Comparing Atoms&

The Periodic Table

Chapter 18.2 and 18.3Neistadt

Physical Science

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

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.

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

Isotopes

• *Isotopes are atoms of the same element that have different numbers of neutrons.

• These are all isotopes of hydrogen:

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.

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.

18.2 Comparing Atoms

Key Question:

What are atoms and how are they put together?

*Read text section 18.2 BEFORE Investigation 18.2

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.

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.

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.

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.

18.3 The Periodic Table of Elements

• The symbols for some elements don’t always obviously match their names.

18.3 The Periodic Table of ElementsKey Question:

What does atomic structure have to do with the periodic table?

*Read text section 18.3 BEFORE Investigation 18.3