Atomic Structure Basics

Grade 7 · Science · Study Guide

An introduction to the structure of atoms, including protons, neutrons, electrons, and how they determine an element's identity.

All matter is made of atoms — tiny particles so small that millions could fit on the period at the end of this sentence. Atoms are the basic building blocks of every substance around you, from the air you breathe to the food you eat.

An atom has two main regions: the nucleus at the center and the electron cloud surrounding it. The nucleus contains protons, which have a positive charge, and neutrons, which have no charge. Electrons, which have a negative charge, occupy the space outside the nucleus in regions called energy levels or shells.

The number of protons in an atom's nucleus is called the atomic number and determines what element the atom is. For example, every carbon atom has exactly 6 protons. Every oxygen atom has 8. If you change the number of protons, you change the element entirely. The periodic table organizes all known elements in order of their atomic numbers.

The mass number of an atom equals the number of protons plus the number of neutrons. Atoms of the same element can have different numbers of neutrons — these are called isotopes. In a neutral atom, the number of electrons equals the number of protons, balancing the charges. If an atom gains or loses electrons, it becomes a charged particle called an ion.

Key terms

Atom: The smallest unit of an element that retains the chemical properties of that element.
Proton: A positively charged particle found in the nucleus of an atom; its count defines the element.
Neutron: A particle with no charge found in the nucleus of an atom; contributes to atomic mass.
Electron: A negatively charged particle that orbits the nucleus in energy levels; responsible for chemical bonding.
Atomic number: The number of protons in the nucleus of an atom, which identifies the element.
Isotope: Atoms of the same element that have the same number of protons but different numbers of neutrons.

Flashcards

Nucleus: The dense central core of an atom containing protons and neutrons.
Atomic number: The number of protons in an atom's nucleus; uniquely identifies each element.
Mass number: The total number of protons and neutrons in an atom's nucleus.
Ion: An atom or molecule with a net electric charge due to gaining or losing one or more electrons.
Element: A pure substance made of only one kind of atom, identified by its atomic number.
Periodic table: A chart that organizes all known elements by increasing atomic number and groups them by similar properties.
Energy level: A region around an atom's nucleus where electrons are likely to be found; electrons in higher levels have more energy.
Isotope: Atoms of the same element with the same number of protons but different numbers of neutrons.

Questions

What determines which element an atom belongs to?
1. The number of neutrons
2. The number of electrons
3. The number of protons
4. The mass number
Show answer
Answer: The number of protons
The number of protons (atomic number) is what defines an element. Change the proton count and you have a completely different element.
Neutrons have a positive charge and are found in the nucleus.
Show answer
Answer: False
Neutrons have NO charge — they are neutral particles. Protons are the positively charged particles in the nucleus.
The central region of an atom that contains protons and neutrons is called the ______.
Show answer
Answer: nucleus
The nucleus is the small, dense core at the center of every atom. It contains protons and neutrons and holds most of the atom's mass.
An atom of carbon has 6 protons and 6 neutrons. What is its mass number?
1. 6
2. 12
3. 18
4. 3
Show answer
Answer: 12
Mass number = protons + neutrons = 6 + 6 = 12. This is why carbon-12 is the most common form of carbon and is used as the standard for atomic mass units.
In a neutral atom, the number of protons equals the number of electrons.
Show answer
Answer: True
In a neutral atom, positive and negative charges balance out. One proton (+1) cancels one electron (-1), so a neutral atom always has equal numbers of each.

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