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Atoms and Elements. Goals: Describe atomic structure and define atomic number and mass number. Understand the nature of isotopes and calculate atomic weight from the isotopic masses and abundances. Explain the concept of the mole and use molar mass in calculations.

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atoms and elements

Atoms and Elements


Describe atomic structure and define atomic number and mass number.

Understand the nature of isotopes and calculate atomic weight from the isotopic masses and abundances.

Explain the concept of the mole and use molar mass in calculations.

Know the terminology of the periodic table.

the atomic structure experimental basis
The Atomic StructureExperimental basis
  • One of the pieces of evidence for the fact that atoms are made of smaller particles came from the work of Marie Curie (1876-1934).
  • She discovered ___________, the spontaneous disintegration of some elements into smaller pieces.

- Rays emitted by polonium and radium – radioactive atoms disintegrate and emit rays: alpha (a), beta (b), gamma (g).

**Contradicted Dalton’s postulate: “atoms are indivisible”.

experiment cathode ray tubes
Experiment: Cathode-Ray Tubes
  • Electrons:

Same charge-to-mass ratio detected for different elements: electrons are present in all atoms.

Millikan (charge of the electron), Goldstein (positive ions).

rutherford s gold foil experiment
Rutherford’s Gold Foil Experiment
  • Nucleus and the protons:


Chadwick (neutrons).

the atomic structure
The Atomic Structure
  • J.J. Thomson and E. Rutherford (England ~1900) established a model of the atom – the basis of modern atomic theory.
  • Three subatomic particles:
    • Electrically positive protons
    • Mass = 1.672622 x 10-24 g
    • Relative mass = 1.007 atomic mass units (u)
    • Electrically neutral neutrons
    • Relative mass = 1.009 u
    • Electrically negative electrons
    • Relative mass = 0.0005 u
    • Which particle is heavier, which particle is lighter?

____________ _______________

atom composition
Atom Composition
  • The atom is mostly empty space.
  • protons and neutrons in the nucleus.
  • the number of electrons is equal to the number of protons.
  • electrons in space around the nucleus.
  • extremely small. One teaspoon of water has 3 times as many atoms as the Atlantic Ocean has teaspoons of water.
how big is an atom
How big is an atom?
  • The radius of the typical atom is between 30 and 300 pm (3 x 10-11 m to 3 x 10-10 m). The radius of the nucleus is about 0.001 pm.
  • If an atom were a macroscopic object, about the size of a football stadium, what would be the radius of the nucleus?

– __________________

what is atomic number

Atomic number

Atom symbol

Atomic weight

What is Atomic Number?

Atomic number (Z) is the number of _____________ in the nucleus of an element.




All atoms of the same element have the same number of __________ in the nucleus.

what is mass number







What is Mass Number?

The mass number (A) is the number of ____________ plus the number of ____________ in the nucleus of an element.

C atom with 6 protons and 6 neutrons is the mass standard = 12 atomic mass units (u)

1 u = 1.661 x 10-24 g

Mass Number (A) = # protons + # neutrons

what is the mass number of an iron atom with 30 neutrons



Atomic number

Atom symbol

Atomic weight

What is the mass number of an iron atom with 30 neutrons?
  • The periodic table gives the atomic number for iron:
  • Why the periodic table gives a WEIGHT of = 55.845 ?
percent abundance
Percent Abundance

50%/50% mixture

90%/10% mixture

what are isotopes how do they affect atomic weight
What are isotopes? How do they affect atomic weight?
  • Isotopes: ___________________________ __________________________________.
  • Because of the presence of atoms, the mass of a collection of atoms is an average of values.

Average mass = ATOMIC WEIGHT

Boron is 19.9% 10B and 80.1% 11B. That is, 11B is 80.1 percent abundant on earth.

  • What is the atomic weight of boron?


how many isotopes for h
How many isotopes for H?

1 proton and 0 neutrons, protium



1 proton and 1 neutron, deuterium



1 proton and 2 neutrons, tritium





Gallium has two isotopes: 69Ga and 71Ga.How many protons and neutrons are in the nuclei of each of these isotopes? If the abundance of 69Ga is 60.1%, what is the abundance of 71Ga?

Students should become familiar with the

use of atomic number and mass number,

especially in isotopes calculations.

how are the masses of isotopes and their abundances determined
How are the masses of isotopes and their % abundances determined?
  • They are experimentally determined by using a ___________________________.

By changing the magnetic field, a beam of charged particles of different mass can be focused on the detector, and a spectrum of masses is observed.


Copper exists as two isotopes: 63Cu (62.9298 u) and 65Cu (64.9278 u). What is the approximate percentage of 63Cu in samples of this element?

congratulations you discovered a new element
Congratulations! You discovered a new element…

you name wyzzlebium (Wz). The average atomic mass of Wz was found to be 303.001 amu and its atomic number is 120.

A) If the masses of the two isotopes of Wz are 300.9326 amu and 303.9303 amu, what is the relative abundance of each isotope?

congratulations you discover a new element
Congratulations! You discover a new element…

B) What are the isotopic notations of the two isotopes? (e.g. W).

C) How many neutrons are in one atom of the more abundant isotope?



what is atomic weight
What is Atomic Weight?
  • This tells us the mass of one atom of an element relative to one atom of another element.
  • OR — the mass of 1000 atoms of one element relative to 1000 atoms of another.
  • Example: an O atom is approximately 1.333 times heavier than an C atom.
  • Define one element as the standard against which all others are measured
  • Standard = carbon
counting atoms
Counting atoms
  • In chemical reactions, how can we predict the amount of product given the mass of reactant?

