The Periodic Table of the Elements

1869~Dmitri Mendeleev1944~Glenn Seaborg

Mendeleev’s Periodic Table

Was organized by two characteristics:

1. Atomic weight or “atomic mass”

2. Their “combining capacity” with other elements, especially with oxygen and chlorine. (The “O in oxide” and “Cl in chloride)

Vocabulary Atomic Number– the number of

protons in an atom’s nucleus Mass Number- the number of

protons and neutrons an atom

has.

Vocabulary Isotope- An element (same proton)that

has different numbers of neutrons.

This is one lithium’s isotope. It has 3 protons and 4

neutrons. 92.5 % of Lithium exists in this isotope.

The other 7.5 % exists in the isotope with 3 protons and 3 neutrons.

This has no effect on the charge.

Vocabulary

Ion – An atom that has gained or lost electrons.

Shown by + or – symbol in top right corner

An atom that has one or more electrons than protons will have a negative charge

An atom that has one or more protons than electrons will have a positive charge.

Metals, Metalloids, Nonmetals

Periodic Variation in Properties

Horizontal rows are called PERIODS Vertical columns are called GROUPS or

FAMILIES. The elements in each group have similar properties. ALKALI METALS—1st column; highly reactive ALKALINE EARTH—2nd column TRANSITION METALS—3rd through 12th column HALOGEN—17th column (2nd from right); readily

form 1- anions NOBLE GASES—18th column (far right column);

very unreactive

What determines properties?

Mostly it is the number and arrangement of the atom’s electrons.

Predicting Properties

Mendeleev used the average of properties of elements just above and below an unknown element to predict its properties.

Average is found by adding all the numbers together then dividing the sum by the number of numbers. Average for 2,4, and 6

4 + 5 + 9 = 18 18/3 = 6

Element #32 was unknown to Mendeleev, but the density of silicon (the element above it on the table) is 2.3 g/cm3 and tin’s (the element below it on the table) density is 7.3 g/cm3. Make an estimate of element #32’s density.

2.3 g/cm³ + 7.3 g/cm³ = x x/2 = Estimation

Predicting Properties

Predicting Properties

When germanium (element #32) was discovered in 1886, its density was found to be 5.3 g/cm3, only about 10% different than the prediction!

Predicting Properties

Formulas for chemical compounds can also be predicted. We know that carbon and oxygen form carbon dioxide (CO2). What formula would you predict for a compound of carbon and sulfur?

Since oxygen and sulfur are in the same group (16), we can predict that the compound would be carbon disulfide (CS2).

Your turn!

1. Krypton (Kr) wasn’t known in Mendeleev’s time. Given that the boiling point of argon (Ar) is -186°C and of xenon (Xe) is -112°C, estimate the boiling point of Kr.

2. The melting points of potassium and cesium (Cs) are 337 K and 302 K respectively.

(a) Estimate the melting point of rubidium (Rb).

(b) Do you expect the melting point of sodium to be higher or lower than that of rubidium? Explain the evidence you used for your prediction.

Solutions to questions

1. -149 degrees C

2. 320 K; Higher because the melting points within this group of elements decrease as you go from top to bottom.

3. Mendeleev knew that silicon tetrachloride (SiCl4) existed. Using his periodic table, he predicted an element just below silicon (germanium). Predict the formula for the compound formed by this predicted element and chlorine.

4. Given these formulas for known compounds– NaI, MgCl2, CaO, Al2O3, and CCl4 –predict the formula for a compound formed from: (a) C and F (b) Al and S

(c) K and Cl (d) Ca and Br(e) Sr and O

Solutions to questions

3. GeCl44. a) CF4;

b) Al2S3;

c) KCl; d) CaBr2;

e) SrO

Reading the periodic table Find the following for Ti (titanium)

1. Atomic Number, or Protons

22

2. Atomic Mass

48

3. Number of Neutrons

26

4. Number of Electrons

22