed up by experiments Definition of the atom:

•Tiny particles that make up matter•They are the building block of matter;•They cannot be chemically divided;•They are extremely small e.g. 1 cm = approximately 50 million atoms lined up!

Approximately 400 B.C. (Greece) approximately 300 B.C.

Democritus

Evolution of the atomic theory through the ages:

1. Matter is discontinuous2. Particles are separated3. Particles are infinitely

small4. Particles cannot be

divided (Atomos)

Aristotle

1. Matter is continuous2. Substance completely fills

up the space it occupies

1214-1294 1627-1691 1783 1799

Joseph Proust:

Chemical reactions happen with definite proportions.

Robert Boyle:

Compression and expansion of gases.

Roger Bacon:

Experimentation is necessary to an understanding of the structure of matter.

Antoine Laurent Lavoisier:

Conservation of matter during a chemical reaction.

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Antoine-Laurent de Lavoisier (August 26, 1743 – May 8, 1794), the father of modern chemistry [1], was a French nobleman prominent in the histories of chemistry, finance, biology, and economics. He stated the first version of the law of conservation of mass, recognized and named oxygen (1778) and hydrogen (1783), disproved the phlogiston theory, introduced the metric system, wrote the first extensive list of elements, and helped to reform chemical nomenclature. He was also an investor and administrator of the "Ferme Générale" a private tax collection company; chairman of the board of the Discount Bank (later the Banque de France); and a powerful member of a number of other aristocratic administrative councils. All of these political and economic activities enabled him to fund his scientific research. But because of his prominence in the pre-revolutionary government in France, he was beheaded at the height of the French Revolution.

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John Dalton

Dalton’s model1.1 DALTON’S ATOMIC MODEL

From grade eight, recall Lavoisier’s law of conservation of mass: the mass of matter present at the beginning of a chemical reaction is equal to the mass of matter present at the end of a chemical reaction, hence: greactants = gproducts

-Dalton’s model is based on scientific experimentation.-It contains the following points:

(1) Matter is formed of atoms which are extremely small and indivisible(2) Each element is made of atoms which are all the same(3) Atoms of one element are different from atoms of another element(4) Atoms of different elements can combine in fixed ratios(5) No atom is created, divided, or destroyed during a chemical reaction, they are only rearranged to form new compounds.

- These wooden spheres (right) were used by Dalton (circa 1810) to demonstrate his theory

J.J. Thomson English Physicist

(1856-1940)

J.J. Thomson’s model1.2 THOMSON’S ATOMIC MODEL

-By experimenting with cathode rays, Thomson theorized that the atom contained indivisible, negatively charged particles (electrons) and that these particles could separate from the atom fairly easily.

-Thomson used gas discharge tubes to conduct his experiments where he studied the flow of electricity through gases at extremely low pressure in a cylindrical glass tube.

- The gas discharge tube consists of a high-voltage source, a vacuum pump, a glass tube, 2 terminals: a positive one (cathode) and a negative one (anode) and it works in the following manner:

Cathode ray tube(1) The tube is filled with a certain gas. (2) The gas is then taken out of the tube

with the help of a vacuum pump thus greatly reducing the pressure in the tube

(3) A high voltage current is sent through the terminals

(4) The gas particles remaining begin to glow (color of the light emitted depends on the gas used, e.g. Ne gas gives off a red light)

(5) It was discovered that the glow was produced by a stream of electrons (called cathode ray) that originated from the cathode and travelled to the positive anode

Gas discharge tube conclusions:

-At very low temperatures gases conduct electricity.

-Because the glow produced migrated towards the positive anode, it was deduced that this glow was made up of a stream of negative particles, namely electrons .- -- Cathode ray tubes were extensively used for TV’s and computer screens before the arrival of liquid crystal and plasma technology. The electron

-J.J. Thomson called the negatively charged particles emitted by the cathode electrons

- He believed the atom to be a positively charged ball covered with electrons, particles that could easily detach themselves from the rest of the atom. He named his model the plum pudding model.

Characteristics of a cathode ray tube:

1. Cathode ray tube have a negative charge.

Power source goes on. The screen is made of a phosphorous derivative that shines lights when hit by the ray.

