This document discusses atomic structure and the periodic table. It explains that the atomic number is the number of protons in an atom, which distinguishes one element from another. The mass number is the total number of protons and neutrons. Isotopes are atoms of the same element with different numbers of neutrons. Electrons orbit the nucleus in energy levels. The periodic table organizes elements based on their properties and places them in rows and columns according to atomic number.

1. Comparing Atoms
&
The Periodic Table
Chapter 18.2 and 18.3
Neistadt
Physical Science

2. How do you tell atoms apart?
• All atoms have neutrons, protons and
electrons.
• *It is the number of protons in an atom that
distinguishes one atom from another

3. Atomic Number
• *Atomic Number
– The number of protons in an element is called its
atomic number.
– Every element has it’s own unique atomic number.

4. Mass Number
• *Mass Number: the total number of
protons and neutrons in the nucleus of an
atom is called the mass number.
• Atoms of the same element have the same
number of protons, but they can have
different numbers of neutrons

5. Isotopes
• *Isotopes are atoms of the same
element that have different numbers of
neutrons.
• These are all isotopes of hydrogen:

7. Where are the electrons?
• *Protons and neutrons are attracted to each
other inside the nucleus of the atom.
• *The electrons of an atom are orbiting around
the nucleus in an electron cloud.

8. Where are the electrons?
• *The electron cloud is divided into energy
levels.
• *The levels have less energy closer to the
nucleus, more energy the further from the
nucleus.
• *The further an electron is from the nucleus,
the more energy it has.

10. 18.2 Comparing Atoms
Key Question:
What are atoms and how
are they put together?
*Read text section 18.2
BEFORE Investigation 18.2

12. 18.3 The Periodic Table of Elements
• Elements are made up of only
one kind of atom.
• Compounds are made up of
combinations of atoms.
• Elements that are part of the
same group act alike.

13. 18.3 The Periodic Table of Elements
• Dimitri Mendeleev
(1834- 1907) organized
information about all the
known elements in a
table that visually
organized the similarities
between them.
• Mendeleev placed each
element on the table in a
certain row and column
based on its properties.

16. 18.3 The Periodic Table of Elements
• The chemical symbol
is an abbreviation of
the element’s name.
• The atomic number is
the number of
protons all atoms of
that element have in
their nuclei.

17. 18.3 The Periodic Table of Elements
• The mass number of
an element is the
total number of
protons and neutrons
in the nucleus.
• The atomic mass is
the average mass of
all the known
isotopes of the
element.

18. 18.3 The Periodic Table of Elements
• The symbols for
some elements don’t
always obviously
match their names.

19. 18.3 The Periodic Table of Elements
Key Question:
What does atomic structure have to do with the
periodic table?
*Read text section 18.3 BEFORE Investigation 18.3