Dalton’s Atomic Theory

John 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

ATOM ATOM COMPOSITIONCOMPOSITION

•protons and neutrons in protons and neutrons in the nucleus.the nucleus.

•the number of electrons is equal to the the number of electrons is equal to the number of protons.number of protons.

•electrons in space around the nucleus.electrons in space around the nucleus.

•extremely small. One teaspoon of water has extremely small. One teaspoon of water has 3 times as many atoms as the Atlantic Ocean 3 times as many atoms as the Atlantic Ocean has teaspoons of water.has teaspoons of water.

The atom is mostlyThe atom is mostlyempty spaceempty space

ATOMIC COMPOSITIONATOMIC COMPOSITION• Protons (pProtons (p++))

– + electrical charge+ electrical charge

– mass = 1.672623 x 10mass = 1.672623 x 10-24-24 g g

– relative mass = 1.007 atomic relative mass = 1.007 atomic mass units mass units (amu) (amu) but we can round to 1but we can round to 1

• Electrons (eElectrons (e--))– negative electrical chargenegative electrical charge

– relative mass = 0.0005 amu relative mass = 0.0005 amu but we can round to 0but we can round to 0

• Neutrons (nNeutrons (noo))– no electrical chargeno electrical charge

– mass = 1.009 amu mass = 1.009 amu but we can round to 1but we can round to 1

Atomic Number, ZAtomic Number, Z

All atoms of the same element All atoms of the same element have the same number of have the same number of protons in the nucleus, protons in the 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 C atom with 6 protons and 6 neutrons

is the mass standard is the 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 NOT on the periodic table…(it is the

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

A = 5 p + 5 n = 10 amuA = 5 p + 5 n = 10 amu

A

Z

10

5B

A

Z

10

5B

IsotopesIsotopes

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

Figure 3.10: Two isotopes of sodium.

Atomic Symbols

Show the name of the element, a hyphen, and

the mass number in hyphen notation

sodium-23

Show the mass number and atomic number

in nuclear symbol formmass number 23 Na

atomic number 11

Isotopes?

Which of the following represent isotopes of the same element? Which element?

234 X 234

X235

X238

X

92 93 92 92

Counting Protons, Neutrons, and Electrons

• Protons: Atomic Number (from periodic table)• Neutrons: Mass Number minus the number of protons

(mass number is protons and neutrons because the mass of electrons is negligible)

• Electrons: – If it’s an atom, the protons and electrons must be

the SAME so that it is has a net charge of zero (equal numbers of + and -)

– If it does NOT have an equal number of electrons, it is not an atom, it is an ION. For each negative charge, add an extra electron. For each positive charge, subtract an electron (Don’t add a proton!!! That changes the element!)

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 is

1) 34X 2) 34X 3) 36X 16 14 14

AVERAGE AVERAGE ATOMIC ATOMIC MASSMASS

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

• Boron is 20% Boron is 20% 1010B and 80% B and 80% 1111B. That is, B. That is, 1111B is B is 80 percent abundant on earth. 80 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 Because of the existence of isotopes, the mass of a collection of atoms has an average value.of a 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 = ______________