What if you have oppositely charged plates around the tube?

Power source goes on.

2. Cathode ray tube proves the “particle” aspect of the electron (kinetics).

So Thomson’s model of the atom is called the plum-pudding model where negative charges called electrons float in a positive sphere.

3. The charge in the cathode ray tube has a rectilinear trajectory and produces light.

And now for something completely different.

Discovery of radioactivity (436)

In 1896, a man by the name of Antoine Henry Becquerel discovered radioactivity when he left radioactive salts over a photographic plate. He noticed that the salts left an imprint on the plates. In 1903, he shared the Nobel Prize in Physics with Pierre and Marie Curie "in recognition of the extraordinary services he has rendered by his discovery of spontaneous radioactivity". In 1908, the year of his death, Becquerel was elected permanent secretary of the Académie des Sciences. He died at the age of 55 in Le Croisic.The SI unit for radioactivity, the becquerel (Bq), is named after him, and there are Becquerel craters on the moon and Becquerel craters on Mars.

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Image of Becquerel's photographic plate which has been fogged by exposure to radiation from uranium salts. The shadow of a metal Maltese Cross placed between the plate and the uranium salts is clearly visible.

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In 1903, the Royal Swedish Academy of Sciences awarded Pierre Curie, Marie Curie, and Henri Becquerel the Nobel Prize in Physics, "in recognition of the extraordinary services they have rendered by their joint researches on the radiation phenomena discovered by Professor Henri Becquerel."Curie was the first woman to be awarded a Nobel Prize. Eight years later, she received the 1911 Nobel Prize in Chemistry, "in recognition of her services to the advancement of chemistry by the discovery of the elements radium and polonium, by the isolation of radium and the study of the nature and compounds of this remarkable element". Her death near Sallanches, Savoy, in 1934 was from aplastic anemia, almost certainly due to exposure to radiation, as the damaging effects of hard radiation were not yet known, and much of her work had been carried out in a shed with no safety measures. She had carried test tubes containing radioactive isotopes in her pocket and stored them in her desk drawer, remarking on the pretty blue-green light the substances gave off in the dark.

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Radiation under the influence of electric fields:

Ernest Rutherford, 1st Baron Rutherford of Nelson (30 August 1871 - 19 October 1937), widely referred to as Lord Rutherford, was a nuclear physicist who became known as the "father" of nuclear physics with the help of his assistant and godfather Spencer Isitt of Condradale. He pioneered the orbital theory of the atom through his discovery of Rutherford scattering off the nucleus with his gold foil experiment.1903: Ernest Rutherford

Using Henri Becquerel’s discovery, Ernest Rutherford observed the effects of electric fields on a radioactive substance.

In science there is only physics; all the rest is stamp collecting

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1.3 RUTHERFORD’S ATOMIC MODEL

- Subsequent to the discovery of cathode rays, was the discovery of X-rays and radioactivity The Atomic Nucleus & the Proton

-At the time Rutherford (1871-1937) conducted his radioactivity experiments ,it was known that radioactive substances could emit 3 types of radioactive particles:

Alpha (+)Beta (-)Gamma (neutral)

-Rutherford attempted to learn more about the location of electrons in atoms by bombarding a very thin gold sheet with a stream of positively charged alpha particles.

-He expected most of the alpha particles would pass right through the very thin (160 atoms thick) gold foil.-He also expected a few alpha particles would be mildly deflected as they would come in contact with an electron; instead a few alpha particles bounced back with considerable force of unexpected strength while the others passed through the gold sheet.

Rutherford’s model

-Because of these findings Rutherford concluded:

(1) Atoms are mostly empty space since the majority of alpha particles passed right through the gold foil and thus through the gold atoms

(2) Atoms contain a very dense and small, solid-like positively charged nucleus since positive alpha particles bounce back with great strength when they come in contact with the nuclear core (thus 2 positives repulse each other)

- Rutherford called the positive particle in the nucleus the proton and he deduced that in an atom the no. protons = the no. electrons since the atom as a whole is neutral .

