Final answer:
Atoms consist of a dense core called the nucleus, holding protons and neutrons, and an extensive region outside the nucleus, where electrons form an electron cloud. These subatomic particles are not randomly arranged; the nucleus carries most of the atom's mass, and the electron cloud indicates probable electron locations.
Step-by-step explanation:
Atoms are fundamental units of matter and consist of three types of subatomic particles: protons, neutrons, and electrons. Protons and neutrons are densely packed in the central part of the atom known as the nucleus, which possesses a positive electric charge. The nucleus provides most of the atom's mass. In contrast, electrons are located outside the nucleus, occupying a region that can be described by an electron cloud. This cloud represents the areas where electrons are most likely to be found as they orbit the nucleus. The arrangement is not random; protons and neutrons cluster tightly in the nucleus, while electrons form a cloud around them.
The concept of the electron cloud stems from quantum mechanics, which describes electrons not as fixed particles orbiting the nucleus like planets around the sun, but as a probabilistic distribution of where they might be at any given time. This understanding replaces earlier atomic models that depicted electrons moving in precise orbits.
The relative sizes of the nucleus and the electron cloud are starkly different. The nucleus is comparable in size to a marble, while the electron cloud's space relative to the nucleus is like a soccer field to a marble. This vast difference illustrates that most of the atom's space is empty, with the electrons whizzing through this largely vacant area.