Atoms

Atoms

by Roy Mathews

​An atom is the smallest neutral particle of any chemical element. Pure substances made up of only one kind of atom are called elements.

• ​All atoms have a central nucleus, which contains a positively (+) charged particle called a proton and a neutral (o) particle called a neutron. Hydrogen is an exception. Most hydrogen atoms have no neutron in the nucleus.

• ​All around the nucleus, there are negatively (-) charged particles located in different energy levels or shells called electrons.

• ​Protons, neutrons, and electrons are called sub-atomic particles.

• ​Protons and neutrons are much heavier than electrons - about 2000 times heavier than electrons.

• ​Atoms have the same number of protons and electrons. It makes the overall charge of an atom zero.

​Structure of a carbon atom

Atomic number

​The atomic number of an element is the total number of protons present in the nucleus of its atom. Atomic number of Lithium is 3. A lithium atom has 3 protons in its nucleus.

​Mass number (atomic mass)

​The mass number the atomic mass of an element is is the total number of protons and neutrons present in the nucleus of its atom. For example, a lithium atom has 3 protons and 4 neutrons, so the mass number of lithium is 7.

​

​A lithium atom

​Remember these basic facts

​In a neutral atom

​atomic number = number of protons

​number of protons = number of electrons

​mass number (atomic mass) = protons + neutrons

​number of neutrons = Mass number - atomic number

​A lithium atom has:

​3 protons

​3 electrons

​4 neutrons

Multiple Choice

Elements are pure substances made up of: _______

• one kind of atoms

• different types of atoms

• one type of molecules

• Several types of molecules.

Multiple Choice

Atoms are neutral particles because they have:

• equal numbers of protons and neutrons.

• equal numbers of protons and electrons

• equal numbers of neutrons and electrons.

• many electron shells.

Multiple Choice

What type of sub-atomic particles are found in the nucleus of an atom?

• Protons and shells

• Electrons and neutrons

• Protons and electrons

• Protons and neutrons

Multiple Choice

Among the three types of sub-atomic particles, which is the lightest?

• Electrons

• Protons

• Neutrons

• ions

Multiple Choice

Mass of an atom (atomic mass) is mainly due to its:

• Protons and neutrons

• Protons and electrons

• Neutrons and electrons

• electron shells

Multiple Choice

An aluminium atom has 13 protons and 14 neutrons in its nucleus. How many electrons does it have?

• 13

• 14

• 27

• 17

Multiple Choice

This is the symbol for chlorine. What is the atomic number of chlorine?

• 35

• 17

• 52

• 18

Multiple Choice

How many protons does a chlorine atom have?

• 35

• 18

• 17

• 52

Multiple Choice

How many electrons does a chlorine atom have?

• 35

• 17

• 18

• 52

Multiple Choice

How many neutrons does a chlorine atom have?

• 35

• 17

• 18

• 52

Open Ended

What are atoms? Write a definition in the space provided.

Open Ended

A lithium atom has positively charged particles and negatively charged particles. Explain why the lithium atom as a whole has no charge.

Fill in the Blank

Three sub-atomic particles present in an atom are protons, _________, and electrons.

Fill in the Blank

If the atomic number of an atom is 18 and its atomic mass is 40, then it must have ___ neutrons in its nucleus.

Fill in the Blank

In a neutral atom, the number of protons is always equal to the number of ___________