Power source

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Lead block

Power source

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β: Beta (-), (electron, e-)

γ: Gamma (Neutral), energy ray

α: Alpha (+), Ionized helium, He+

To give you an idea of the radiation effect on us:

Rutherford’s experiment on alpha particle dispersion (Scattering) (pp 67)

In 1907, E. Rutherford performed a famous experiment at McGill University.

Power source

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So the alpha radiation which has a positive charge floats into a chamber surrounded by fluorescent material. The alpha radiation encounters a gold sheet.

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http://en.wikipedia.org/wiki/Image:Niels_Bohr.jpg

1.4 THE RUTHERFORD-BOHR ATOMIC MODEL

-Rutherford’s model was problematic. Scientists wondered why protons and electrons did not collide with one another. Two years later, Bohr expanded on Rutherford’s model and provided some explanation.

- Bohr’s experiments consisted of shining a glowing hydrogen-filled discharge tube through a plate containing a slit. This slit would break the incoming glow into 4 bands of differing color (just like a prism would separate white light into the colorful electromagnetic spectrum)

Rutherford-Bohr’s model

-To explain the 4 differently colored bands Bohr concluded:

(1) That electrons were found in specific areas he called orbitals as opposed to being randomly found. Bohr speculated that electrons could jump from one orbit to the next.

(2) An orbit correspond to a level of energy.(3) An electron can “jump” to another orbital (farther from the nucleus) when it receives extra

energy and gets “excited”.(4) If a previously excited electron returns to its original orbital, it emits light thus releasing

energy.

1.5 THE SIMPLIFIED ATOMIC MODEL

-Bohr’s improvement still did not explain all questions, e.g. why being all positive, the nucleus does not cause an explosion (excessive repulsion bet. + charges)

The Neutron

- Chadwick (1891-1974) discovered the neutron, a neutral nuclear particle; he speculated that the neutron acted as the ‘glue’ that holds protons in place in the nucleus - The simplified atomic model is a representation of the atom containing the no. of protons and neutrons in the nucleus and the no. of electrons in their respective shells

The simplified model

The simplified atomic model:

At the legendary First Solvay Conference (1911), Skłodowska-Curie (seated, 2nd from right), the only woman present, confers with Henri Poincaré. Standing, 4th from right, is Ernest Rutherford; 2nd from right, Albert Einstein; at far right, Paul Langevin.

At the legendary First Solvay Conference (1911), Skłodowska-Curie (seated, 2nd from right), the only woman present, confers with Henri Poincaré. Standing, 4th from right, is Ernest Rutherford; 2nd from right, Albert Einstein; at far right, Paul Langevin.

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(2) THE PERIODIC CLASSIFICATION OF THE ELEMENTS

Definition: periodic classification: a method whereby objects are classify according to their characteristics in order to make things more comprehensible.

-Today’s periodic table is based on a version developed by Dmitri Mendeleev (1834-1907) who offered the best classification solution for elements.

-Definition: periodic table of elements: a visual model whereby the elements are classified according to their physical and chemical properties; makes the relationships between elements more understandable.

The periodic table

Dimitri Mendeleev (Russian: Дм трий Ив нович и́� а́�Мендел еве́� , Dimitriy Ivanovich Mendeleyev (8 February [O.S. 27 January] 1834 in Tobolsk – 2 February [O.S. 20 January] 1907 in Saint Petersburg), was a Russian chemist. He is credited as being the primary creator of the first version of the periodic table of elements. Unlike other contributors to the table, Mendeleev predicted the properties of elements yet to be discovered.

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General info: As a better understanding of atomic weights was developed and better data became available, Mendeleev made for himself the following table:

Cl 35.5 K 39 Ca 40

Br 80 Rb 85 Sr 88

I 127 Cs 133 Ba 137

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By adding additional elements following this pattern, he developed his version of the periodic table.On March 6, 1869, Mendeleev made a formal presentation to the Russian Chemical Society, entitled The Dependence between the Properties of the Atomic Weights of the Elements, which described elements according to both weight and valence. This presentation stated that

