The basic building block of matter. The way atoms interact each other is a critical part of electrical engineering but isn’t directly applicable until dealing with semiconductor devices.
The smallest particle of an element that possesses the unique characteristics of that element.
Electronic Devices : Conventional Current Version, 9th Edition by Thomas L. Floyd
The smallest particle of an element that still has the same characteristics as the element.
Grob’s Basic Electronics, 11th Edition by Mitchel E. Schultz
An atom is the smallest constituent unit of ordinary matter that constitutes a chemical element. Every solid, liquid, gas, and plasma is composed of neutral or ionized atoms. Atoms are extremely small; typical sizes are around 100 picometers (1×10−10 m, a ten-millionth of a millimeter, or 1/254,000,000 of an inch). They are so small that accurately predicting their behavior using classical physics – as if they were billiard balls, for example – is not possible. This is due to quantum effects. Current atomic models now use quantum principles to better explain and predict this behavior.