Periodic TrendsChemCom Unit 1B

Phases of the moon

The phases of the moon repeat at regular intervals or periods.- This is termed “periodicity”

The history of the periodic tableJohann Dobereiner – Law of Triads (1817)

The history of the periodic table1862- Alexandre-Emile Béguyer de

Chancourtois was the first person to organize the elements by atomic mass

John Newlands – Law of Octaves 1865

Dmitri Mendeleev – 1872

Early Periodic tablesThe tables of the 1800’s were organized

according to two characteristicsAtomic mass

Physical property Hydrogen atoms have the lowest mass, oxygen

atoms are about 16 times more massive, sulfur was about 32 times more massive

An average atomic mass was assigned to each element based on these comparisons

Combining capacity with other elements, such as chlorine and oxygen Chemical property One atom of potassium (K) or cesium (Cs) combines

with only one atom of chlorine to produce KCl or CsCl One atom of magnesium (Mg) or strontium (Sr)

combines with two atoms of chlorine to produce MgCl2 or SrCl2

Vertical columns Elements with similar chemical properties

Horizontal arrangements Based on increasing atomic mass

Creators of early periodic tables were unable to explain the similarities in properties among neighboring elements 50 years after Mendeleev, the reason for these patterns

was proposed

The Electrical Nature of MatterLike charges repel

Unlike charges attract

+

+

+

-

--

Every electrically neutral (uncharged) atom contains equal numbers of positively and negatively charged particles:Protons – positively charged particles found in the

nucleus (center) of the atom The number of protons identifies what element the atom

isElectrons – negatively charged particles found

outside the nucleus of the atomAn electrically neutral atom of chlorine contains

17 protons 17 electrons

Ions (atoms or groups of atoms that are NOT electrically neutral) contain different numbers of protons and electronsA potassium atom with 19 protons and 18

electrons will have a charge of: +1 Because there is one more proton than there are

electrons.

NeutronsElectrically neutral particles within the nucleus

of most atomsOnly hydrogen contains no neutronsDoes not affect the charge of the atom

The Modern Periodic tableElements are placed in sequence according

to their increasing atomic number (number of protons) rather than their atomic massEarly periodic tables used atomic mass rather

than atomic number Works well for relatively small atoms Does not work well for larger/ more massive atoms Note that Cobalt (Co) and Nickel (Ni) are not in

mass order

The nucleus of the atomA concentrated region of positive charge in the

center of the atomContains both protons (positive charge) and

neutrons (neutral charge)The total number of protons and neutrons in

the nucleus is called the mass numberElectrons are not included in the mass number

because each electron is only about 1/2000th the mass of a proton or neutron

Atoms of the same elementAll have the same number of protonsMay have differing numbers of neutronsAtoms with the same number of protons but

different numbers of neutrons are known as isotopes Isotopes are atoms of the same element with

different mass numbers

Atomic massesAre decimals because it represents the average

mass of all of the different isotopes of that particular element.

Counting Subatomic ParticlesAtomic number:

Equals the number of protonsEquals the number of electrons (in a neutral

atom)Atomic mass:

Is an average of the masses of all the possible isotopes of a particular element

Rounded to the nearest whole number, it equals the mass number for the most common isotope

Mass number:Equals the number of protons + number of

neutrons

In a neutral atom:# Protons = # electrons = atomic number# Neutrons = mass number – # protons

In an ion (charged atom):# Protons = atomic number# Electrons = # protons – charge

Ex. 1 Na+

11 protons 11 – 1 = 10 electrons

Ex. 2 Cl- 17 protons 17 – (-1) = 18 electrons

# Neutrons = mass number - # protons

To find the charge of an ion when the number of protons and electrons is known: Charge = # Protons - # electrons

Ex. 1 an ion of cobalt with 25 electrons will have what charge? 27 -25 = +2

Ex. 2 an ion of sulfur has 18 electrons. What is its charge? 16 – 18 = -2

You can now complete B.5 Extra Practice packet