1. The elements, if arranged according to their atomic mass, exhibit an apparent periodicity of properties. 2. Elements which are similar as regards to their chemical properties have atomic weights which are either of nearly the same value (e.g., Pt, Ir, Os) or which increase regularly (e.g., K, Rb, Cs). 3. The arrangement of the elements in groups of elements in the order of their atomic weights corresponds to their so-called valencies, as well as, to some extent, to their distinctive chemical properties; as is apparent among other series in that of Li, Be, B, C, N, O, and F. 4. The elements which are the most widely diffused have small atomic weights. 5. The magnitude of the atomic weight determines the character of the element, just as the magnitude of the molecule determines the character of a compound body. 6. We must expect the discovery of many yet unknown elements–for example, two elements, analogous to aluminium and silicon, whose atomic weights would be between 65 and 75. 7. The atomic weight of an element may sometimes be amended by a knowledge of those of its contiguous elements. Thus the atomic weight of tellurium must lie between 123 and 126, and cannot be 128. Here Mendeleev was wrong as the atomic mass of tellurium (127.6) remains higher than that of iodine (126.9). 8. Certain characteristic properties of elements can be foretold from their atomic weights.

General info

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One form of Mendeleev's periodic table, from the 1st English edition of his textbook (1891, based on the Russian 5th edition)

Sculpture in honor of Mendeleev and the periodic table, located in Bratislava, Slovakia

2.1 METALS, NONMETALS, & METALLOIDS

-The staircase divides the periodic table into metals to the left and nonmetals to the right; except for Al and Po (metals), metalloids are found immediately to the right and left of the staircase.

-Metals: are usually good conductors of heat and electricity. They are often ductile and malleable. Usually shiny. Solid at room Temperature except for Hg which is liquid. Many react with acids.

- Nonmetals: usually weak heat and electricity conductors. Many are gases at room Temperature. Solid nonmetals are brittle and can easily be reduced to powder.

- Metalloids or Semimetals: 7 elements (B, Si, As, Sb, Te, I, Po, At) have properties from each , category; maybe be good electrical conductors in some conditions and weak conductors under other conditions, because their conductance can vary with current and voltage, metalloids are used extensively for the control of electrical current in devices that use radio waves; called semiconductors.

Categories of elements

2.2 THE GROUPS OF THE PERIODIC TABLE

-The columns of the periodic table form groups; vertical groupings

-Elements of a group have similar chemical properties and have the same number of valence electrons (electrons on the last shell)

-Note: hydrogen doesn’t belong to a group

-Groups are identified with the letter A or B and a Roman numeral that indicates the number of electrons in the outermost shell; e.g. elements of group IIA ,all have 2 valence electrons and elements of group VIIB have 7 valence electrons

Families in the table

Li

Na

K

Rb

Cs

Fr

Be

Mg

Ca

Sr

Ba

RaF

Cl

Br

I

At

He

Ne

Ar

Kr

Xe

Rn

Alkaline family Alkaline-Earth family

Halogen familyNoble or inert gases family

(1) Alkali Metals (group IA; 1st column): e.g. Na, Li, K…; are soft and reactive; must be stored in oil since they will react with humidity in the air; so reactive that they are never found in their elemental form in nature, found only in compound form.

(2) Alkaline Earth Metals (group IIA; 2nd column): e.g. Be, Mg, Ca…; highly malleable & reactive; burn easily in the presence of heat; although can be exposed to air, are not found in their elemental state in nature (too reactive); form many compounds formed in rocks

(3) Halogens (group VIIA; before last column): e.g. F, Cl, Br…; react easily to form compounds, including salts (compounds formed when an acid reacts with a base or with a metal); nonmetals; several are powerful cleaners;

(4) Noble Gases or Rare Gases or Inert Gases (group VIIIA; last column): ex. He, Ne, Ar…; very stable; rarely react, therefore are found in their elemental form in nature; image shows the color emitted by noble gases as an electric current is passed through them, explaining why they are used in glowing signs

Families in the table

2.3 THE PERIODS OF THE PERIODIC TABLE

- The rows of the periodic table; horizontal groupings-Elements of a same period will all have the same number of shells around their nucleus

The Periodicity of Properties

- Refers to how properties vary within a period.-E.g. the size of an atom will decrease from left to right within a period; as there are more protons and electrons in each element as you go to the right of a period, the attraction between the + and - forces increase causing the compacting of the atom

- Properties include: melting point, boiling point, density, atomic radius, first ionization energy (energy needed to move the outmost electron away from an atom), electronegativity (the degree of ability an atom has to attract electrons from other atoms in order to form bonds – the higher the electronegativity, the more readily an atom will react – F has the highest electronegativity value at 4.0)

Periods

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Na

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LiRadius gets bigger is you go down a family.

