What do we call the numbers that cannot be arranged into 2-row arrays?

The numbers that cannot be arranged into 2-row arrays are known as prime numbers. Prime numbers are integers greater than 1 that are divisible only by 1 and themselves.

To determine if a number is prime or not, you can follow these steps:

1. Take the number you want to test for primality.
2. Check if the number is divisible by any integer starting from 2 up to its square root (rounded up to the nearest whole number).
- If the number is divisible by any integer within this range, it is not prime.
- If the number is not divisible by any integer within this range, it is prime.

For example, let's check if the number 17 is prime:

1. Start with the number 17.
2. Determine the square root of 17, which is approximately 4.12 (rounded up to 5).
3. Check divisibility by any integer from 2 to 5:
- 17 is not divisible by 2.
- 17 is not divisible by 3.
- 17 is not divisible by 4.
- 17 is not divisible by 5.
4. Since there are no divisors within the range of 2 to 5, we can conclude that 17 is a prime number.

So, the numbers that cannot be arranged into 2-row arrays are called prime numbers.