Atoms, Elements and Isotopes

Describe atomic structure and explain isotopes.

~18 min Chemistry

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Atoms are the building blocks of every element.

In this lesson you will:

Name the subatomic particles and their charges.
Define atomic number and mass number.
Explain what an isotope is.

Inside the Atom

An atom has a tiny central nucleus containing protons and neutrons, surrounded by electrons in shells.

Proton — positive charge, mass number 1.
Neutron — no charge, mass number 1.
Electron — negative charge, negligible mass.

Numbers That Identify an Atom

Atomic number (Z) = number of protons. Mass number (A) = protons + neutrons. In a neutral atom, electrons equal protons.

Isotopes

Isotopes are atoms of the same element with the same number of protons but different numbers of neutrons. Carbon-12 and carbon-14 are isotopes; carbon-14 is radioactive and used in dating.

Count the particles

A sodium atom has Z = 11 and A = 23.

Protons = 11; electrons = 11; neutrons = 23 − 11 = 12.

Tip Mass number is always the larger number written at the top-left of the symbol: ²³Na.

Practice

State the charge and location of protons, neutrons and electrons.
An atom has 17 protons and 18 neutrons. Find its atomic and mass numbers.
Explain why isotopes of an element have the same chemical properties.

Recap

Atoms contain protons, neutrons and electrons. Z counts protons; A counts protons plus neutrons. Isotopes differ only in neutrons.

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Course contents

Atomic Structure and Bonding

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