isotopes

The Periodic Table

An atom consists of aAn atom consists of a • nucleusnucleus

– (of (of protonsprotons and and neutronsneutrons) ) • electronselectrons in space about the nucleus. in space about the nucleus.

The The AtomAtom

NucleusNucleus

Electron cloudElectron cloud

Dalton’s Atomic TheoryJohn Dalton (1766-1844) proposed an atomic theory

While this theory was not completely correct, it revolutionized how chemists looked at matter and brought about chemistry as we know it today instead of alchemy

Thus, it’s an important landmark in the history of science.

Dalton’s Atomic Theory - Summary

1. matter is composed, indivisible particles (atoms)2. all atoms of a particular element are identical3. different elements have different atoms4. atoms combine in certain whole-number ratios 5. In a chemical reaction, atoms are merely

rearranged to form new compounds; they are not created, destroyed, or changed into atoms of any other elements.

Problems with Dalton’s Atomic Theory?1. matter is composed, indivisible particles

Atoms Can Be Divided, but only in a nuclear reaction2. all atoms of a particular element are identical

Does Not Account for Isotopes (atoms of the same element but a different mass due to a different number of neutrons)!

3. different elements have different atomsYES!

4. atoms combine in certain whole-number ratiosYES! Called the Law of Definite Proportions

5. In a chemical reaction, atoms are merely rearranged to form new compounds; they are not created, destroyed, or changed into atoms of any other elements.Yes, except for nuclear reactions that can change atoms of one element to a different element

Atomic Number, ZAtomic Number, Z

All atoms of the same element have All atoms of the same element have the same number of protons in the the same number of protons in the nucleus, nucleus, ZZ

1313

AlAl

26.98126.981

Atomic numberAtomic number

Atom symbolAtom symbol

AVERAGE Atomic MassAVERAGE Atomic Mass

Mass Number, AMass Number, A• C atom with 6 protons and 6 neutrons is the C atom with 6 protons and 6 neutrons is the

mass standard mass standard • = 12 atomic mass units= 12 atomic mass units• Mass NumberMass Number (A) (A)

= # protons + # neutrons= # protons + # neutrons• NOT on the periodic table…(it is the AVERAGE NOT on the periodic table…(it is the AVERAGE

atomic mass on the table)atomic mass on the table)• A boron atom can have A boron atom can have A A

= 5 p + 5 n = 10 amu = 5 p + 5 n = 10 amu

A

Z

10

5B

A

Z

10

5B

IsotopesIsotopes

• Atoms of the same element (same Z) but Atoms of the same element (same Z) but different mass number (A).different mass number (A).

• Boron-10 (Boron-10 (1010B) has 5 p and 5 nB) has 5 p and 5 n• Boron-11 (Boron-11 (1111B) has 5 p and 6 nB) has 5 p and 6 n

10B

11B

• Isotopes are two forms of an element with the same atomic number but different mass number.

• All atoms contain three kinds of basic particles: protons, neutrons, and electrons.

• The protons and neutrons in an atom are found in the atomic nucleus, while the electrons are found in the space around the nucleus.

• The number of protons in a nucleus defines an atom.

• Hydrogen atoms all have one proton in their nucleus; helium atoms all have two protons in their nucleus; lithium atoms all have three protons in their nucleus; and so on.

• The number of protons in an atom's nucleus is called its atomic number. Hydrogen has an atomic number of 1; helium, an atomic number of 2; and lithium, an atomic number of 3.

• But atoms of the same element can have different numbers of neutrons.

• Some helium nuclei, for example, have two neutrons; others have only one.

• The mass number of an atom is the total number of protons and neutrons in the atom's nucleus.

• The two-neutron atom of helium has a mass number of four (two protons plus two neutrons)

• The one-neutron atom of helium has a mass number of three (two protons plus one neutron).

Figure 3.10: Two isotopes of sodium.

Representing isotopes• Isotopes are commonly represented in one of

two ways. First, they may be designated by writing the name of the element followed by the mass number of the isotope.

• The two forms of helium are called helium-4 and helium-3.

• Second, isotopes may be designated by the chemical symbol of the element with a superscript that shows their mass number. The designations for the two isotopes of helium are 4He and 3He.

Learning Check – Counting

Naturally occurring carbon consists of three isotopes, 12C, 13C, and 14C. State the number of protons, neutrons, and electrons in each of these carbon atoms.

12C 13C 14C 6 6 6

#p+ _______ _______ _______

#no _______ _______ _______

#e- _______ _______ _______

Answers

12C 13C 14C 6 6 6

#p+ 6 6 6

#no 6 7 8

#e- 6 6 6

Learning Check

An atom has 14 protons and 20 neutrons.A. Its atomic number is

1) 14 2) 16 3) 34

B. Its mass number is1) 14 2) 16 3) 34

C. The element is1) Si 2) Ca 3) Se

D. Another isotope of this element is1) 34X 2) 34X 3) 36X

16 14 14

AVERAGE AVERAGE ATOMIC ATOMIC MASSMASS

• Because of the existence of isotopes, the mass of a Because of the existence of isotopes, the mass of a collection of atoms has an average value.collection of atoms has an average value.

• Boron is 20% Boron is 20% 1010B and 80% B and 80% 1111B. That is, B. That is, 1111B is 80 B is 80 percent abundant on earth. percent abundant on earth.

• For boron atomic weightFor boron atomic weight = 0.20 (10 amu) + 0.80 (11 amu) = 10.8 amu= 0.20 (10 amu) + 0.80 (11 amu) = 10.8 amu

10B

11B

Isotopes & Average Atomic Isotopes & Average Atomic MassMass

• Because of the existence of isotopes, the mass of a Because of the existence of isotopes, the mass of a collection of atoms has an average value.collection of atoms has an average value.

• 66Li = 7.5% abundant and Li = 7.5% abundant and 77Li = 92.5%Li = 92.5%

– Avg. Atomic mass of Li = ______________Avg. Atomic mass of Li = ______________

• 2828Si = 92.23%, Si = 92.23%, 2929Si = 4.67%, Si = 4.67%, 3030Si = 3.10%Si = 3.10%

– Avg. Atomic mass of Si = ______________Avg. Atomic mass of Si = ______________

Radioactive isotopes• A radioactive isotope is an isotope that spontaneously

breaks apart (decay), changing into some other isotope.

• As an example, potassium has a radioactive isotope with mass number 40, 40K. This isotope breaks down into a stable isotope of potassium, 39K .

• Radioisotope, have an unstable nucleus that decays, emitting alpha, beta, or gamma rays until stability is reached

• The stable end product is a nonradioactive isotope of another element, i.e., radium-226 decays finally to lead-206