Atoms and the Periodic Table

The AtomCh. 4

Democritus (400 BC)Greek philosopher.

All things are composed of tiny,indivisible particles: called:Atomos means

“something thatcannot be cut”.

Lavoisier (1789)French scientist.

Came up with the lawof conservation ofMass.

“Matter is neithercreated nor destroyed.”

Dalton (1803)Came up with the first“official” atomicTheory:

• Elements are made of tiny particles called atoms.

• All atoms of a given element are the same.

Dalton

• Atoms of different elements are different.

• Atoms of different elements combine to form compounds.

• Atoms cannot be subdivided, created, or destroyed.

Thomson (1906)Experimented withcathode rays

Discovered thatcathode rays arenegatively charged

Developed the“plum pudding”model of the atom

Cathode Ray Experiment

Thomson passed negatively charged Cathode rays through a tube of Hydrogen: the rays bent toward the positively charged plate.He tried it again with different gases and always got the same result.He realized that all matter must contain these negatively charged particles (electrons).

“Plum Pudding”

• Thomson said atoms were probably composed of negatively charged particles mixed with a positively charged substance.

History Summary:

• Democritus – Atomos• Lavoisier – Law of Conservation of

Mass• Dalton – First official Atomic Theory• Thomson – Plum Pudding Model,

theory of electrons

– Electrons are about 1837 times smaller than protons, so they do not significantly contribute to an atom’s mass.

– 1 neutron has about the same mass as 1 proton, so we calculate the mass of an atom to be protons + neutrons.

– Ex. Hydrogen – 1.00794 AMU• Hydrogen’s mass is essentially that of 1 proton• H has 1 proton, 0 neutrons and 1 electron

Subatomic Particles

Subatomic Particles Charge Symbol Mass (kg)

Electron 1- e- 9.02x10-31

Proton 1+ p+ 1.673x10-27

Neutron 0 no 1.675x10-27

Atomic Number

• An element is defined by its number of protons

• Ex. All examples of Nitrogen (number 7 on the periodic table) have 7 protons. If you change this number, you change the element!– Ex. If we change to 8 protons, what do we now

have?– Oxygen

Calculating Number of Neutrons• All the atoms of one element have the same

number of protons in their nucleus, but electrons & neutrons differ

• From the Periodic Table:Atomic Number (Z) = # of

protonsMass Number (A) = # of

protons + # of neutronsAtomic Symbol = X

Calculating Number of Neutron

• Example: For Ca (Calcium)Z = 20 = (20 protons)A = 40 = (20 protons +

neutrons)Number of Neutrons = A - ZA – Z = 40 – 20 = 20 neutrons Ca has 20 protons and 20

neutrons

The Nucleus & Standard Atomic Notation

X = CaA

Z

40

20

Z = # of protons

A = # of protons + # of neutrons

Inferring the Number of Electrons

• You can infer the number of electrons from Z

• Protons have a positive charge = p+ But, the atom is neutral on the periodic table Therefore, there must be enough electrons to

BALANCE the number of protons

• If the atom has p+ = 5 and NO CHARGE• e- = 5 BALANCE

Atomic Mass Number

• mass # = protons + neutrons

¨ always a whole number¨ ROUND from atomic

mass on the Periodic Table!

© Addison-Wesley Publishing Company, Inc.

Isotopes

• Isotopes are atoms of the same element with different mass numbers/different number of neutrons.

C126Mass # (P + N)

Atomic # (P)

¨ Nuclear symbol:

¨ Hyphen notation: carbon-12 (C-12)

Isotopes

© Addison-Wesley Publishing Company, Inc.

Isotopes

• Chlorine-37– atomic #:

– mass #:

– # of protons:

– # of electrons:

– # of neutrons:

17

37

17

17

20

Cl3717

Relative Atomic Mass

• 12C atom = 1.992 × 10-23 g – that’s small!

¨ 1 p = 1.007276 amu1 n = 1.008665 amu1 e- = 0.0005486 amu

¨ atomic mass unit (amu) – used by scientists to compare the size of really small particles

¨ 1 amu = 1/12 the mass of a 12C atom

I’m so small nobody notices me

Average Atomic Mass

• Weighted average of all isotopes.• This is the crazy number on the Periodic Table.

– You should round to the nearest tenth.

OR

100

(%)(mass(mass)(%) )

Avg.AtomicMass

Avg.AtomicMass

= (mass)(% in nature) + (mass)(% in nature)

Average Atomic Mass

• EX: Calculate the avg. atomic mass of oxygen if its abundance in nature is 99.76% 16O, 0.04% 17O, and 0.20% 18O.

Avg.AtomicMass

100

(18)(0.20)(17)(0.04))(16)(99.76 16.0g/mol

OR

Avg.AtomicMass

= (16)(.9976) + (17)(.0004) + (18)(.020) = 16.0 g/mol

This is the same number you get when you round the atomic mass to the nearest tenth on the Periodic Table!

Avg.AtomicMass

Average Atomic Mass

• EX: Find chlorine’s average atomic mass if approximately 8 of every 10 atoms are chlorine-35 and 2 are chlorine-37.

10

(37)(2)(35)(8)35.4 g/mol

Avg.AtomicMass

= (35)(.8) + (37)(.2) = 35.4 g/mol

OR

Electrostatic forces

• These are the forces of attraction between electrons and protons

Nuclear force

• Different from electrostatic forces, this is the force that keeps nucleons (particles found in the nucleus of an atom) together.

• Fun fact: the total mass of the nucleus is actually slightly greater than the mass of the protons and the neutrons together- when you break a nucleus down, some of that mass is released as energy, which we use in nuclear power plants and nuclear weapons