Introduction to Atoms, Ions and Isotopes Chapter 5: Atomic Structure

What are atoms? The atom is a basic unit of matter that consists of a dense, central nucleus surrounded by a cloud of negatively charged electrons. While all atoms have protons, neutrons and electrons, atoms of different elements have different numbers of these.

Atoms Each element on the periodic table has an atomic number, which tells how many protons an atom of that element has. Each element also has an mass number. Mass number = # of Protons + # of Neutrons Amu = atomic mass unit

Electrons Neutral atoms have the same number of electrons (negative charge) as they do protons (positive charge). # of electrons = atomic number ONLY IN NEUTRAL ATOMS

Let’s Practice! Find Boron on the periodic table. How many protons does an atom of Boron have? How many neutrons? Assume the mass number is 11 How many electrons?

Isotopes All neutral atoms of the same element have the same number of protons, and therefore the same number of electrons. The number of neutrons CAN vary. Isotopes: atoms of an element that have the same number of protons, but a different number of neutrons.

Average Atomic Mass vs. Mass Number The mass shown on your periodic table as a decimal is the average atomic mass of all isotopes of an atom in nature. By contrast, Mass Numbers are not on the periodic table – but must be given for each atom. The mass number is the sum of protons and neutrons in one atom of that element

Example: Lithium 2 Isotopes are found in nature: Lithium-6 and Lithium-7 (Lithium-8 is radioactive and quickly decays) Atomic mass is listed as 6.94. How is that possible? 94% of Li atoms have an atomic mass of 7, and 6% have an atomic mass of 6. Avg. Atomic Mass = (7 x .94) + (6 x .06)

Ions An atom or small molecule with an overall positive or negative charge due to an imbalance of protons and electrons. # of protons does not change # of electrons changes. Positive charge = loss of electrons Negative charge = gain of electron

Ions Continued… Ions are represented by a superscript charge on the element symbol. Examples: F-, Mg2+, Al3+ Practice: How many protons, neutrons, and electrons does Al3+ have?

Let’s Review! ________ are charged atoms. _________ ions have LOST electrons. _________ ions have GAINED electrons. An atom of Cu2+ with mass number 63 has: ________ protons ________ neutrons ________ electrons

More Review ____________ are atoms of the same element that have different numbers of neutrons in the nucleus. What does average atomic mass represent?

Complete Atomic Structure Practice Problems Hint: Chromium-52 means Chromium with a mass number of 52. 26Fe means iron with atomic number 26.