General info

2.4 ATOMIC NUMBER

- Whole number in the squares of the periodic table; ex. phosphorous: 15- Equal to the number of protons in the nucleus of an atom (and electrons in a neutral atom)- Symbol e.g. P

Atomic number

Atomic mass

Name

Symbol

2.5 RELATIVE ATOMIC MASS

- The mass of one atom of a particular element derived by using carbon-12 atom as a reference; ex. phosphorous: 30.97 u

-Expressed in amu or u, atomic mass units

-1 u = approx. 1.66 x 10-24g

- Because atomic masses are very small, they are difficult to measure directly. As a solution to this problem, a reference atom was used to establish the meaning of the atomic mass unit and to determine the mass of all other atoms; scientists agreed to give 1 carbon-12 atom (a carbon atom which has 6 protons and 6 electrons) a mass of 12u; by extension it was calculated that the mass of 1 proton or 1 neutron was about 1u (electrons being extremely light have an almost negligible mass)

Relative mass

Mass number

-Whole number that represents the mass of protons + the mass of neutrons

-Determined by rounding off the relative atomic mass, ex. P = 30.97 = 31

-Atoms are often represented with a notation; where A is mass no; Z is atomic no and E is element symbol; ex. for phosphorus:

-To calculate no. of neutrons, you must subsrtracted the atomic number from the atomic mass:

No. of neutrons = mass number – atomic number

Mass number

2.6 ISOTOPES

-An atom may exist in different versions where the number of neutrons will vary from one atom to the next

-Ex. hydrogen has 3 isotopes:

• most common form, atomic mass = 1, also called: hydrogen-1• (deuterium): heavy hydrogen, atomic mass = 2, also called: hydrogen-2• (tritium): super-heavy hydrogen, atomic mass = 3, also called: hydrogen-3

- All isotopes of an element have the same number of protons and the same chemical properties but they have different physical properties.

Isotopes

Isotopes (436)

Hydrogen

1 Proton (+)

1 Electron (-)

Zero Neutron (Ø)

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Deuterium

1 Proton (+)

1 Electron (-)

1 Neutron (Ø)

e-

Tritium

1 Proton (+)

1 Electron (-)

2 Neutrons (Ø)

e-

(3) REPRESENTING ATOMS

-Usually electrons fill the shell closest to the positively charged nucleus before filling a shell that is farther away from the nucleus

-The 1st electron shell can contain a maximum of 2 electrons and the 2nd shell a maximum of 8 electrons

-The 3rd shell also can carry 8 electrons.

-Once 8 electrons are filled in the 3rd shell, the 9th electron jumps over to the 4th shell followed by other electrons.

Visualizing atoms

3.1 LEWIS NOTATION

- Atoms are represented by their symbol and paired dots. The dots represent the valence electrons. The dots will make up a cross shape before starting to double up

Lewis Diagram

3.1 LEWIS NOTATION

Lewis notation can also be used for compounds. This is the Lewis structure for water.

Lewis Diagram

3.2 REPRESENTING AN ATOM ACCORDING TO THE RUTHERFORD-BOHR MODEL

-Must know the period (its no. indicates the no. of shells), the group (indicates the no. of valence electrons), and the atomic no. (indicates the no. of protons and electrons)

-The small sphere is drawn and the no. of protons is written in it; each electron shell is represented by a circle around the nucleus; electrons are drawn on the electron shell circles to indicate their location

- This type of model is sometimes called the planetary model (right); what is inaccurate about this representation?

