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 Blanks

Three sub-atomic particles present in an atom are protons,

, and electrons.

Fill in the Blanks

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 Blanks

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