Atomic Structure and the Periodic Table

Grade 10 · Science · Study Guide

Understand subatomic particles, electron configuration, and how the periodic table is organized by atomic structure.

An atom is the smallest unit of an element that retains that element’s chemical properties. Atoms consist of three types of subatomic particles: protons and neutrons are packed into the tiny, dense nucleus, while electrons occupy regions of space called orbitals outside the nucleus. Protons carry a positive charge (+1) and determine an element’s identity — an atom’s atomic number equals its proton count. Neutrons carry no charge and contribute to atomic mass. Electrons carry a negative charge (-1) and determine how an element behaves chemically.

Isotopes are atoms of the same element that differ in their number of neutrons, giving them different mass numbers (protons + neutrons) but identical chemical behavior. The atomic mass listed on the periodic table is a weighted average of all naturally occurring isotopes.

The modern periodic table arranges elements in order of increasing atomic number. Elements in the same vertical column (group or family) share similar chemical properties because they have the same number of valence electrons — the electrons in the outermost energy level. Horizontal rows are called periods; as you move across a period, atomic radius decreases and electronegativity generally increases. The periodic table is divided into metals, nonmetals, and metalloids, and into blocks (s, p, d, f) based on electron configuration.

Key terms

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.
Isotope: Atoms of the same element with the same number of protons but different numbers of neutrons.
Valence Electrons: Electrons in the outermost energy level of an atom; they determine chemical reactivity.
Period (Periodic Table): A horizontal row in the periodic table; elements in the same period have the same number of electron shells.
Group (Periodic Table): A vertical column in the periodic table; elements share the same number of valence electrons and similar properties.

Flashcards

Proton: Positively charged particle in the nucleus; the proton count equals the atomic number.
Neutron: Neutral particle in the nucleus; contributes to mass number and defines isotopes.
Electron: Negatively charged particle found outside the nucleus; involved in chemical bonding.
Atomic Number: Number of protons in the nucleus; uniquely identifies each element.
Mass Number: Protons + neutrons; used to identify specific isotopes (e.g., carbon-14).
Valence Electrons: Outermost electrons that participate in chemical reactions and bonding.
Period: Horizontal row in the periodic table; elements share the same number of electron shells.
Group/Family: Vertical column in the periodic table; elements share valence electron count and chemical behavior.

Questions

What determines the identity of an element — that is, what makes carbon always carbon?
1. Number of neutrons
2. Number of electrons
3. Number of protons
4. Atomic mass
Show answer
Answer: Number of protons
The number of protons (atomic number) uniquely identifies an element. Carbon always has 6 protons — change that and it becomes a different element.
Atoms of the same element that have different numbers of neutrons are called ___.
Show answer
Answer: isotopes
Isotopes are variants of an element with the same proton count but differing neutron counts, giving different mass numbers.
Elements in the same group of the periodic table have the same number of valence electrons.
Show answer
Answer: True
Group membership is defined by having the same number of valence electrons (e.g., all Group 1 elements have 1 valence electron), which drives their similar reactivity.
An atom of carbon-14 has how many neutrons?
1. 6
2. 8
3. 12
4. 14
Show answer
Answer: 8
Carbon has atomic number 6 (6 protons). Carbon-14 has a mass number of 14, so neutrons = 14 − 6 = 8.
As you move left to right across a period in the periodic table, what generally happens to atomic radius?
1. It increases
2. It stays the same
3. It decreases
4. It fluctuates randomly
Show answer
Answer: It decreases
Moving across a period, nuclear charge increases while electrons are added to the same shell, pulling them closer to the nucleus and shrinking the atomic radius.

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