Lesson 1 – Introduction to Atoms

Atoms are made of even smaller particles called neutrons, protons, and electrons.

An atom consists of a nucleus surrounded by one or more electrons.

The number of protons equals the number of electrons.

As a result, the positive charge from the protons equals the negative charge from the electrons. The charges balance, making the atom neutral.

 

Electrons move within a sphere-shaped region surrounding the nucleus.

Most of an atom’s volume is the space in which electrons move.

Although electrons occupy most of an atom’s volume, they don’t account for much of its mass.

Together, the protons and neutrons make up nearly all the mass of an atom.

Atoms are too small to be measured in everyday units of mass, such as grams or kilograms, so scientists use units known as atomic mass units (amu)

An element can be identified by the number of protons in the nucleus of its atom.

Every atom of an element has the same number of protons.

Each element has a unique atomic number – the number of protons in its nucleus.

 

Although all atoms of an element have the same number of protons, their number of neutrons can vary.

An isotope is identified by its mass number, which is the sum of the protons and neutron in the nucleus of an atom.

Because atoms are so small, scientists create models to describe them.

Scientists use models to study objects and events that are too small, too large, too slow, too fast, too dangerous, or too far away to see.

In chemistry, models of atoms are used to explain how matter behaves.

  

The chemical elements can be organized into something like a calendar. The name of the “chemists” calendar Is the periodic table.

A Russian scientist, Dmitri Mendeleev discovered a set of patterns that applied to all the elements.

He noticed that a pattern of properties appeared when he arranged the elements in order of increasing atomic mass.

To find a pattern, Mendeleev wrote each element’s melting point, density, and color on individual cards. Along with the atomic mass and number of chemical bonds it could form.

Mendeleev found that the properties of elements repeated.

Mendeleev found that arranging the known elements strictly by increasing atomic mass did not always group similar elements together.

Mendeleev published the first periodic table in 1869.