atomic structure. atom definition – smallest particle of an element that retains the chemical...

Atomic Structure

Atom

Definition – smallest particle of an element that retains the chemical identity of that element

Term proposed by Democritus in 450 BCDemocritus proposed that all matter is

composed of tiny, indivisible particlesRejected by Aristotle because Democritus

could not explain how particles “stuck together”

Dalton’s Atomic Theory of Matter- 1803

Based on observations made by scientists during the 1700’s, John Dalton proposed a theory that could explain the properties of matter

Four postulates:1. Each element is made of small particles called atoms2. All atoms of a given element are identical, but different

from those of any other element3. Atoms are neither created nor destroyed in a chemical

reaction4. A given compound always has the same relative

numbers and kinds of atoms

Are the four postulates all true?

What do we know about atoms that means we might need to re-state one of the postulates?

Postulate Number 2 – not all atoms of the same element are the same

You can have different Isotopes of the same element

Example: Iodine-125 and Iodine-127 are both iodine, but are different from each other. Iodine-125 is radioactive and Iodine-127 is not.

History of Discovering about Atomic Structure

Dalton thought atoms were hard and round like tiny marbles

Early 1800’s –

Franklin- two kinds of electric charges

(positive and negative)

Electrons were First Particles Found

An English scientist discovered electrons

and named them. An Amercian scientist found their mass.

Mass of an electron to be 9.11 x 10-28 grams

Protons were next particles found

Atoms are electrically neutral so the negative electrons must be balanced by an equal number of positive particles

An English scientist found these particles and called them protons.

He also discovered these positively charged particles were all concentrated in the center of the atom – called it the nucleus

Modern Atomic Theory

Atoms are composed of protons, neutrons, and electrons

At first, scientists visualized electrons orbiting the nucleus much like planets orbiting the sun

Last particle found was the neutron

Neutrons have a mass approximately equal to that of a proton

Neutrons have no charge

Neutrons help “glue” the nucleus together

Fundamental Subatomic Particles

Particle Location Charge Mass (amu) Proton Nucleus +1 1

Neutron Nucleus 0 1

Electron Outside -1 0 Nucleus

Moseley’s Discovery

Found that atoms of each element contain a unique positive charge in their nucleus

Conclusion: atom’s identity comes from the number of protons in the nucleus

Called “Atomic Number” Neutral atoms – positive charges must

equal negative charges Conclusion – for neutral atoms, no. of

electrons equals no. of protons

Information in Periodic Table

7 Atomic Number (sometime not in center)

N Chemical Symbol

Nitrogen

14.0067 Atomic Mass

Mass Number

Hydrogen -1 The 1 after the hydrogen is the mass number. Used

to help keep track of what isotope you have.Definition Sum of the neutrons and protons

Always a whole numberAnother way to represent

Mass Number 1 2 3

H H H 1 1 1

Atomic Number

Mass of an Atom

Atomic Mass Unit – approximately the weight of one neutron or proton

Used because the weight in grams is very small

1 a.m.u. = 1.66 x 10-24 g.

Element’s atomic mass

Weighted average of all of the isotopes of an element

7

N Nitrogen

14.0067 Atomic Mass

Nitrogen contains mostly Nitrogen-14 with a small amount of Nitrogen-15

Practice

Find sodium (Na) on the periodic table I gave you

Find the atomic number. What does that number mean?

Find the atomic mass. What does that number mean?

How many electrons does one sodium atom have?

I have one atom of sodium represented by 46

V

23

How many protons does it have?

How many neutrons does it have?