Rutherford-Bohr model

3.3 REPRESENTING AN ATOM ACCORDING TO THE SIMPLIFIED ATOMIC MODEL

-In this model, protons, neutrons and electrons are represented; ex. oxygen below which has 8 protons, electrons and(in this case also) 8 neutrons

- The no. of neutrons is calculated by rounding the atomic mass up/down and subtracting the atomic number; ex. Be, beryllium (the 4th element) has an atomic mass of 9.01; this would be rounded down to 9; the no. of neutrons would be calculated by subtracting 4 from the nine, giving 5; therefore a Be atom would have 4 protons , 4 electrons and 5 neutrons

Simplified atomic model

Reminder: Chemical group and electron configuration: Simplified atomic model

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Nucleus:

Composed of the proton and the neutron.

Electron shells:

The most probable positions occupied by electrons in an atom.

Valence electron:

An electron located on the outermost electron shell..

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Lithium:

Group number one, hence one valence electron

Period two, hence two electron shells or levels

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NaSodium:

Group number one, hence one valence electron

Period three, hence three electron shells or levels

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Shorthand technique:

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3.4 THE ‘BALL-AND-STICK’ MODEL

- In this model the whole atom is represented by a ball & bonds are represented by a line (stick); ex. image; different colors indicate different atoms; plastic ball & stick of methane (CH4)

Ball-and-Stick method

(4) THE CONCEPT OF MOLE

-A mole is a quantity used to express the amount of atoms & molecules present in a sample

-It is a concept similar to a pair, a dozen, or a hundred, where each notion equates a specific amount, except that it refers to a much greater quantity

-Once again the carbon-12 atom is used as a reference; to be exact:

1 mole is equal to number of atoms in exactly 12g of carbon-12

- The symbol for the mole is mol

Mole

4.1 MOLAR MASS

-Molar mass is the mass of one mole of a substance

-From above we know that 1 mole carbon-12 = 12g (from 12u which is the relative atomic mass for carbon-12); because of this knowledge we can to say that the mass of a mole of a substance = to its atomic mass

-Molar mass is expressed in g/mol

-Therefore molar mass is determined by using the relative atomic mass; because one atom of N has a relative atomic mass of 14.01u then the mass for one mole of N is 14.01g/mol

-If we wanted the molecular molar mass for N, meaning the molar mass for N2, then we would multiply 14.01 x 2 to obtain 28.02g/mol

-Can you figure out H2O & CO2’s molar mass?

- There is a formula associated with molar mass, it is:

Molar mass

4.2 AVOGADRO’S NUMBER

-The Avogadro constant was named in honor of Amedeo Avagadro’s (physicist, 1776-1856) contribution to the field; image

-Through experimentation it was determined 6.02 x 1023 atoms are contained in 12g of carbon-12; this number is the Avogadro constant or Avogadro number

-This means that 1 mole of anything will contain 6.02 x 1023 entities; ex. 1 mole of apples represents 6.02 x 1023 apples

- By extension the molar mass (derived by the relative atomic mass) of any element will equal to 6.02 x 1023 atoms; 24.31g of Mg & 6.94g of Li will respectively equal 6.02 x 1023 atoms of Mg & Li respectively

Avogadro’s number

General info

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Using Avogadro's result that any gas under the same conditions has the same number of molecules per Mole (unit), Loschmidt determined that number, now called Avogadro's number as being 6.023 × 1023 molecules. This is why on rare occasions this "Avogadro number" is called the "Loschmidt number" in English (in German, though, "Loschmidt'sche Zahl" is the commonly used name).

General info

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General info

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- Can you figure out H2O & CO2’s molar mass?

- There is a formula associated with molar mass, it is:

M=m/n, where M=molar mass in g/mol; m=mass in g; n=number of moles in mol- Ex. How many moles are there in 84 g of CO2?n = m/M = 84g/44.01g/mol

= 1.9mol of CO2

MMM: 2 (H) + 1 (S) + 4(O)

2 (1) + 1 (32) + 4(16)

2 + 32 + 64

98 g/mol

MMM: 2 (1 (N) + 4 (H)) + 1(C) + 3(O)

2 (1 (14) + 4 (1)) + 1(12) + 3(16)

2 (14 + 4 ) + 12 + 48

2 (18 ) + 12 + 48

36 + 12 + 48

96 g/mol

m ___

M n

n: number of moles (mol)

M: Molecular molar mass (g/mol)

m: mass (g)