Mg burns in air (O2) to produce white magnesium oxide, MgO.

the periodic table
The Periodic Table
  • Dmitri Mendeleev developed the modern periodic table. Argued that element properties are periodic functions of their atomic weights.
  • We now know that element properties are periodic functions of their __________ __________ (G. J. Moseley) . – Law of Chemical Periodicity

Students should become familiar with the organization of elements in the periodic table.

the periodic table23
The Periodic Table

Family or



  • Metals
    • Solid (except for mercury)
    • Conduct electricity
    • Ductile (can be drawn into wires)
    • Malleable (can be rolled into sheets)
    • Alloys (solutions of one metal into another metal)
  • Metalloids (semimetals)
    • Physical characteristics of metals, but chemical characteristics of nonmetals.
  • Nonmetals
    • Do not conduct electricity (except from C – graphite)
  • Non-metal.
  • Most abundant element in the universe.
  • Lightest element.
group ia alkali metals
Group IA: Alkali Metals
  • React with water to give alkaline solutions.
  • Found in nature only combined in compounds.
group iia alkaline earth metals
Group IIA: Alkaline Earth Metals
  • React with water to give alkaline

solutions (except Be).

  • Found in nature only combined in compounds.
group iiia
Group IIIA

Al resists corrosion (in HNO3).

Cu Al

Gallium can be a liquid at room temp.

  • All metals, but boron is a metalloid.
  • Aluminum is the most abundant metal in the earth’s crust by mass (8.2%).
group iva
Group IVA


Quartz, SiO2

  • C has large biological importance.
  • Nonmetal: C, Metalloids: Si, Ge, Metals: Sn, Pb.
  • All form analogous compounds such as their oxides: CO2, SiO2, GeO2, SnO2, PbO2.
what are allotropes
What are allotropes?
  • Characteristic of the chemistry of nonmetals.
  • Allotropes: _________________________ _______________________ each having its own properties.
  • C: graphite, diamond, buckminsterfullerene.
  • Decide which represents more mass:
  • 10 atoms of Fe or 10 atoms of K

b) 9 g of Na or 0.5 mol of Na

group va
Group VA
  • N2: makes up about ______ of earth’s atmosphere.
  • N and P have biological importance.
  • P has several allotropes.
  • Nonmetals: N2 and P, Metalloids: As and Sb, Metal: Bi.
  • Analogous oxides: N2O5, P2O5, As2O5.

White and red P

group via
Group VIA
  • O2: makes up ______of earth’s atmosphere.
  • S, Se, Te are the chalcogens (present in cupper ores).
  • O2 has an allotrope: O3 (ozone).
  • Nonmetals: O2, S and Se, Metalloids: Te, Metal: Po (radioactive).
  • Analogous oxides: SO2, SeO2, TeO2.
group viia halogens
Group VIIA: Halogens
  • All nonmetals, named the halogens (salt forming, they react violently with metals of group IA.
  • All exist as diatomic molecules.
  • F2 and Cl2 are gases. Br2 is a liquid. I2 is a solid.
group viiia noble gases
Group VIIIA: Noble Gases
  • All are gases.
  • The least reactive elements (called inert gases).
  • Helium is the second most abundant element in the universe. But non is abundant on earth or earth’s atmosphere (also called rare gases).
the transition elements
The Transition Elements



the transition elements44
The Transition Elements
  • Most occur naturally in compounds. Ag, Au, and Pt are much less reactive and can be found in nature as pure elements.
  • Some are of biological importance: Fe, Zn, Cu, Co.
essential elements
Essential Elements
  • Found in hemoglobin (Fe), vitamin B12 (Co).
  • Element in the 2th period, group 3A
  • Alkali metal in the 4th period
  • Metalloid in group 6A
  • Noble gas in 3th period
units for counting
Units for counting

1 dozen = 12 objects

Eggs are measured by the dozen

1 ream = 500 objects

Paper is measured by the ream, 500 sheets

counting atoms48
Counting atoms

Chemistry is a quantitative science—we need a “counting unit.”


Definition: 1 MOLE is the amount of substance that contains as many particles (atoms, molecules) as there are in __________________.

how many particles in one mole
How many Particles in one Mole?
  • Amedeo Avogadro (1776-1856).
  • There is Avogadro’s number of particles in a mole of any substance.

6.02214199 x 1023

1 mole = 6.022 x 1023 items

what is molar mass
What is Molar Mass?

1 mol of 12C = 12.00 g of C = 6.022 x 1023 atoms of C

12.00 g of 12C is its MOLAR MASS

Taking into account all of the isotopes of C, the molar mass of C is 12.011 g/mol.

  • The atomic mass expressed in grams is the molar mass of an element. It is different for each element.

1 molar mass = atomic mass of an element in grams

  • One mole of any element contains Avogadro’s number of atoms.

1 mol of atoms = 6.022 x 1023 atoms

You can use these relationships to make conversions between number of atoms, mass, and moles.

mass moles conversion
Mass Moles Conversion

It is essential to convert from moles to mass and from mass to moles.

1 mol




= moles



= grams


1 mol

1/molar mass

molar mass

what amount of mg in moles is represented by 0 200 g how many atoms

Atomic number

Atom symbol

Atomic weight

What amount of Mg in moles is represented by 0.200 g? How many atoms?

Mg has a molar mass of 24.3050 g/mol.




Students should become familiar with calculations using molar mass and avogadro’s number.

  • Go over all the contents of your textbook.
  • Practice with examples and with problems at the end of the chapter.
  • Practice with OWL tutor.
  • Practice with the quiz on CD of Chemistry Now.
  • Work on your assignment for Chapter 2.