Atoms & Molecules

Atoms, Molecules and the Periodic Table

The classification of matter

We already know how matter can be classified, according to its composition. Substances can be either pure substances or mixtures. Pure substances can be elements or compounds, while mixtures can be homogeneous or heterogeneous.

Here, we will concentrate on elements and compounds.

Atoms and molecules, a review

We already talked a bit about them, but let's make things clear.

An atom is the smallest indivisible piece of matter we can have. It is so small that we can't see individual atoms through a light microscope. Atoms have two parts, the nucleus (where we can find protons, with positive electric charge, and neutrons, with no electric charge) and the electrons, with negative electric charge, and which move around the nucleus. Elements are composed of multiple copies of the same atom.

A molecule is a grouping of two or more atoms which is electrically neutral. Molecules can be quite small (like the water molecule H₂O) or very large (like the molecules of proteins). Compounds are composed of multiple copies of the same molecule.

It is very important to distinguish atoms from molecules, as they are different things.

Multiple Select

Which of these pictures represent a molecule?

Multiple Select

Which part of the atom has more mass?

• The proton

• The neutron

• The electron

Multiple Select

Which part of the atom has electrical charge?

• The proton

• The neutron

• The electron

Multiple Select

Which of these contain molecules?

• The oxygen we breathe

• The gold of my earrings

• The water we drink

• The sugar we eat

Different atoms

There are many different atoms, depending on the number of protons, neutrons and electrons they have.

The simplest possible atom is the Hydrogen atom. It is composed by 1 proton and 1 electron: remember the atom needs to be electrically neutral, and so the positive charge of the proton needs to be balanced by the negative charge of the electron. All atoms will have as many protons as electrons, in order to be electrically neutral.

If we change the number of protons in an atom, the atom changes its name. For example, an atom with 2 protons is called a Helium atom; an atom with 8 protons is an Oxygen atom, an atom with 79 protons is a Gold atom... and the largest known atom, Oganesson, has 118 protons in its nucleus.

Multiple Select

Which of these is true?

• The atomic number represents the number of protons in an atom.

• The atomic number represents the number of neutrons in an atom.

• The atomic number represents the number of electrons in an atom.

Multiple Select

What happens when we change the number of protons of an atom?

• The atom changes its name.

• The atom changes its atomic number

• The atom remains the same.

• The atom changes its number of electrons.

The Periodic Table

As we said, the number of protons defines the name of the atom. We have thus 118 different atoms, from the smallest, Hydrogen, to the largest, Oganesson.

If we have a substance with only one type of atom, we call this substance an element.

Elements are usually arranged in a table, known as the Periodic Table of the Elements. In it, each atom is represented by one or two letters. The number on top, called the atomic number represents the number of protons in an atom of a given element. For example, an atom of the element Carbon (C) will have 6 protons, an atom of the element Calcium (Ca) will have 20 protons, and so on.

The atomic number is usually called Z; therefore, an atom with Z=26 will have 26 protons in its nucleus and 26 electrons around it, and it will be an Iron (Fe) atom.

Multiple Select

Which of these are true?

Use the Periodic Table of your agenda or search "Periodic Table" on Google Images to get one.

• Hydrogen is the atom with the lowest atomic number Z.

• Carbon has a larger atomic number than Oxygen.

• Lithium (Li), Sodium (Na) and Magnesium (K) are in the same column of the Periodic Table

• The Aluminium (Al) atom has more protons than the Nitrogen (N) atom.

Multiple Choice

Who created the first Periodic Table of the Elements?

Fill in the Blanks

Multiple Choice

Which of these images represents an oxygen atom ?