Structure of an atom and the
Atomic timeline

Review
Element
Two or more elements
Compound
What is an element made up of?

Today we will learn…
• That an element is made up of atoms
• Parts of an atom
• How the atom was discovered – atomic timeline

What is an atom?
• An atom is the smallest particle into which an element can be divided and still be the same substance.
rkm.com.au

Atoms are building blocks of matter because…
MATTER
• Atoms make up elements
• Elements make up compounds
• Elements and compounds make up matter.
Compound element 2 element 1
Atom 2
Atom 1

Parts of an atom
• Nucleus is the small, dense, positively charged center of the atom.
• Protons are positively charged particles in the nucleus.
• Neutrons are particles in the nucleus of an atom that have no charge.
• Electrons are negatively charged particles found in electron clouds outside the nucleus.
http://www.chem4kids.com/files/atom_structure.html

Let’s compare the parts of the atom
Charge Mass Location
P roton P ositive 1 amu Nucleus
Neu tron No charge/
Neu tral
1 amu Nucleus
E lectron Negative Almost zero
Electron clouds
( ex ternal to nucleus)
1 atomic mass unit (amu) = 1.7 X 10 -24 g

Properties of an atom
• Most of the atomic mass is present in the nucleus because the mass of the electron is almost equal to zero.
• Mass of a proton = mass of a neutron=
1a.m.u
• The atom has no charge (neutral) because number of protons = number of electrons .
http://www.chem4kids.com/files/atom_structure.html

There is something even smaller than protons and neutrons…
• Quarks are smaller particles that make up protons and neutrons.

Quick review
• Which of these is NOT one of the basic pieces of an atom?
1. Electron
2. Proton
3. Quantron
4. Neutron

Quick review
• If I ask you to draw a nitrogen atom, what would you show in the nucleus of the atom?
1. Molecules
2. Neutrons
3. Protons
4. Electrons

Story of the atom
• Also known as the Atomic Timeline
440
BC
1911
1803 1897
1913
20 th century

Democritus
440 BC
• Proposed the “atom”
• Atoms
– are small, hard particles
– constantly move
– are “uncuttable”
440
BC
1911
1803 1897
1913
20 th century

John Dalton
1803
• Conducted experiments in combining elements
• All substances are made up of atoms
• Atoms of the same element are exactly alike
• Atoms of different elements are different
• Elements combine in specific proportions
1911
440
BC
1803 1897
1913
20 th century

440
BC
J. J. Thomson
1897
• Conducted the cathode ray tube experiment
• There are small, negatively charged particles inside the atom
• “Plum pudding” model
1911
1803 1897
1913
20 th century

E. Rutherford
1911
• Gold foil experiment
• Atoms contain mostly empty space
• Small, dense positively charged nucleus
• Most of the atom’s mass is in the nucleus
• Electrons move around in empty space in the atom
1911
440
BC
1803 1897
1913
20 th century

N. Bohr
1913
• Electrons travel in definite paths
• Electrons jump between levels from path to path
440
BC
1911
1803 1897
1913
20 th century

440
BC
Schr ödinger and Heisenberg
20 th century
• Electron paths cannot be predicted
• Electrons are found in electron clouds, not paths
1911
1803 1897
1913
20 th century

Quick Review
• How is the modern model of an atom different from the Bohr atomic model?
1. The masses of the atomic particles are different.
2. The numbers of electrons are different.
3. The shapes of the nuclei are different.
4. The arrangements of the electrons are different.

Atomic Timeline worksheet
• Complete this in class. If it is not done in class then it will become a homework?

Wrapping up
• What we learned today
– What is an atom?
– Parts of an atom
– Atomic timeline
• Homework
– Timeline?
I’m positive!
I’m negative!
I could care less!
• Next time
– Continue with more information about the atom
– Learn about how atoms help elements find their spot on the Periodic Table