Section 1: Introduction to Atoms
 A.) What
does all matter consist of?
 B.) How many elements are there?
 C.)What are elements made of?
 D.)What is a molecule?
 E.)How many atoms are in a water
molecule?
 F.)How many atoms would fit in a period
at the end of a sentence?
 G.)Can we see an atom?
 Nucleus: very
small center core of an atom
where protons and neutrons are located;
contains most of the mass of the atom
Proton: positively charged subatomic particle found
in the nucleus
Neutron: neutral (no charge) subatomic particle
found in the nucleus
 Electron: negatively
charged subatomic
particle found moving rapidly outside the
nucleus in electron cloud
Sphere-shaped area outside the nucleus where
electrons move
 Negatively charged
 Electrons may be anywhere within it
 Electrons with lower energy usually move in the
space (orbitals) near the atom’s nucleus and
ones with higher energy are further from the
nucleus
 Accounts for most of the volume of the atom (if
nucleus were the size of a pencil eraser in the
middle of a baseball stadium, the electrons could
be as far away as the top row of seats)

 What
are the three subatomic particles?
 What
are the three subatomic particles?
 Protons, neutrons, and electrons
 Protons
and neutrons have approximately
equal masses
 Electrons have a mass 1/2000th that of a
proton or neutron
 Almost all of the mass of an atom is in the
nucleus even though it is so much smaller
than the electron cloud (nucleus is very
dense!)
 Atoms measured in atomic mass units
(amu)
 Which
is the smallest of the subatomic
particles?
 Which
is the smallest of the subatomic
particles?
 electron
 1808: John
Dalton concluded that each
element is made of atoms that are all
alike. He thought that different elements
have atoms of different mass and
imagined atoms as tiny, solid balls.
 1897: J.J. Thomson
discovered the
electron. Later proposed that an atom is a
positively charged sphere with electrons
embedded in it. Called the plum
pudding model.
 1904: Hantaro
Nagaoka proposed a
model of the atom that had a large sphere
in the center with a positive charge. His
model showed the electrons revolving
around this sphere like the planets
around the sun.
 1911: Ernest
Rutherford concluded that
the atom is mostly empty space.
Electrons orbit randomly around a small,
positively charged nucleus.
 1913: Niels
Bohr proposed a model that
showed electrons moving in specific
layers, or shells, rather than randomly. He
said that atoms absorb or give off energy
when the electrons move from one shell
to another.
 1932: James
Chadwick discovered the
neutron. The existence of neutrons
explained why atoms were heavier than
the total mass of their protons and
electrons.
 Present
model: Electrons form a
negatively charged cloud around the
nucleus. It is impossible to determine
exactly where an electron is at a given
time.
 Who
discovered that electrons are found
in shells?
 Who
discovered that electrons are found
in shells?
 Niels Bohr
 Each
element consists of atoms that differ
from atoms of all other elements
 An element can be identified by the
number of protons in the nucleus (all
hydrogen atoms have 1 proton, all helium
atoms have 2, etc)
 Atomic number: the number of protons in
the nucleus (usually big number in box
on periodic table)
 All
atoms of same element have same
number of protons, but can have different
number of neutrons
 Isotope: atoms with the same number of
protons and different number of neutrons
are called isotopes
 Mass
number: sum of the protons and
neutrons in the nucleus of an atom
 Used to identify isotopes of the same
elements
 Ex. Carbon-14 (carbon with a mass
number of 14) has 6 protons (as do all
carbon atoms) and so, must have 8
neutrons.
 Atoms
hard to study because of size (a
sheet of paper is 10,000 atoms thick)
 Scientists create models to describe
them
 Model: a diagram, mental picture,
mathematical statement, or object that
helps explain ideas about the natural
world
 Oxygen
has an atomic number of 8. How
many neutrons does the isotope oxygen15 have?
 Oxygen
has an atomic number of 8. How
many neutrons does the isotope oxygen15 have?
 Mass number – atomic number =
neutrons
 15 – 8 = 7 neutrons
1. An element is identified by its atomic
number. What is atomic number equal to?
a. neutrons
b. electrons
c. protons
d. quarks
2. Which two subatomic particles are
located in the nucleus?
a. electrons and neutrons
b. protons and neutrons
c. electrons and protons
d. protons and quarks
3. Which number is equal to the number of
protons and neutrons?
a. atomic number
b. mass number
c. atomic mass
d. weight number
4. What are atoms with the same number of
protons and different number of neutrons
called?
a. atoms
b. ions
c. molecules
d. isotopes
5. What do scientists call a diagram, mental
picture, mathematical statement, or object
that helps explain ideas about the natural
world?
a. a model
b. a graph
c. a formula
d. an equation